295 results on '"Hegerl, Gabriele"'
Search Results
252. A large-scale perspective on the meteorological modulation of air quality over China in winter
- Author
-
Jia, Zixuan, Doherty, Ruth, Bollasina, Massimo, and Hegerl, Gabriele
- Subjects
Chinese air pollution ,East Asian winter monsoon ,El Nin~o-Southern Oscillation ,air quality ,PM2.5 concentrations ,ENSO-EAWM relationship ,air pollutant emissions - Abstract
Rapid economic and industrial growth in China has led to serious air pollution with high concentrations of suspended fine particulate matter (PM2.5), in particular during winter. On a regional scale, meteorological conditions play a major role in modulating the accumulation, transport, removal and transformation of air pollutants. These meteorological conditions are affected by large-scale circulation patterns over China, dominated by the East Asian winter monsoon (EAWM). However, previous studies of how the large-scale winter circulation modulates air quality in China primarily focused on the North China Plain. The study of regional differences in the dominant large-scale circulation patterns needed to project future climate-driven PM2.5 concentration changes is far from complete. On longer inter-annual timescales, the EAWM is in turn influenced by El Niño-Southern Oscillation (ENSO) and the ensuing Pacific-East Asia teleconnection pattern. Better understanding of the ENSO-EAWM relationship and changes in this relationship under global warming is needed. Firstly, the influence of large-scale circulation on daily PM2.5 variability through its direct effect on key regional meteorological variables over three major populated regions of China (Beijing-Tianjin-Hebei, BTH; the Yangtze River Delta, YRD; the Pearl River Delta, PRD) is examined, based on a new high-resolution air quality reanalysis dataset for China for five winters from December 2013 to February 2018. In BTH, a shallow East Asian trough curbs northerly cold and dry air from the Siberian High, enhancing PM2.5 pollution levels. Weak near-surface southerly winds in eastern and southern China, associated with a weakened Siberian High, suppress horizontal dispersion, contributing to air pollution accumulation over YRD. In PRD, weak southerly winds and precipitation deficits over southern China are conducive to high PM2.5 concentrations. To account for these dominant large-scale circulation-PM2.5 relationships, we propose three new circulation-based indices: a 500 hPa geopotential height-based index for BTH, a sea level pressure-based index for YRD and an 850 hPa meridional wind-based index for PRD. These three indices can effectively distinguish clean days from heavily polluted days in these regions, assuming PM2.5 variability is solely due to meteorology. Subsequently, the influence of the winter large-scale circulation on daily PM2.5 concentrations and on the sensitivity of PM2.5 to emissions over major populated regions of China with a focus on YRD is investigated, using the United Kingdom Earth System Model, UKESM1. Weak flow of near-surface cold, dry air from the north and weak inflow of maritime air are conducive to air pollution over YRD for 1999-2019. These provide favourable conditions for the accumulation of local pollution but limit the transport of air pollutants into YRD from the north for 2014- 2019. Based on the dominant large-scale circulation, we construct a new index using the north-south pressure gradient to project PM2.5 concentrations over the region. We show that this index can effectively distinguish different levels of pollution over YRD and explain changes in PM2.5 sensitivity to emissions from local and northern regions. We then project future changes in PM2.5 concentrations using this index under the weak climate and air pollutant mitigation scenario (SSP3- 7.0). We find an increase in PM2.5 concentrations over YRD due to climate-driven circulation changes that is expected to partially offset the effect of emission control measures in the near-term future. Finally, changes in the relationship between ENSO and the EAWM at various global warming levels during the 21st century are examined based on experiments from the Max Planck Institute Grand Ensemble (MPI-GE) that represent the upper boundary of the range of emissions scenario (RCP 8.5). The externally forced component of this relationship (i.e. forced by greenhouse gases and anthropogenic aerosol emissions) strengthens under moderate warming (+1.5 ◦ C), and then weakens for +3 ◦ C warming. These changes are characterised by variations in strength and location of the core of El Niño-related warming and associated deep convection anomalies over the equatorial Pacific leading to circulation anomalies across the Asian-Pacific region. Under global warming, the ENSO-EAWM relationship is strongly related to the background mean state of both the EAWM and ENSO, through changesin the EAWM strength and a shift of the ENSO pattern. Anthropogenic aerosols also play a key role in influencing the ENSO-EAWM relationship under moderate warming (up to 1.5 ◦C). These results demonstrate the importance of understanding the occurrence of days with elevated PM2.5 concentrations and explaining changes in the sensitivity of PM2.5 to emissions from local and surrounding regions from a large-scale perspective. These findings could help project the occurrence of heavily polluted PM2.5 days during wintertime and assess future emission control strategies for PM2.5 air quality improvement under climate change. Furthermore, the ENSO-EAWM relationship is shown to have a substantial inter-decadal variation under global warming. This may further improve the accuracy of future predictions of air quality in China.
- Published
- 2023
- Full Text
- View/download PDF
253. Technical Summary
- Author
-
Solomon, Susan, Qin, Dahe, Manning, Martin, Alley, Richard B., Berntsen, Terje, Nathaniel Bindoff, Chen, Zhenlin, Chidthaisong, Amnat, Gregory, Jonathan M., Hegerl, Gabriele C., Heimann, Martin, Hewitson, Bruce, Hoskins, Brian J., Joos, Fortunat, Jouzel, Jean, Kattsov, Vladimir, Lohmann, Ulrike, Matsuno, Taroh, Molina, Mario, Nicholls, Neville, Overpeck, Jonathan, Raga, Graciela, Ramaswamy, Venkatachalam, Ren, Jiawen, Rusticucci, Matilde, Somerville, Richard, Stocker, Thomas F., Stouffer, Ronald J., Whetton, Penny, Wood, Richard A., and Wratt, David
254. An assessment of Earth's climate sensitivity using multiple lines of evidence
- Author
-
Sherwood, Steven, Webb, Mark J., Annan, James D., Armour, Kyle, Forster, Piers M., Hargreaves, Julia C., Hegerl, Gabriele, Klein, Stephen A., Marvel, Kate D., Rohling, Eelco J., Watanabe, Masahiro, Andrews, Timothy, Braconnot, Pascale, Bretherton, Christopher S., Foster, Gavin L., Hausfather, Zeke, von der Heydt, Anna S., Knutti, Reto, Mauritsen, Thorsten, Norris, J.R., Proistosescu, Cristian, Rugenstein, Maria, Schmidt, Gavin A., Tokarska, Katarzyna B., and Zelinka, Mark D.
- Subjects
Bayesian methods ,13. Climate action ,Climate ,climate sensitivity ,global warming - Abstract
We assess evidence relevant to Earth's equilibrium climate sensitivity per doubling of atmospheric CO2, characterized by an effective sensitivity S. This evidence includes feedback process understanding, the historical climate record, and the paleoclimate record. An S value lower than 2 K is difficult to reconcile with any of the three lines of evidence. The amount of cooling during the Last Glacial Maximum provides strong evidence against values of S greater than 4.5 K. Other lines of evidence in combination also show that this is relatively unlikely. We use a Bayesian approach to produce a probability density (PDF) for S given all the evidence, including tests of robustness to difficult‐to‐quantify uncertainties and different priors. The 66% range is 2.6‐3.9 K for our Baseline calculation, and remains within 2.3‐4.5 K under the robustness tests; corresponding 5‐95% ranges are 2.3‐4.7 K, bounded by 2.0‐5.7 K (although such high‐confidence ranges should be regarded more cautiously). This indicates a stronger constraint on S than reported in past assessments, by lifting the low end of the range. This narrowing occurs because the three lines of evidence agree and are judged to be largely independent, and because of greater confidence in understanding feedback processes and in combining evidence. We identify promising avenues for further narrowing the range in S, in particular using comprehensive models and process understanding to address limitations in the traditional forcing‐feedback paradigm for interpreting past changes., Reviews of Geophysics, 58 (4), ISSN:8755-1209, ISSN:0096-1043, ISSN:1944-9208
255. The Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP): Experimental design and forcing input data for CMIP6
- Author
-
Zanchettin, Davide, Khodri, Myriam, Timmreck, Claudia, Toohey, Matthew, Schmidt, Anja, Gerber, Edwin P., Hegerl, Gabriele, Robock, Alan, Pausata, Francesco S.R., Ball, William T., Bauer, Susanne E., Bekki, Slimane, Dhomse, Sandip S., Le Grande, Allegra N., Mann, Graham W., Marshall, Lauren, Mills, Michael, Marchand, Marion, Niemeier, Ulrike, Poulain, Virginie, Rozanov, Eugene, Rubino, Angelo, Stenke, Andrea, Tsigaridis, Kostas, and Tummon, Fiona
- Subjects
13. Climate action - Abstract
The enhancement of the stratospheric aerosol layer by volcanic eruptions induces a complex set of responses causing global and regional climate effects on a broad range of timescales. Uncertainties exist regarding the climatic response to strong volcanic forcing identified in coupled climate simulations that contributed to the fifth phase of the Coupled Model Intercomparison Project (CMIP5). In order to better understand the sources of these model diversities, the Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP) has defined a coordinated set of idealized volcanic perturbation experiments to be carried out in alignment with the CMIP6 protocol. VolMIP provides a common stratospheric aerosol data set for each experiment to minimize differences in the applied volcanic forcing. It defines a set of initial conditions to assess how internal climate variability contributes to determining the response. VolMIP will assess to what extent volcanically forced responses of the coupled ocean–atmosphere system are robustly simulated by state-of-the-art coupled climate models and identify the causes that limit robust simulated behavior, especially differences in the treatment of physical processes. This paper illustrates the design of the idealized volcanic perturbation experiments in the VolMIP protocol and describes the common aerosol forcing input data sets to be used., Geoscientific Model Development, 9 (8), ISSN:1991-9603, ISSN:1991-959X
256. Decadal Prediction: Can It Be Skillful?
- Author
-
Meehl, Gerald A., Goddard, Lisa M., Murphy, James, Stouffer, Ronald J., Boer, George, Danabasoglu, Gokhan, Dixon, Keith, Giorgetta, Marco A., Greene, Arthur M., Hawkins, Ed, Hegerl, Gabriele, Karoly, David, Keenlyside, Noel, Kimoto, Masahide, Kirtman, Ben, Navarra, Antonio, Pulwarty, Roger, Smith, Doug, Stammer, Detlef, and Stockdale, Timothy
- Subjects
13. Climate action ,Climatic changes--Forecasting ,Atmosphere ,Climatic changes ,Climatic changes--Research - Abstract
A new field of study, “decadal prediction,” is emerging in climate science. Decadal prediction lies between seasonal/interannual forecasting and longer-term climate change projections, and focuses on time-evolving regional climate conditions over the next 10–30 yr. Numerous assessments of climate information user needs have identified this time scale as being important to infrastructure planners, water resource managers, and many others. It is central to the information portfolio required to adapt effectively to and through climatic changes. At least three factors influence time-evolving regional climate at the decadal time scale: 1) climate change commitment (further warming as the coupled climate system comes into adjustment with increases of greenhouse gases that have already occurred), 2) external forcing, particularly from future increases of greenhouse gases and recovery of the ozone hole, and 3) internally generated variability. Some decadal prediction skill has been demonstrated to arise from the first two of these factors, and there is evidence that initialized coupled climate models can capture mechanisms of internally generated decadal climate variations, thus increasing predictive skill globally and particularly regionally. Several methods have been proposed for initializing global coupled climate models for decadal predictions, all of which involve global time-evolving three-dimensional ocean data, including temperature and salinity. An experimental framework to address decadal predictability/prediction is described in this paper and has been incorporated into the coordinated Coupled Model Intercomparison Model, phase 5 (CMIP5) experiments, some of which will be assessed for the IPCC Fifth Assessment Report (AR5). These experiments will likely guide work in this emerging field over the next 5 yr.
257. Influence of human and natural forcing on European seasonal temperatures
- Author
-
Hegerl, Gabriele, Tett, Simon F.B., Luterbacher, Jürg, Crowley, Thomas, González-Rouco, Fidel, and Xoplaki, Elena
- Subjects
13. Climate action
258. Causes of climate change over the historical record
- Author
-
Hegerl, Gabriele C, Brönnimann, Stefan, Cowan, Tim, Friedman, Andrew R, Hawkins, Ed, Iles, Carley, Müller, Wolfgang, Schurer, Andrew, and Undorf, Sabine
- Subjects
13. Climate action ,910 Geography & travel - Abstract
This review addresses the causes of observed climate variations across the industrial period, from 1750 to present. It focuses on long-term changes, both in response to external forcing and to climate variability in the ocean and atmosphere. A synthesis of results from attribution studies based on palaeoclimatic reconstructions covering the recent few centuries to the 20th century, and instrumental data shows how greenhouse gases began to cause warming since the beginning of industrialization, causing trends that are attributable to greenhouse gases by 1900 in proxy-based temperature reconstructions. Their influence increased over time, dominating recent trends. However, other forcings have caused substantial deviations from this emerging greenhouse warming trend: volcanic eruptions have caused strong cooling following a period of unusually heavy activity, such as in the early 19th century; or warming during periods of low activity, such as in the early-to-mid 20th century. Anthropogenic aerosol forcing most likely masked some global greenhouse warming over the 20th century, especially since the accelerated increase in sulphate aerosol emissions starting around 1950. Based on modelling and attribution studies, aerosol forcing has also influenced regional temperatures, caused long-term changes in monsoons and imprinted on Atlantic variability. Multi-decadal variations in atmospheric modes can also cause long-term climate variability, as apparent for the example of the North Atlantic Oscillation, and have influenced Atlantic ocean variability. Long-term precipitation changes are more difficult to attribute to external forcing due to spatial sparseness of data and noisiness of precipitation changes, but the observed pattern of precipitation response to warming from station data supports climate model simulated changes and with it, predictions. The long-term warming has also led to significant differences in daily variability as, for example, visible in long European station data. Extreme events over the historical record provide valuable samples of possible extreme events and their mechanisms.
259. Possible causes of data model discrepancy in the temperature history of the last Millennium
- Author
-
Neukom, Raphael, Schurer, Andrew P., Steiger, Nathan. J., and Hegerl, Gabriele C.
- Subjects
13. Climate action ,550 Earth sciences & geology ,910 Geography & travel - Abstract
Model simulations and proxy-based reconstructions are the main tools for quantifying pre-instrumental climate variations. For some metrics such as Northern Hemisphere mean temperatures, there is remarkable agreement between models and reconstructions. For other diagnostics, such as the regional response to volcanic eruptions, or hemispheric temperature differences, substantial disagreements between data and models have been reported. Here, we assess the potential sources of these discrepancies by comparing 1000-year hemispheric temperature reconstructions based on real-world paleoclimate proxies with climate-model-based pseudoproxies. These pseudoproxy experiments (PPE) indicate that noise inherent in proxy records and the unequal spatial distribution of proxy data are the key factors in explaining the data-model differences. For example, lower inter-hemispheric correlations in reconstructions can be fully accounted for by these factors in the PPE. Noise and data sampling also partly explain the reduced amplitude of the response to external forcing in reconstructions compared to models. For other metrics, such as inter-hemispheric differences, some, although reduced, discrepancy remains. Our results suggest that improving proxy data quality and spatial coverage is the key factor to increase the quality of future climate reconstructions, while the total number of proxy records and reconstruction methodology play a smaller role.
260. Toward Consistent Observational Constraints in Climate Predictions and Projections
- Author
-
Hegerl, Gabriele C., Ballinger, Andrew P., Booth, Ben B.B., Borchert, Leonard F., Brunner, Lukas, Donat, Markus G., Doblas-Reyes, Francisco J., Harris, Glen R., Lowe, Jason, Mahmood, Rashed, Mignot, Juliette, Murphy, James M., Swingedouw, Didier, and Weisheimer, Antje
- Subjects
13. Climate action ,Observational constraints ,Climate modeling ,Climate change ,Climate predictions ,Model evaluation ,Future projections - Abstract
Observations facilitate model evaluation and provide constraints that are relevant to future predictions and projections. Constraints for uninitialized projections are generally based on model performance in simulating climatology and climate change. For initialized predictions, skill scores over the hindcast period provide insight into the relative performance of models, and the value of initialization as compared to projections. Predictions and projections combined can, in principle, provide seamless decadal to multi-decadal climate information. For that, though, the role of observations in skill estimates and constraints needs to be understood in order to use both consistently across the prediction and projection time horizons. This paper discusses the challenges in doing so, illustrated by examples of state-of-the-art methods for predicting and projecting changes in European climate. It discusses constraints across prediction and projection methods, their interpretation, and the metrics that drive them such as process accuracy, accurate trends or high signal-to-noise ratio. We also discuss the potential to combine constraints to arrive at more reliable climate prediction systems from years to decades. To illustrate constraints on projections, we discuss their use in the UK's climate prediction system UKCP18, the case of model performance weights obtained from the Climate model Weighting by Independence and Performance (ClimWIP) method, and the estimated magnitude of the forced signal in observations from detection and attribution. For initialized predictions, skill scores are used to evaluate which models perform well, what might contribute to this performance, and how skill may vary over time. Skill estimates also vary with different phases of climate variability and climatic conditions, and are influenced by the presence of external forcing. This complicates the systematic use of observational constraints. Furthermore, we illustrate that sub-selecting simulations from large ensembles based on reproduction of the observed evolution of climate variations is a good testbed for combining projections and predictions. Finally, the methods described in this paper potentially add value to projections and predictions for users, but must be used with caution., Frontiers in Climate, 3, ISSN:2624-9553
261. Comparing Methods to Constrain Future European Climate Projections Using a Consistent Framework
- Author
-
Brunner, Lukas, Mc Sweeney, Carol, Ballinger, Andrew P., Befort, Daniel J., Benassi, Marianna, Booth, Ben, Coppola, Erika, de Vries, Hylke, Harris, Glen, Hegerl, Gabriele C., Knutti, Reto, Lenderink, Geert, Lowe, Jason, Nogherotto, Rita, O'Reilly, Chris, Qasimi, Saïd, Ribes, Aurélien, Stocchi, Paolo, and Undorf, Sabine
- Subjects
13. Climate action - Abstract
Political decisions, adaptation planning, and impact assessments need reliable estimates of future climate change and related uncertainties. To provide these estimates, different approaches to constrain, filter, or weight climate model projections into probabilistic distributions have been proposed. However, an assessment of multiple such methods to, for example, expose cases of agreement or disagreement, is often hindered by a lack of coordination, with methods focusing on a variety of variables, time periods, regions, or model pools. Here, a consistent framework is developed to allow a quantitative comparison of eight different methods; focus is given to summer temperature and precipitation change in three spatial regimes in Europe in 2041–60 relative to 1995–2014. The analysis draws on projections from several large ensembles, the CMIP5 multimodel ensemble, and perturbed physics ensembles, all using the high-emission scenario RCP8.5. The methods’ key features are summarized, assumptions are discussed, and resulting constrained distributions are presented. Method agreement is found to be dependent on the investigated region but is generally higher for median changes than for the uncertainty ranges. This study, therefore, highlights the importance of providing clear context about how different methods affect the assessed uncertainty—in particular, the upper and lower percentiles that are of interest to risk-averse stakeholders. The comparison also exposes cases in which diverse lines of evidence lead to diverging constraints; additional work is needed to understand how the underlying differences between methods lead to such disagreements and to provide clear guidance to users., Journal of Climate, 33 (20), ISSN:0894-8755, ISSN:1520-0442
262. Last phase of the Little Ice Age forced by volcanic eruptions
- Author
-
Brönnimann, Stefan, Franke, Jörg, Nussbaumer, Samuel U., Zumbühl, Heinz J., Steiner, Daniel, Trachsel, Mathias, Hegerl, Gabriele C., Schurer, Andrew, Worni, Matthias, Malik, Abdul, Flückiger, Julian, and Raible, Christoph C.
- Subjects
13. Climate action ,530 Physics ,910 Geography & travel
263. The early 20th century warming: Anomalies, causes, and consequences
- Author
-
Hegerl, Gabriele C., Brönnimann, Stefan, Schurer, Andrew, and Cowan, Tim
- Subjects
13. Climate action
264. Disentangling the causes of the 1816 European year without a summer
- Author
-
Schurer, Andrew P, Hegerl, Gabriele C, Luterbacher, Jürg, Brönnimann, Stefan, Cowan, Tim, Tett, Simon F B, Zanchettin, Davide, and Timmreck, Claudia
- Subjects
13. Climate action ,910 Geography & travel - Abstract
The European summer of 1816 has often been referred to as a 'year without a summer' due to anomalously cold conditions and unusual wetness, which led to widespread famines and agricultural failures. The cause has often been assumed to be the eruption of Mount Tambora in April 1815, however this link has not, until now, been proven. Here we apply state-of-the-art event attribution methods to quantify the contribution by the eruption and random weather variability to this extreme European summer climate anomaly. By selecting analogue summers that have similar sea-level-pressure patterns to that observed in 1816 from both observations and unperturbed climate model simulations, we show that the circulation state can reproduce the precipitation anomaly without external forcing, but can explain only about a quarter of the anomalously cold conditions. We find that in climate models, including the forcing by the Tambora eruption makes the European cold anomaly up to 100 times more likely, while the precipitation anomaly became 1.5–3 times as likely, attributing a large fraction of the observed anomalies to the volcanic forcing. Our study thus demonstrates how linking regional climate anomalies to large-scale circulation is necessary to quantitatively interpret and attribute post-eruption variability.
265. Understanding, modeling and predicting weather and climate extremes: Challenges and opportunities
- Author
-
Sillmann, Jana, Thorarinsdottir, Thordis, Keenlyside, Noel, Schaller, Nathalie, Alexander, Lisa V., Hegerl, Gabriele, Seneviratne, Sonia I., Vautard, Robert, Zhang, Xuebin, and Zwiers, Francis W.
- Subjects
13. Climate action - Abstract
Weather and climate extremes are identified as major areas necessitating further progress in climate research and have thus been selected as one of the World Climate Research Programme (WCRP) Grand Challenges. Here, we provide an overview of current challenges and opportunities for scientific progress and cross-community collaboration on the topic of understanding, modeling and predicting extreme events based on an expert workshop organized as part of the implementation of the WCRP Grand Challenge on Weather and Climate Extremes. In general, the development of an extreme event depends on a favorable initial state, the presence of large-scale drivers, and positive local feedbacks, as well as stochastic processes. We, therefore, elaborate on the scientific challenges related to large-scale drivers and local-to-regional feedback processes leading to extreme events. A better understanding of the drivers and processes will improve the prediction of extremes and will support process-based evaluation of the representation of weather and climate extremes in climate model simulations. Further, we discuss how to address these challenges by focusing on short-duration (less than three days) and long-duration (weeks to months) extreme events, their underlying mechanisms and approaches for their evaluation and prediction., Weather and Climate Extremes, 18, ISSN:2212-0947
266. Last phase of the Little Ice Age forced by volcanic eruptions
- Author
-
Brönnimann, Stefan, Franke, Jörg, Nussbaumer, Samuel U., Zumbühl, Heinz J., Steiner, Daniel, Trachsel, Mathias, Hegerl, Gabriele C., Schurer, Andrew, Worni, Matthias, Malik, Abdul, Flückiger, Julian, Raible, Christoph C., Brönnimann, Stefan, Franke, Jörg, Nussbaumer, Samuel U., Zumbühl, Heinz J., Steiner, Daniel, Trachsel, Mathias, Hegerl, Gabriele C., Schurer, Andrew, Worni, Matthias, Malik, Abdul, Flückiger, Julian, and Raible, Christoph C.
- Abstract
During the first half of the nineteenth century, several large tropical volcanic eruptions occurred within less than three decades. The global climate effects of the 1815 Tambora eruption have been investigated, but those of an eruption in 1808 or 1809 whose source is unknown and the eruptions in the 1820s and 1830s have received less attention. Here we analyse the effect of the sequence of eruptions in observations, global three-dimensional climate field reconstructions and coupled climate model simulations. All the eruptions were followed by substantial drops of summer temperature over the Northern Hemisphere land areas. In addition to the direct radiative effect, which lasts 2–3 years, the simulated ocean–atmosphere heat exchange sustained cooling for several years after these eruptions, which affected the slow components of the climate system. Africa was hit by two decades of drought, global monsoons weakened and the tracks of low-pressure systems over the North Atlantic moved south. The low temperatures and increased precipitation in Europe triggered the last phase of the advance of Alpine glaciers. Only after the 1850s did the transition into the period of anthropogenic warming start. We conclude that the end of the Little Ice Age was marked by the recovery from a sequence of volcanic eruptions, which makes it difficult to define a single pre-industrial baseline.
267. Linearity of the Climate Response to Increasingly Strong Tropical Volcanic Eruptions in a Large Ensemble Framework.
- Author
-
Timmreck, Claudia, Olonscheck, Dirk, Ballinger, Andrew P., D'Agostino, Roberta, Fang, Shih-Wei, Schurer, Andrew P., and Hegerl, Gabriele C.
- Abstract
Large explosive volcanic eruptions cause short-term climatic impacts on both regional and global scales. Their impact on tropical climate variability, in particular El Niño–Southern Oscillation (ENSO), is still uncertain, as is their combined and separate effect on tropical and global precipitation. Here, we investigate the relationship between large-scale temperature and precipitation and tropical volcanic eruption strength, using 100-member MPI-ESM ensembles for idealized equatorial symmetric Northern Hemisphere summer eruptions of different sulfur emission strengths. Our results show that for idealized tropical eruptions, global and hemispheric mean near-surface temperature and precipitation anomalies are negative and linearly scalable for sulfur emissions between 10 and 40 Tg S. We identify 20 Tg S emission as a threshold where the global ensemble-mean near-surface temperature and precipitation signals exceed the range of internal variability, even though some ensemble members emerge from variability for lower eruption strengths. Seasonal and ensemble mean patterns of near-surface temperature and precipitation anomalies are highly correlated across eruption strengths, in particular for larger emission strengths in the tropics, and strongly modulated by ENSO. There is a tendency to shift toward a warm ENSO phase for the first postvolcanic year as the emission strength increases. Volcanic cooling emerges on a hemisphere-wide scale, while the precipitation response is more localized, and emergence is mainly confined to the tropics and subtropics. Significance Statement: The purpose of this study is to investigate at which strength the climate responses of volcanic forcing can be distinguished from the internal climate variability and whether the responses will linearly increase as the emission strengths become stronger. We ran 100-member MPI-ESM ensembles of idealized equatorial volcanic eruptions of different sulfur emission strengths and find that seasonal and ensemble mean patterns of near-surface temperature and precipitation anomalies are distinguishable and linearly scalable for sulfur emissions from 10 to 40 Tg S if their forcing patterns are similar. The identification of volcanic fingerprints is important for seasonal to decadal forecasts in the case of potential future eruptions and could help to prepare society for the regional climatic consequences of such an event. [ABSTRACT FROM AUTHOR]
- Published
- 2024
- Full Text
- View/download PDF
268. Climate change detection and attribution using high resolution paleoclimate observations.
- Author
-
Franke, Jörg, Evans, Michael, Hegerl, Gabriele, Schurer, Andrew, and Brönnimann, Stefan
- Published
- 2018
269. Multiple constraints revisited.
- Author
-
Annan, James, Forster, Piers, Hargreaves, Julia, Hegerl, Gabriele, Klein, Steve, Knutti, Reto, Sherwood, Steven, Watanabe, Masahiro, and Webb, Mark
- Published
- 2018
270. Statistical modelling of environmental extremes
- Author
-
Auld, Graeme, Papastathopoulos, Ioannis, and Hegerl, Gabriele
- Abstract
This thesis is concerned with the development of theory and statistical methodologies that may be used to analyse environmental extremes. As extreme environmental events are often associated with large economic costs and loss of human life, accurate statistical modelling of such events is crucial in order to be able to accurately estimate their frequency and intensity. A key feature of environmental time series is that they display serial correlation which must be modelled in order for valid inferences to be drawn. One line of research in this thesis is the development of flexible time series models that may be used to simulate the behaviour of an environmental process after entering an extreme state. This allows us to estimate quantities such as the mean duration of an extreme event. We illustrate our modelling approach and methodology by simulating the behaviour of daily maximum temperature in Orleans, France, over a three week period given that the temperature exceeds 35C at the start of the period. Much of extreme value theory for time series has been developed under the assumption of strict stationarity, a mathematically convenient but often unrealistic assumption for environmental data. Our second project extends some well known classical results for strictly stationary time series to a more general setting that allows for non-stationarity. We show that for weakly dependent time series with common marginal distributions, the distribution of the sample maximum at large thresholds is characterized by a parameter that plays an analogous role to the extremal index of a stationary time series, and may be estimated similarly. Our results are applied to the particular case where non-stationarity arises through periodicity in the dependence structure as may be expected in certain environmental time series. We also show how our results may be further generalized to allow for different marginal distributions. Another strand of research in this thesis concerns the detection and quantification of changes in the distribution of the annual maximum daily maximum temperature (TXx) in a large gridded data set of European daily temperature during the years 1950-2018. We model TXx throughout Europe using a generalized extreme value distribution, with the log of the atmospheric concentration of CO2 as a covariate. It is commonplace in the geoscientific literature for such models to be fit separately at each spatial location over the domain of interest. To reflect the fact that nearby locations are expected to be similarly affected by any climate change, we instead consider models that incorporate spatial dependence, and thus increase efficiency in parameter estimation compared to separate model fits. We find strong evidence for shifts towards hotter temperatures throughout Europe. Averaged across our spatial domain, the 100-year return temperature based on the 2018 climate is approximately 2C hotter than that based on the 1950 climate. Our final project concerns the evaluation of bias in climate model output and how such biases contribute to biases in hazard indices. Based on copula theory we develop a multivariate bias-assessment framework, which allows us to disentangle the biases in hazard indicators in terms of biases in the underlying univariate drivers and their statistical dependence. Based on this framework, we dissect biases in fire and heat stress hazards in a set of global climate models by considering two simplified hazard indicators: the wet-bulb globe temperature (WBGT) and the Chandler burning index (CBI).
- Published
- 2022
- Full Text
- View/download PDF
271. I. INTRODUCTION TO EXPLAINING EXTREME EVENTS OF 2013 FROM A CLIMATE PERSPECTIVE.
- Author
-
STOTT, PETER A., HEGERL, GABRIELE C., HERRING, STEPHANIE C., HOERLING, MARTIN P., PETERSON, THOMAS C., XUEBIN ZHANG, and ZWIERS, FRANCIS W.
- Subjects
- *
CLIMATE change , *FLOODS , *STORMS - Abstract
An introduction to the journal dedicated on several extreme weather events in 2013 is presented.
- Published
- 2014
272. Using the Past to Predict the Future?
- Author
-
Hegerl, Gabriele C. and Russon, Tom
- Subjects
- *
CLIMATE change research , *PALEOCLIMATOLOGY , *LAST Glacial Maximum , *EQUILIBRIUM , *ATMOSPHERIC carbon dioxide , *GLOBAL warming , *OCEAN temperature , *EARTH temperature - Abstract
The article discusses a report in the same issue by Schmittner and colleagues regarding the use of paleoclimate data to estimate future climate change. Researchers believe that observing climate responses to past long-term events can help to predict the equilibrium climate sensitivity (ECS), or the equilibrium response of surface temperature to increasing atmospheric carbon dioxide. Reconstructions of ocean and land temperatures from the Last Glacial Maximum were used to determine the ECS range for that time period and help predict likely ESC values for recent climate data.
- Published
- 2011
- Full Text
- View/download PDF
273. The Importance of Accounting for the North Atlantic Oscillation When Applying Observational Constraints to European Climate Projections.
- Author
-
Ballinger, Andrew P., Schurer, Andrew P., O'Reilly, Christopher H., and Hegerl, Gabriele C.
- Subjects
- *
CLIMATE change models , *ATMOSPHERIC models , *CLIMATE change , *NORTH Atlantic oscillation , *SEA level - Abstract
Variability in the North Atlantic Oscillation (NAO) has contributed to the recent multidecadal trends observed in European climate, especially to trends in winter precipitation over Northern Europe. However, the current generation of coupled climate models struggle to reproduce the NAO's contribution to multidecadal trends, which has important implications for deriving constraints based on the comparison of observed and modeled trends. An observational constraint based on attribution results, both with and without the contribution of variability associated with the NAO, is applied to projections of Northern European precipitation and temperature, and observed NAO variability is shown to lead to a constraint that overestimates future forced changes. Only after removing the NAO variability is the observed climate change consistent with model simulations, and a tighter, unbiased observational constraint based on the forced signal (without the NAO) can be applied to future projections. Plain Language Summary: The observed precipitation and temperature across Northern Europe in winter has been increasing over the past several decades. However, the magnitude of this trend is generally not well reproduced by the latest set of global climate models. A part of the observed increase in precipitation can be related to the variability in the North Atlantic Oscillation (NAO), which is a large‐scale pattern of sea level pressure in the North Atlantic region that varies in strength each season and year, and is strongly linked to temperature and precipitation patterns across Europe, particularly in winter. In this study, we show that if the variability related to the NAO is first removed from the precipitation time series, from both the observations and the model simulations, the difference between the long‐term observed and modeled trends in precipitation is reduced. Removing the NAO variability also has an impact on the difference in observed and modeled trends in temperature. This has important implications for approaches that use the difference in observations and model simulations as a way of constraining future projections of climate change. Key Points: Variability in the North Atlantic Oscillation (NAO) has contributed to the recent multidecadal trends observed in European climateThe suite of current comprehensive global models (CMIP6) struggle to reproduce the NAO's contribution to multidecadal trendsRemoving the NAO from both observations and models provides a tighter, unbiased constraint, which can then be applied to model projections [ABSTRACT FROM AUTHOR]
- Published
- 2023
- Full Text
- View/download PDF
274. Isolating the impact of North American and European anthropogenic aerosol emissions since the early instrumental period
- Author
-
Undorf, Sabine, Bollasina, Massimo, and Hegerl, Gabriele
- Subjects
climate change during the twentieth century ,aerosol-climate interactions ,external forcing and internal climate variability ,atmospheric circulation changes ,drivers of Atlantic variability ,monsoon precipitation ,climate modelling - Abstract
Anthropogenic aerosols have been identified as an important driver of global and regional climate. Globally, aerosols are estimated to have offset much of the positive forcing due to greenhouse gases; regionally, their effect can be dominating, and can potentially drive climate anomalies far from the emission sources due to changes in the atmospheric circulation. Aerosols emitted from North America (NA) and Europe (EU) dominated the global aerosol loading until the late twentieth century. Despite recent progress, our knowledge of the climate imprint of NA and EU aerosols is still incomplete, especially regarding the decades before the mid-twentieth century, in which emissions were still lower and did not yet change as rapidly as later, but might have been more effective due to non-linearities in the aerosol-cloud interactions. The overarching goal of this work is thus to determine robust features of the impact of NA and EU aerosols on regional and large-scale climate and to advance current understanding of the underlying mechanisms, compared to those generated by other forcing agents as well as aerosols from other geographical regions. The study focuses mainly on the period of increasing sulphur dioxide (SO2) emissions -precursor of sulphate aerosols, the most abundant anthropogenic aerosol species- from NA and EU sources (1850-1975), and on identifying the aerosol impact over the Atlantic and Eurasian domain, where North American and European aerosols are presumed to have relevant impact. Along with observations, existing historical simulations from a range of coupled climate models are studied and complementary experiments performed and analysed. First, the boreal summer climate response to North American and European (NAEU) anthropogenic aerosol emissions during the twentieth century is characterised using a suite of models from the Coupled Model Inter-Comparison Project 5 (CMIP5). Supported by the co-variability of aerosol optical depth and near-surface climate, long-term variations in aerosol-only and all-forcing simulations are attributed to NAEU aerosol forcing if they undergo a significant reversal coinciding with the peak in NAEU SO2 emissions, measured by inter-model agreement on the sign of linear trends before and after 1975. Regionally, robust aerosol impact is found on Eurasian near-surface temperature, pressure, and diurnal temperature range; remotely, robust aerosol impact is found on the Inter-Tropical Convergence Zone (ITCZ) position and the subtropical jet stream. The contribution of anthropogenic aerosol forcing to the forced component of simulated inter-decadal climate variability of European-mean near-surface temperature is furthermore estimated to be more than a third throughout the twentieth century. Observed variations also of European-mean sea level pressure and diurnal temperature range tend to agree better with simulations that include aerosols. These findings highlight significant aerosol impact on Eurasian climate already in the first half of the twentieth century. The aerosol impact on observed West African and South Asian monsoon precipitation is then investigated by using a detection and attribution (D&A) approach. The aerosol source regions (NAEU, South Asia, or China) which are most important for explaining the observed 1920-2005 changes are identified. For this, fingerprints of the response to regional-aerosol forcing are derived from historical simulations with the GFDL-CM3 model along with CMIP5 simulations. It is found that in precipitation observations for West Africa, the only anthropogenic forcing which can be detected are NAEU emissions. In precipitation observations for South Asia, in contrast, local emissions are the only external forcing detected. Changes in West Africa are related to a meridional shift in the ITCZ due to aerosol-induced changes in the inter-hemispheric temperature gradient. Changes in South Asia, in contrast, are associated with a weakening of the monsoon circulation, driven by the increase of remote NAEU aerosol emissions until 1975 and since then by the increase in local emissions offsetting the decrease in NAEU emissions. These findings show for the first time that the aerosol forcing from individual emission regions is strong and distinct enough to be detected in the presence of internal variability. Finally, the dynamical impact of NA and EU sulphate aerosol emissions is fully analysed in the coupled Community Earth System model (CESM1-CAM5), focusing on the Atlantic. For this, multi-member ensemble simulations covering the period 1850- 1975 are performed, and the response to emissions from NA and EU is contrasted. The results show that sulphate aerosols from either source cause a long-term cooling of North Atlantic sea-surface temperatures (SSTs), with the patterns a combination of atmospheric aerosol effects and an aerosol-induced strengthening of the Atlantic Meridional Overturning Circulation (AMOC). The North Atlantic response to NA emissions is larger than that to EU emissions, with stronger indirect aerosol effects due to a wider aerosol spread over the Atlantic and collocation with climatological cloud cover. A southward shift of the ITCZ, affecting tropical precipitation globally, is also found. The (multi)decadal variability components of Atlantic SSTs and of the AMOC are furthermore both found to be externally forced. A suppression of Atlantic Tropical Hurricane frequency and a north-eastward shift of Atlantic extra-tropical storms in response to both NA and EU emissions are finally shown. The analysis provides novel insights into the mechanisms of aerosol impact on the Atlantic. Overall, the results from this work represent a significant contribution to advance our understanding of the historical impact of anthropogenic aerosols over the entire twentieth century and in particular that of aerosols from NA and EU by finding robust signals across models, using statistically rigorous methods to detect forced impact in observations, and analysing new model experiments. The findings emphasise the importance of historical anthropogenic aerosol emissions already before the late twentieth century and shed light on differences in the climate response to aerosols depending on their emission region, which will also be relevant for understanding future patterns of change related to further emission reductions.
- Published
- 2019
275. The First 30 Years of GEWEX.
- Author
-
Stephens, Graeme, Polcher, Jan, Zeng, Xubin, van Oevelen, Peter, Poveda, Germán, Bosilovich, Michael, Ahn, Myoung-Hwan, Balsamo, Gianpaolo, Duan, Qingyun, Hegerl, Gabriele, Jakob, Christian, Lamptey, Benjamin, Leung, Ruby, Piles, Maria, Su, Zhongbo, Dirmeyer, Paul, Findell, Kirsten L., Verhoef, Anne, Ek, Michael, and L'Ecuyer, Tristan
- Subjects
- *
WEATHER forecasting , *HYDROLOGIC cycle , *LAND-atmosphere interactions , *CLIMATE research , *COMMUNITIES , *ENERGY budget (Geophysics) - Abstract
The Global Energy and Water Cycle Exchanges (GEWEX) project was created more than 30 years ago within the framework of the World Climate Research Programme (WCRP). The aim of this initiative was to address major gaps in our understanding of Earth's energy and water cycles given a lack of information about the basic fluxes and associated reservoirs of these cycles. GEWEX sought to acquire and set standards for climatological data on variables essential for quantifying water and energy fluxes and for closing budgets at the regional and global scales. In so doing, GEWEX activities led to a greatly improved understanding of processes and our ability to predict them. Such understanding was viewed then, as it remains today, essential for advancing weather and climate prediction from global to regional scales. GEWEX has also demonstrated over time the importance of a wider engagement of different communities and the necessity of international collaboration for making progress on understanding and on the monitoring of the changes in the energy and water cycles under ever increasing human pressures. This paper reflects on the first 30 years of evolution and progress that has occurred within GEWEX. This evolution is presented in terms of three main phases of activity. Progress toward the main goals of GEWEX is highlighted by calling out a few achievements from each phase. A vision of the path forward for the coming decade, including the goals of GEWEX for the future, are also described. [ABSTRACT FROM AUTHOR]
- Published
- 2023
- Full Text
- View/download PDF
276. Effects of forcing differences and initial conditions on inter-model agreement in the VolMIP volc-pinatubo-full experiment.
- Author
-
Zanchettin, Davide, Timmreck, Claudia, Khodri, Myriam, Schmidt, Anja, Toohey, Matthew, Abe, Manabu, Bekki, Slimane, Cole, Jason, Fang, Shih-Wei, Feng, Wuhu, Hegerl, Gabriele, Johnson, Ben, Lebas, Nicolas, LeGrande, Allegra N., Mann, Graham W., Marshall, Lauren, Rieger, Landon, Robock, Alan, Rubinetti, Sara, and Tsigaridis, Kostas
- Subjects
- *
CLIMATE feedbacks , *EXPERIMENTAL design ,EL Nino - Abstract
This paper provides initial results from a multi-model ensemble analysis based on the volc-pinatubo-full experiment performed within the Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP) as part of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). The volc-pinatubo-full experiment is based on an ensemble of volcanic forcing-only climate simulations with the same volcanic aerosol dataset across the participating models (the 1991–1993 Pinatubo period from the CMIP6-GloSSAC dataset). The simulations are conducted within an idealized experimental design where initial states are sampled consistently across models from the CMIP6-piControl simulation providing unperturbed preindustrial background conditions. The multi-model ensemble includes output from an initial set of six participating Earth system models (CanESM5, GISS-E2.1-G, IPSL-CM6A-LR, MIROC-E2SL, MPI-ESM1.2-LR and UKESM1). The results show overall good agreement between the different models on the global and hemispheric scales concerning the surface climate responses, thus demonstrating the overall effectiveness of VolMIP's experimental design. However, small yet significant inter-model discrepancies are found in radiative fluxes, especially in the tropics, that preliminary analyses link with minor differences in forcing implementation; model physics, notably aerosol–radiation interactions; the simulation and sampling of El Niño–Southern Oscillation (ENSO); and, possibly, the simulation of climate feedbacks operating in the tropics. We discuss the volc-pinatubo-full protocol and highlight the advantages of volcanic forcing experiments defined within a carefully designed protocol with respect to emerging modelling approaches based on large ensemble transient simulations. We identify how the VolMIP strategy could be improved in future phases of the initiative to ensure a cleaner sampling protocol with greater focus on the evolving state of ENSO in the pre-eruption period. [ABSTRACT FROM AUTHOR]
- Published
- 2022
- Full Text
- View/download PDF
277. Effect of volcanic eruptions on the hydrological cycle
- Author
-
Iles, Carley Elizabeth, Hegerl, Gabriele, and Tett, Simon
- Subjects
551.21 ,rainfall pattern ,volcanic eruptions ,water cycle ,climate models ,precipitation response ,river discharge change - Abstract
Large explosive volcanic eruptions inject SO2 into the stratosphere where it is oxidised to sulphate aerosols which reflect sunlight. This causes a reduction in global temperature and precipitation lasting a few years. Here the robust features of this precipitation response are investigated, using superposed epoch analysis that combines results from multiple eruptions. The precipitation response is first analysed using the climate model HadCM3 compared to two gauge based land precipitation datasets. The analysis is then extended to a large suite of state-of-the art climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5). This is the first multi-model study focusing on the precipitation response to volcanoes. The large ensemble allows analysis of a short satellite based dataset which includes ocean coverage. Finally the response of major world rivers to eruptions is examined using historical records. Whilst previous studies focus on the response of just a few rivers or global discharge to single eruptions, here the response of 50 major world rivers is averaged across multiple eruptions. Results are applicable in predicting the precipitation response to future eruptions and to geoengineering schemes that seek to counteract global warming through reducing incoming solar radiation. The main model-simulated features of the precipitation response include a significant global drying over both land and ocean, which is dominated by the wet tropical regions, whilst the dry tropical ocean regions get significantly wetter following eruptions. Monsoon rainfall decreases, whilst in response to individual eruptions the Intertropical Convergence Zone shifts away from the hemisphere with the greater concentration of volcanic aerosols. The ocean precipitation response is longer lived than that over land and correlates with near surface air temperature, whilst the land response correlates with aerosol optical depth and a reduction in land-ocean temperature gradient Many of these modelled features are also seen in observational data, including the decrease in global mean and wet tropical regions precipitation over land and the increase of precipitation over dry tropical ocean regions, all of which are significant in the boreal cold season. The land precipitation response features were robust to choice of dataset. Removing the influence of the El Nino Southern Oscillation (ENSO) reduces the magnitude of the volcanic response, as several recent eruptions coincided with El Nino events. However, results generally remain significant after subtraction of ENSO, at least in the cold season. Over ocean, observed results only match model expectations in the cold season, whilst data are noisy in the warm season. Results are too noisy in both seasons to confirm whether a long ocean precipitation response occurs. Spatial patterns of precipitation response agree well between observational datasets, including a decrease in precipitation over most monsoon regions. A positive North Atlantic Oscillation-like precipitation response can be seen in all datasets in boreal winter, but this is not captured by the models. A detection analysis is performed that builds on previous detection studies by focusing specifically on the influence of volcanoes. The influence of volcanism on precipitation is detectable using all three observational datasets in boreal winter, including for the first time in a dataset with ocean coverage, and marginally detectable in summer. However, the models underestimate the size of the winter response, with the discrepancy originating in the wet tropics. Finally, the number of major rivers that undergo a significant change in discharge following eruptions is slightly higher than expected by chance, including decreased flow in the Amazon, Congo, Nile, Orange, Ob and Yenisey. This proportion increases when only large or less humanly influenced basins are considered. Results are clearer when neighbouring basins are combined that undergo the same sign of CMIP5 simulated precipitation response. In this way a significant reduction in flow is detected for northern South American, central African and less robustly for high-latitude Asian rivers, along with a significant increase for southern South American and SW North American rivers, as expected from the model simulated precipitation response.
- Published
- 2014
278. Impact of externally forced changes on temperature extremes
- Author
-
Morak, Simone, Hegerl, Gabriele, Merchant, Chris, and Tudhope, Sandy
- Subjects
temperature extremes ,detection ,climate change - Abstract
This thesis investigates changes in temperature extremes between 1950-2005, analysing gridded data sets of observations and climate model simulations. It focuses on changes in the frequency of extreme temperatures occurring in single days or over periods of six or more consecutive days. The study aims to quantify the significance of changes in extreme temperature events and answer the following questions. Are external or human-induced forcings together with natural forcings responsible for the observed change in temperature extremes or can these changes be explained due to natural climate variability alone? Are the observed changes consistent with those from climate model simulations? And are the changes in extremes linked only to changes in the mean climate, or only to those in climate variability or both? The analysis concentrates on changes from global to regional scale and from annual mean to seasonal scale. A detection method is applied to assess if changes are significantly different with respect to the internal climate variability. Results show that there has been a significant increase in warm daily extremes and a decrease in cold ones, both on large and small spatial scales. The increase in warm extremes has been found to be highly correlated with the increase in mean temperature. The changes in daily extremes are well represented in climate model simulations. Changes in the persistent extremes show a detectable increase in the frequency of warm and a decrease in cold events and are reproducible by models.
- Published
- 2013
279. U.K. Climate Projections: Summer Daytime and Nighttime Urban Heat Island Changes in England's Major Cities.
- Author
-
LO, Y. T. EUNICE, MITCHELL, DANIEL M., BOHNENSTENGEL, SYLVIA I., COLLINS, MAT, HAWKINS, ED, HEGERL, GABRIELE C., JOSHI, MANOJ, and STOTT, PETER A.
- Abstract
In the United Kingdom, where 90% of residents are projected to live in urban areas by 2050, projecting changes in urban heat islands (UHIs) is essential to municipal adaptation. Increased summer temperatures are linked to increased mortality. Using the new regional U.K. Climate Projections, UKCP18-regional, we estimate the 1981--2079 trends in summer urban and rural near-surface air temperatures and in UHI intensities during day and at night in the 10 most populous built-up areas in England. Summer temperatures increase by 0.45°0.81°C per decade under RCP8.5, depending on the time of day and location. Nighttime temperatures increase more in urban than rural areas, enhancing the nighttime UHI by 0.01°-0.05°C per decade in all cities. When these upward UHI signals emerge from 2008-18 variability, positive summer nighttime UHI intensities of up to 1.8°C are projected in most cities. However, we can prevent most of these upward nighttime UHI signals from emerging by stabilizing climate to the Paris Agreement target of 2°C above preindustrial levels. In contrast, daytime UHI intensities decrease in nine cities, at rates between -0.004° and -0.05°C per decade, indicating a trend toward a reduced daytime UHI effect. These changes reflect different feedbacks over urban and rural areas and are specific to UKCP18-regional. Future research is important to better understand the drivers of these UHI intensity changes. [ABSTRACT FROM AUTHOR]
- Published
- 2020
- Full Text
- View/download PDF
280. Comparing Methods to Constrain Future European Climate Projections Using a Consistent Framework.
- Author
-
BRUNNER, LUKAS, MCSWEENEY, CAROL, BALLINGER, ANDREW P., BEFORT, DANIEL J., BENASSI, MARIANNA, BOOTH, BEN, COPPOLA, ERIKA, DE VRIES, HYLKE, HARRIS, GLEN, HEGERL, GABRIELE C., KNUTTI, RETO, LENDERINK, GEERT, LOWE, JASON, NOGHEROTTO, RITA, O'REILLY, CHRIS, QASMI, SAÏD, RIBES, AURÉLIEN, STOCCHI, PAOLO, and UNDORF, SABINE
- Abstract
Political decisions, adaptation planning, and impact assessments need reliable estimates of future climate change and related uncertainties. To provide these estimates, different approaches to constrain, filter, or weight climate model projections into probabilistic distributions have been proposed. However, an assessment of multiple such methods to, for example, expose cases of agreement or disagreement, is often hindered by a lack of coordination, with methods focusing on a variety of variables, time periods, regions, or model pools. Here, a consistent framework is developed to allow a quantitative comparison of eight different methods; focus is given to summer temperature and precipitation change in three spatial regimes in Europe in 2041-60 relative to 1995-2014. The analysis draws on projections from several large ensembles, the CMIP5 multimodel ensemble, and perturbed physics ensembles, all using the high-emission scenario RCP8.5. The methods' key features are summarized, assumptions are discussed, and resulting constrained distributions are presented. Method agreement is found to be dependent on the investigated region but is generally higher for median changes than for the uncertainty ranges. This study, therefore, highlights the importance of providing clear context about how different methods affect the assessed uncertainty--in particular, the upper and lower percentiles that are of interest to risk-averse stakeholders. The comparison also exposes cases in which diverse lines of evidence lead to diverging constraints; additional work is needed to understand how the underlying differences between methods lead to such disagreements and to provide clear guidance to users. [ABSTRACT FROM AUTHOR]
- Published
- 2020
- Full Text
- View/download PDF
281. Circulation analogues and uncertainty in the time-evolution of extreme event probabilities: evidence from the 1947 Central European heatwave.
- Author
-
Harrington, Luke J., Otto, Friederike E. L., Cowan, Tim, and Hegerl, Gabriele C.
- Subjects
- *
HEAT waves (Meteorology) , *EFFECT of human beings on climate change , *CLIMATE extremes , *UNCERTAINTY , *PROBABILITY theory , *TWENTIETH century - Abstract
The science of extreme event attribution has rapidly expanded in recent years, with numerous studies dedicated to determining whether and to what extent anthropogenic climate change has increased the likelihood of specific extreme weather events occurring. However, the majority of such studies have focussed on extreme events which have occurred in the recent past (usually within the past 10 years) while minimal research efforts have considered the multitude of high-impact extreme climatic events which occurred throughout the instrumental record. This study proposes a framework to quantify how the likelihood of witnessing meteorological characteristics reminiscent of the 1947 Central European heatwave have evolved over time. We specifically examine circulation analogues as a tool to understand the relative role of dynamical and thermodynamic contributions to changes in the probability of experiencing similar heatwave events. Using a reanalysis-based dataset, our results show changes in the frequency of 1947-like extreme heat throughout the twentieth century to be highly sensitive to methodological choices, particularly in the context of disaggregating dynamic and thermodynamic changes in the risk of extreme heat. Evidence also suggests clear decadal variability in the occurrence of circulation patterns conducive to the 1947 heatwaves. Finally, we discuss how to appropriately consider the time-evolution of attribution statements, as well as the broader limitations of employing circulation analogues as a method to interrogate the dynamical contribution to the probability ratio of an extreme event. [ABSTRACT FROM AUTHOR]
- Published
- 2019
- Full Text
- View/download PDF
282. Evaluation of the HadGEM3-A simulations in view of detection and attribution of human influence on extreme events in Europe.
- Author
-
Vautard, Robert, Christidis, Nikolaos, Ciavarella, Andrew, Alvarez-Castro, Carmen, Bellprat, Omar, Christiansen, Bo, Colfescu, Ioana, Cowan, Tim, Doblas-Reyes, Francisco, Eden, Jonathan, Hauser, Mathias, Hegerl, Gabriele, Hempelmann, Nils, Klehmet, Katharina, Lott, Fraser, Nangini, Cathy, Orth, René, Radanovics, Sabine, Seneviratne, Sonia I., and van Oldenborgh, Geert Jan
- Subjects
- *
ATMOSPHERIC models , *METEOROLOGICAL precipitation , *ATMOSPHERIC temperature , *STORM surges - Abstract
A detailed analysis is carried out to assess the HadGEM3-A global atmospheric model skill in simulating extreme temperatures, precipitation and storm surges in Europe in the view of their attribution to human influence. The analysis is performed based on an ensemble of 15 atmospheric simulations forced with observed sea surface temperature of the 54 year period 1960-2013. These simulations, together with dual simulations without human influence in the forcing, are intended to be used in weather and climate event attribution. The analysis investigates the main processes leading to extreme events, including atmospheric circulation patterns, their links with temperature extremes, land-atmosphere and troposphere-stratosphere interactions. It also compares observed and simulated variability, trends and generalized extreme value theory parameters for temperature and precipitation. One of the most striking findings is the ability of the model to capture North-Atlantic atmospheric weather regimes as obtained from a cluster analysis of sea level pressure fields. The model also reproduces the main observed weather patterns responsible for temperature and precipitation extreme events. However, biases are found in many physical processes. Slightly excessive drying may be the cause of an overestimated summer interannual variability and too intense heat waves, especially in central/northern Europe. However, this does not seem to hinder proper simulation of summer temperature trends. Cold extremes appear well simulated, as well as the underlying blocking frequency and stratosphere-troposphere interactions. Extreme precipitation amounts are overestimated and too variable. The atmospheric conditions leading to storm surges were also examined in the Baltics region. There, simulated weather conditions appear not to be leading to strong enough storm surges, but winds were found in very good agreement with reanalyses. The performance in reproducing atmospheric weather patterns indicates that biases mainly originate from local and regional physical processes. This makes local bias adjustment meaningful for climate change attribution. [ABSTRACT FROM AUTHOR]
- Published
- 2019
- Full Text
- View/download PDF
283. Large-scale perspective on the meteorological modulation of air quality over China in winter
- Author
-
Jia, Zixuan, Doherty, Ruth, Bollasina, Massimo, and Hegerl, Gabriele
- Subjects
El Niño–Southern Oscillation ,East Asian winter monsoon ,PM2.5 concentrations ,air pollutant emissions ,Chinese air pollution ,ENSO-EAWM relationship ,air quality - Abstract
Rapid economic and industrial growth in China has led to serious air pollution with high concentrations of suspended fine particulate matter (PM2.5), in particular during winter. On a regional scale, meteorological conditions play a major role in modulating the accumulation, transport, removal and transformation of air pollutants. These meteorological conditions are affected by large-scale circulation patterns over China, dominated by the East Asian winter monsoon (EAWM). However, previous studies of how the large-scale winter circulation modulates air quality in China primarily focused on the North China Plain. The study of regional differences in the dominant large-scale circulation patterns needed to project future climate-driven PM2.5 concentration changes is far from complete. On longer inter-annual timescales, the EAWM is in turn influenced by El Niño–Southern Oscillation (ENSO) and the ensuing Pacific-East Asia teleconnection pattern. Better understanding of the ENSO-EAWM relationship and changes in this relationship under global warming is needed. Firstly, the influence of large-scale circulation on daily PM2.5 variability through its direct effect on key regional meteorological variables over three major populated regions of China (Beijing–Tianjin–Hebei, BTH; the Yangtze River Delta, YRD; the Pearl River Delta, PRD) is examined, based on a new high-resolution air quality reanalysis dataset for China for five winters from December 2013 to February 2018. In BTH, a shallow East Asian trough curbs northerly cold and dry air from the Siberian High, enhancing PM2.5 pollution levels. Weak near-surface southerly winds in eastern and southern China, associated with a weakened Siberian High, suppress horizontal dispersion, contributing to air pollution accumulation over YRD. In PRD, weak southerly winds and precipitation deficits over southern China are conducive to high PM2.5 concentrations. To account for these dominant large-scale circulation–PM2.5 relationships, we propose three new circulation-based indices: a 500 hPa geopotential height-based index for BTH, a sea level pressure-based index for YRD and an 850 hPa meridional wind-based index for PRD. These three indices can effectively distinguish clean days from heavily polluted days in these regions, assuming PM2.5 variability is solely due to meteorology. Subsequently, the influence of the winter large-scale circulation on daily PM2.5 concentrations and on the sensitivity of PM2.5 to emissions over major populated regions of China with a focus on YRD is investigated, using the United Kingdom Earth System Model, UKESM1. Weak flow of near-surface cold, dry air from the north and weak inflow of maritime air are conducive to air pollution over YRD for 1999–2019. These provide favourable conditions for the accumulation of local pollution but limit the transport of air pollutants into YRD from the north for 2014– 2019. Based on the dominant large-scale circulation, we construct a new index using the north-south pressure gradient to project PM2.5 concentrations over the region. We show that this index can effectively distinguish different levels of pollution over YRD and explain changes in PM2.5 sensitivity to emissions from local and northern regions. We then project future changes in PM2.5 concentrations using this index under the weak climate and air pollutant mitigation scenario (SSP3- 7.0). We find an increase in PM2.5 concentrations over YRD due to climate-driven circulation changes that is expected to partially offset the effect of emission control measures in the near-term future. Finally, changes in the relationship between ENSO and the EAWM at various global warming levels during the 21st century are examined based on experiments from the Max Planck Institute Grand Ensemble (MPI-GE) that represent the upper boundary of the range of emissions scenario (RCP 8.5). The externally forced component of this relationship (i.e. forced by greenhouse gases and anthropogenic aerosol emissions) strengthens under moderate warming (+1.5 ◦ C), and then weakens for +3 ◦ C warming. These changes are characterised by variations in strength and location of the core of El Niño-related warming and associated deep convection anomalies over the equatorial Pacific leading to circulation anomalies across the Asian-Pacific region. Under global warming, the ENSO–EAWM relationship is strongly related to the background mean state of both the EAWM and ENSO, through changesin the EAWM strength and a shift of the ENSO pattern. Anthropogenic aerosols also play a key role in influencing the ENSO–EAWM relationship under moderate warming (up to 1.5 ◦C). These results demonstrate the importance of understanding the occurrence of days with elevated PM2.5 concentrations and explaining changes in the sensitivity of PM2.5 to emissions from local and surrounding regions from a large-scale perspective. These findings could help project the occurrence of heavily polluted PM2.5 days during wintertime and assess future emission control strategies for PM2.5 air quality improvement under climate change. Furthermore, the ENSO–EAWM relationship is shown to have a substantial inter-decadal variation under global warming. This may further improve the accuracy of future predictions of air quality in China.
- Published
- 2023
284. Physical storylines for very rare climate extremes
- Author
-
Gessner, Claudia, Knutti, Reto, Fischer, Erich Markus, and Hegerl, Gabriele C.
- Subjects
Earth sciences ,ddc:550 ,climate extremes, heat wave, drought, heavy precipitation, soil moisture, storyline, ensemble boosting, climate change - Published
- 2022
285. The Detection and Attribution Model Intercomparison Project (DAMIP v1.0) contribution to CMIP6.
- Author
-
Gillett, Nathan P., Shiogama, Hideo, Funke, Bernd, Hegerl, Gabriele, Knutti, Reto, Matthes, Katja, Santer, Benjamin D., Stone, Daithi, and Tebaldi, Claudia
- Subjects
- *
CLIMATE change , *ANTHROPOGENIC effects on nature , *GLOBAL warming , *AEROSOLS , *ATMOSPHERIC chemistry - Abstract
Detection and attribution (D&A) simulations were important components of CMIP5 and underpinned the climate change detection and attribution assessments of the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. The primary goals of the Detection and Attribution Model Intercomparison Project (DAMIP) are to facilitate improved estimation of the contributions of anthropogenic and natural forcing changes to observed global warming as well as to observed global and regional changes in other climate variables; to contribute to the estimation of how historical emissions have altered and are altering contemporary climate risk; and to facilitate improved observationally constrained projections of future climate change. D&A studies typically require unforced control simulations and historical simulations including all major anthropogenic and natural forcings. Such simulations will be carried out as part of the DECK and the CMIP6 historical simulation. In addition D&A studies require simulations covering the historical period driven by individual forcings or subsets of forcings only: such simulations are proposed here. Key novel features of the experimental design presented here include firstly new historical simulations with aerosols-only, stratosphericozone- only, CO2-only, solar-only, and volcanic-only forcing, facilitating an improved estimation of the climate response to individual forcing, secondly future single forcing experiments, allowing observationally constrained projections of future climate change, and thirdly an experimental design which allows models with and without coupled atmospheric chemistry to be compared on an equal footing. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
- View/download PDF
286. The Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP): experimental design and forcing input data for CMIP6.
- Author
-
Zanchettin, Davide, Khodri, Myriam, Timmreck, Claudia, Toohey, Matthew, Schmidt, Anja, Gerber, Edwin P., Hegerl, Gabriele, Robock, Alan, Pausata, Francesco S. R., Ball, William T., Bauer, Susanne E., Bekki, Slimane, Dhomse, Sandip S., LeGrande, Allegra N., Mann, Graham W., Marshal, Lauren, Mills, Michael, Marchand, Marion, Niemeier, Ulrike, and Poulain, Virginie
- Subjects
- *
STRATOSPHERIC aerosols , *VOLCANIC eruptions , *EFFECT of volcanic eruptions on weather , *OCEAN-atmosphere interaction , *ATMOSPHERIC models - Abstract
The enhancement of the stratospheric aerosol layer by volcanic eruptions induces a complex set of responses causing global and regional climate effects on a broad range of timescales. Uncertainties exist regarding the climatic response to strong volcanic forcing identified in coupled climate simulations that contributed to the fifth phase of the Coupled Model Intercomparison Project (CMIP5). In order to better understand the sources of these model diversities, the Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP) has defined a coordinated set of idealized volcanic perturbation experiments to be carried out in alignment with the CMIP6 protocol. VolMIP provides a common stratospheric aerosol data set for each experiment to minimize differences in the applied volcanic forcing. It defines a set of initial conditions to assess how internal climate variability contributes to determining the response. VolMIP will assess to what extent volcanically forced responses of the coupled ocean-atmosphere system are robustly simulated by state-of-the-art coupled climate models and identify the causes that limit robust simulated behavior, especially differences in the treatment of physical processes. This paper illustrates the design of the idealized volcanic perturbation experiments in the VolMIP protocol and describes the common aerosol forcing input data sets to be used. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
- View/download PDF
287. How can event attribution science underpin financial decisions on Loss and Damage?
- Author
-
Coumou D, Arias PA, Bastos A, Gonzales CKG, Hegerl GC, Hope P, Jack C, Otto F, Saeed F, Serdeczny O, Shepherd TG, and Vautard R
- Abstract
With climate extremes hitting nations across the globe, disproportionately burdening vulnerable developing countries, the prompt operation of the Loss and Damage fund is of paramount importance. As decisions on resource disbursement at the international level, and investment strategies at the national level, loom, the climate science community's role in providing fair and effective evidence is crucial. Attribution science can provide useful information for decision makers, but both ethical implications and deep uncertainty cannot be ignored. Considering these aspects, we articulate a vision that integrates established attribution methods and multiple lines of evidence within a coherent logical framework., (© The Author(s) 2024. Published by Oxford University Press on behalf of National Academy of Sciences.)
- Published
- 2024
- Full Text
- View/download PDF
288. Last year's summer was the warmest in 2,000 years.
- Author
-
Hegerl GC and Taylor KL
- Subjects
- History, Ancient, Humans, Hot Temperature, Animals, History, 20th Century, Seasons, Global Warming prevention & control, Global Warming statistics & numerical data
- Published
- 2024
- Full Text
- View/download PDF
289. The most at-risk regions in the world for high-impact heatwaves.
- Author
-
Thompson V, Mitchell D, Hegerl GC, Collins M, Leach NJ, and Slingo JM
- Subjects
- Temperature, Afghanistan, Central America, Hot Temperature, Climate Change
- Abstract
Heatwaves are becoming more frequent under climate change and can lead to thousands of excess deaths. Adaptation to extreme weather events often occurs in response to an event, with communities learning fast following unexpectedly impactful events. Using extreme value statistics, here we show where regional temperature records are statistically likely to be exceeded, and therefore communities might be more at-risk. In 31% of regions examined, the observed daily maximum temperature record is exceptional. Climate models suggest that similar behaviour can occur in any region. In some regions, such as Afghanistan and parts of Central America, this is a particular problem - not only have they the potential for far more extreme heatwaves than experienced, but their population is growing and increasingly exposed because of limited healthcare and energy resources. We urge policy makers in vulnerable regions to consider if heat action plans are sufficient for what might come., (© 2023. The Author(s).)
- Published
- 2023
- Full Text
- View/download PDF
290. Large-scale emergence of regional changes in year-to-year temperature variability by the end of the 21 st century.
- Author
-
Olonscheck D, Schurer AP, Lücke L, and Hegerl GC
- Abstract
Global warming is expected to not only impact mean temperatures but also temperature variability, substantially altering climate extremes. Here we show that human-caused changes in internal year-to-year temperature variability are expected to emerge from the unforced range by the end of the 21
st century across climate model initial-condition large ensembles forced with a strong global warming scenario. Different simulated changes in globally averaged regional temperature variability between models can be explained by a trade-off between strong increases in variability on tropical land and substantial decreases in high latitudes, both shown by most models. This latitudinal pattern of temperature variability change is consistent with loss of sea ice in high latitudes and changes in vegetation cover in the tropics. Instrumental records are broadly in line with this emerging pattern, but have data gaps in key regions. Paleoclimate proxy reconstructions support the simulated magnitude and distribution of temperature variability. Our findings strengthen the need for urgent mitigation to avoid unprecedented changes in temperature variability., (© 2021. The Author(s).)- Published
- 2021
- Full Text
- View/download PDF
291. Ocean and land forcing of the record-breaking Dust Bowl heatwaves across central United States.
- Author
-
Cowan T, Hegerl GC, Schurer A, Tett SFB, Vautard R, Yiou P, Jézéquel A, Otto FEL, Harrington LJ, and Ng B
- Abstract
The severe drought of the 1930s Dust Bowl decade coincided with record-breaking summer heatwaves that contributed to the socio-economic and ecological disaster over North America's Great Plains. It remains unresolved to what extent these exceptional heatwaves, hotter than in historically forced coupled climate model simulations, were forced by sea surface temperatures (SSTs) and exacerbated through human-induced deterioration of land cover. Here we show, using an atmospheric-only model, that anomalously warm North Atlantic SSTs enhance heatwave activity through an association with drier spring conditions resulting from weaker moisture transport. Model devegetation simulations, that represent the wide-spread exposure of bare soil in the 1930s, suggest human activity fueled stronger and more frequent heatwaves through greater evaporative drying in the warmer months. This study highlights the potential for the amplification of naturally occurring extreme events like droughts by vegetation feedbacks to create more extreme heatwaves in a warmer world.
- Published
- 2020
- Full Text
- View/download PDF
292. Importance of the Pre-Industrial Baseline in Determining the Likelihood of Exceeding the Paris Limits.
- Author
-
Schurer AP, Mann ME, Hawkins E, Tett SFB, and Hegerl GC
- Abstract
During the Paris Conference in 2015, nations of the world strengthened the United Nations Framework Convention on Climate Change by agreeing to holding "the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C"1. However, "pre-industrial" was not defined. Here we investigate the implications of different choices of the pre-industrial baseline on the likelihood of exceeding these two temperature thresholds. We find that for the strongest mitigation scenario RCP2.6 and a medium scenario RCP4.5 the probability of exceeding the thresholds and timing of exceedance is highly dependent on the pre-industrial baseline, for example the probability of crossing 1.5°C by the end of the century under RCP2.6, varies from 61% to 88% depending on how the baseline is defined. In contrast, in the scenario with no mitigation, RCP8.5, both thresholds will almost certainly be exceeded by the middle of the century with the definition of the pre-industrial baseline of less importance. Allowable carbon emissions for threshold stabilisation are similarly highly dependent on the pre-industrial baseline. For stabilisation at 2°C, allowable emissions decrease by as much as 40% when earlier than 19th century climates are considered as a baseline.
- Published
- 2017
- Full Text
- View/download PDF
293. Climate change. Using the past to predict the future?
- Author
-
Hegerl GC and Russon T
- Published
- 2011
- Full Text
- View/download PDF
294. Climate change. Risks of climate engineering.
- Author
-
Hegerl GC and Solomon S
- Published
- 2009
- Full Text
- View/download PDF
295. Ocean science. Warming the world's oceans.
- Author
-
Hegerl GC and Bindoff NL
- Published
- 2005
- Full Text
- View/download PDF
Catalog
Discovery Service for Jio Institute Digital Library
For full access to our library's resources, please sign in.