Search

Future precipitation changes impact the availability of water resources and related flood and drought events. Recent studies have focused on the variations in precipitation based on the Representative Concentration Pathways scenarios, which do not reflect the current mitigation commitments of the Paris Agreement to stabilize the global mean temperature below the 2.0 °C threshold. This paper analyzes the Asian precipitation response to emissions scenarios from warming. This response is a result of the Intended Nationally Determined Contributions (INDC) pledges (as of August 2018), which would satisfy the Paris Agreement target over the next few decades. Our results demonstrate that by the end of 21st century, mean precipitation in Asia will increase by 12.9% (11.7–14.7%) for a delayed action (No‐policy) scenario, 8.0% (6.7–8.8%) for a continued mitigation action (continued INDC‐pledge) scenario, and 2.4% (1.9–3.5%) and 4.4% (3.7–5.4%) for scenarios that stabilize the global mean temperature at the 1.5 °C and 2.0 °C levels, respectively. The spatial heterogeneity of precipitation changes, however, reflects the complexity of precipitation responses in future climate projections. Furthermore, heavy rainfall events will become stronger as warming increases; however, the trend of dry spell events varies depending on the region. Considering the impacts of precipitation‐related extremes, we determined that the projected population exposure to heavy rainfall and dry spell events will significantly increase in most Asian regions. Limiting warming to lower levels (such as 1.5 or 2.0 °C) would reduce the population exposure to heavy rainfall, thereby avoiding impacts associated with precipitation extremes. These results contribute to an improved understanding of the risks related to future climate extremes. Such information is crucial for planning mitigation and adaptation activities in Asia, which is home to nearly 60% of the global population.

CO2-induced warming is approximately proportional to the total amount of CO2 emitted. This emergent property of the climate system, known as the Transient Climate Response to cumulative CO2 Emissions (TCRE), gave rise to the concept of a remaining carbon budget that specifies a cap on global CO2 emissions in line with reaching a given temperature target, such as those in the Paris Agreement (e.g., Matthews et al. 2020). However, estimating the policy-relevant TCRE metric directly from the observation-based data products remains challenging due to non-CO2 forcing and land-use change emissions present in the real-world climate conditions. Here, we present preliminary results for applying and comparing different statistical learning methods to determine TCRE (and later, remaining carbon budgets) from: (i) climate models’ output and (ii) the observational data products. First, we make use of a ‘perfect-model’ setting, i.e. using output from physics-based climate models (CMIP5 and CMIP6) under historical forcing (treated as pseudo-observations). This output is used to train different statistical-learning models, and to make predictions of TCRE (which are known from climate model simulations under CO2-only forcing, per experimental design). Next, we use such trained statistical learning models to make TCRE predictions directly from the observation-based data products. We also explore interpretability of the applied techniques, to determine where the statistical models are learning from, what the regions of importance are, and the key input features and weights. Explainable AI methods (e.g., McGovern et al. 2019; Molnar 2019; Samek et al. 2019) present a promising way forward in linking data-driven statistical and machine learning methods with traditional physical climate sciences, while leveraging from the large amount of data from the observational data products to provide more robust estimates of, often policy relevant, climate metrics. References: Matthews et al. (2020). Opportunities and challenges in using carbon budgets to guide climate policy. Nature Geoscience, 13, 769–779. https://doi.org/10.1038/s41561-020-00663-3 McGovern et al. (2019). Making the Black Box More Transparent: Understanding the Physical Implications of Machine Learning, B. Am. Meteorol. Soc., 100, 2175–2199, https://doi.org/10.1175/BAMS-D-18-0195.1 Molnar, C. (2019) Interpretable Machine Learning -A Guide for Making Black Box Models Explainable. https://christophm.github.io/interpretable-ml-book/ Samek, W. et al. (2019) Explainable AI: Interpreting, explaining and visualizing deep learning. https://doi.org/10.1007/978-3-030-28954-6

The remaining carbon budget quantifies the future CO2 emissions to limit global warming below a desired level. Carbon budgets are subject to uncertainty in the Transient Climate Response to Cumulative CO2 Emissions (TCRE), as well as to non-CO2 climate influences. Here we estimate the TCRE using observational constraints, and integrate the geophysical and socioeconomic uncertainties affecting the distribution of the remaining carbon budget. We estimate a median TCRE of 0.44 °C and 5–95% range of 0.32–0.62 °C per 1000 GtCO2 emitted. Considering only geophysical uncertainties, our median estimate of the 1.5 °C remaining carbon budget is 440 GtCO2 from 2020 onwards, with a range of 230–670 GtCO2, (for a 67–33% chance of not exceeding the target). Additional socioeconomic uncertainty related to human decisions regarding future non-CO2 emissions scenarios can further shift the median 1.5 °C remaining carbon budget by ±170 GtCO2.

We assess evidence relevant to Earth's equilibrium climate sensitivity per doubling of atmospheric CO 2, 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 function (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.

The remaining carbon budget represents the total amount of CO2 that can still be emitted in the future while limiting global warming to a given temperature target. Remaining carbon budget estimates range widely, however, and this uncertainty can be used to either trivialize the most ambitious mitigation targets by characterizing them as impossible, or to argue that there is ample time to allow for a gradual transition to a low-carbon economy. Neither of these extremes is consistent with our best understanding of the policy implications of remaining carbon budgets. Understanding the scientific and socio-economic uncertainties affecting the size of the remaining carbon budgets, as well as the methodological choices and assumptions that underlie their calculation, is essential before applying them as a policy tool. Here we provide recommendations on how to calculate remaining carbon budgets in a traceable and transparent way, and discuss their uncertainties and implications for both international and national climate policies. ISSN:1752-0908 ISSN:1752-0894

Remaining carbon budget specifies the cap on global cumulative CO2 emissions from the present-day onwards that would be in line with limiting global warming to a specific maximum level. In the context of the Paris Agreement, global warming is usually interpreted as the externally-forced response to anthropogenic activities and emissions, but it excludes the natural fluctuations of the climate system known as internal variability. A remaining carbon budget can be calculated from an estimate of the anthropogenic warming to date, and either (i) the ratio of CO2-induced warming to cumulative emissions, known as the Transient Climate Response to Emissions (TCRE), in addition to information on the temperature response to the future evolution of non-CO2 emissions; or (ii) climate model scenario simulations that reach a given temperature threshold. Here we quantify the impact of internal variability on the carbon budgets consistent with the Paris Agreement derived using either approach, and on the TCRE diagnosed from individual models. Our results show that internal variability contributes approximately ±0.09 °C to the overall uncertainty range of the human-induced warming to-date, leading to a spread in the remaining carbon budgets as large as ±50 PgC, when using approach (i). Differences in diagnosed TCRE due to internal variability in individual models can be as large as ±0.1 °C/1000 PgC (5%–95% range). Alternatively, spread in the remaining carbon budgets calculated from (ii) using future concentration-driven simulations of large ensembles of CMIP6 and CMIP5 models is estimated at ±30 PgC and ±40 PgC (5%–95% range). These results are important for model evaluation and imply that caution is needed when interpreting small remaining budgets in policy discussions. We do not question the validity of a carbon budget approach in determining mitigation requirements. However, due to intrinsic uncertainty arising from internal variability, it may only be possible to determine the exact year when a budget is exceeded in hindsight, highlighting the importance of a precautionary approach. Horizon 2020 Framework Programmehttp://dx.doi.org/10.13039/100010661 Swiss National Science Foundation

Carbon budgets are a policy-relevant tool that provides a cap on global total CO2 emissions to limit global mean warming at the desired level, for example, to meet the Paris Agreement target. Internal variability due to natural fluctuations of the climate system affects the temperature and carbon uptake on land and in the ocean. However, uncertainties arising from internal variability have not been quantified in the Transient Climate Response on Cumulative Emissions (TCRE) framework and related carbon budgets. Here we show that even though land carbon uptake exhibits the highest internal variability, most of the uncertainty in TCRE and carbon budgets arises from the temperature component, in concentration-driven simulations. Resulting remaining carbon budgets for 1.5 and 2.0 °C temperature targets differ even up to ±10 PgC (± 36.7 GtCO2; 5-95% range), due to internal variability, which is approximately equivalent to one year of global annual CO2 emissions. Our results suggest that calculating carbon budgets directly from climate models’ output does not introduce significant biases in TCRE and remaining carbon budgets due to internal variability.

The remaining carbon budget quantifies the allowable future CO2 emissions to keep global mean warming below a desiredlevel. Carbon budget estimates are subject to uncertainty in the Transient Climate Response to Cumulative CO2 Emissions (TCRE), which measures the warming resulting from a given total amount of CO2 emitted. Moreover, other sources of uncertainty linked to non-CO2 emissions have been shown to also strongly affect estimates of the remaining carbon budget. Here we present a new framework that estimates the TCRE using geophysical constraints derived from observations, and integrates the effect of geophysical and socioeconomic pathway uncertainties on the distribution of the remaining carbon budget. We estimate a median TCRE of 0.40 °C and likely range of 0.3 to 0.5 °C (17-83%) per 1000 GtCO2 emitted. Our 1.5 °C remaining carbon budget has a median value of 710 GtCO2 from 2020 onwards, with a range of 470 to 960 GtCO2, (for a 67% to 33% chance of not exceeding the target). Uncertainty in the amount of current warming from non-CO2 forcing is the dominant geophysical contributor to the spread in both the TCRE and remaining carbon budget estimates. The remaining carbon budget distribution is also strongly affected by current and future mitigation decisions, where the range of non-CO2forcing across scenarios has the potential to increase or decrease the median 1.5 °C remaining carbon budget by 740 GtCO2.

Following the Paris Agreement, many countries are enacting targets to achieve net-zero GHG emissions. Stringent mitigation will have clear societal benefits in the second half of this century by limiting peak warming and stabilizing climate. However, the near-term benefits of mitigation are generally thought to be less clear because forced surface temperature trends can be masked by internal variability. Here we use observationally constrained projections from the latest comprehensive climate models and a simple climate model emulator to show that pursuing stringent mitigation consistent with holding long-term warming below 1.5 °C reduces the risk of unprecedented warming rates in the next 20 years by a factor of 13 compared with a no mitigation scenario, even after accounting for internal variability. Therefore, in addition to long-term benefits, stringent mitigation offers substantial near-term benefits by offering societies and ecosystems a greater chance to adapt to and avoid the worst climate change impacts. GHG mitigation is not likely to be detectable in global mean temperature before mid-century. However, a simple climate emulator and an Earth system model ensemble suggest that strong mitigation greatly decreases the likelihood of high rates of 20-year warming over the next two decades.

Perspective Published: 28 November 2019 Recommended temperature metrics for carbon budget estimates, model evaluation and climate policy Katarzyna B. Tokarska, Carl-Friedrich Schleussner, Joeri Rogelj, Martin B. Stolpe, H. Damon Matthews, Peter Pfleiderer & Nathan P. Gillett Nature Geoscience volume 12, pages964–971(2019)Cite this article Article metrics 570 Accesses 28 Altmetric Metricsdetails Abstract Recent estimates of the amount of carbon dioxide that can still be emitted while achieving the Paris Agreement temperature goals are larger than previously thought. One potential reason for these larger estimates may be the different temperature metrics used to estimate the observed global mean warming for the historical period, as they affect the size of the remaining carbon budget. Here we explain the reasons behind these remaining carbon budget increases, and discuss how methodological choices of the global mean temperature metric and the reference period influence estimates of the remaining carbon budget. We argue that the choice of the temperature metric should depend on the domain of application. For scientific estimates of total or remaining carbon budgets, globally averaged surface air temperature estimates should be used consistently for the past and the future. However, when used to inform the achievement of the Paris Agreement goal, a temperature metric consistent with the science that was underlying and directly informed the Paris Agreement should be applied. The resulting remaining carbon budgets should be calculated using the appropriate metric or adjusted to reflect these differences among temperature metrics. Transparency and understanding of the implications of such choices are crucial to providing useful information that can bridge the science–policy gap.

The amount of additional future temperature change following a complete cessation of CO2 emissions is a measure of the unrealized warming to which we are committed due to CO2 already emitted to the atmosphere. This "Zero Emissions Commitment" (ZEC) is also an important quantity when estimating the remaining carbon budget – a limit on the total amount of CO2 emissions consistent with limiting global mean temperature at a particular level. In the recent IPCC Special Report on Global Warming of 1.5 °C, the carbon budget framework used to calculate the remaining carbon budget for 1.5 °C included the assumption that the ZEC due to CO2 emissions is negligible and close to zero. Previous research has shown significant uncertainty even in the sign of the ZEC. To close this knowledge gap, we propose the Zero Emissions Commitment Model Intercomparison Project (ZECMIP), which will quantify the amount of unrealized temperature change that occurs after CO2 emissions cease and investigate the geophysical drivers behind this climate response. Quantitative information on ZEC is a key gap in our knowledge, and one that will not be addressed by currently planned CMIP6 simulations, yet it is crucial for verifying whether carbon budgets need to be adjusted to account for any unrealized temperature change resulting from past CO2 emissions. We request only one top priority simulation from comprehensive general circulation Earth System Models (ESMs) and Earth System Models of Intermediate Complexity (EMICs) – a branch from the 1 % CO2 run with CO2 emissions set to zero at the point of 1000 PgC of total CO2 emissions in the simulation – with the possibility for additional simulations, if resources allow. ZECMIP is part of CMIP6, under joint sponsorship by C4MIP and CDRMIP, with associated experiment names to enable data submissions to Earth System Grid Federation. All data will be published and made freely available.

Climate projections1–3 and observations over recent decades4,5 indicate that precipitation in subtropical latitudes declines in response to anthropogenic warming, with significant implications for food production and population sustainability. However, this conclusion is derived from emissions scenarios with rapidly increasing radiative forcing to the year 21001,2, which may represent very different conditions from both past and future ‘equilibrium’ warmer climates. Here, we examine multi-century future climate simulations and show that in the Southern Hemisphere subtropical drying ceases soon after global temperature stabilizes. Our results suggest that twenty-first century Southern Hemisphere subtropical drying is not a feature of warm climates per se, but is primarily a response to rapidly rising forcing and global temperatures, as tropical sea-surface temperatures rise more than southern subtropical sea-surface temperatures under transient warming. Subtropical drying may therefore be a temporary response to rapid warming: as greenhouse gas concentrations and global temperatures stabilize, Southern Hemisphere subtropical regions may experience positive precipitation trends. Warming is altering subtropical precipitation; however, it is not clear whether this will continue in an equilibrium climate. Using projections to 2300, Southern Hemisphere drying is shown to be a transient response to the meridional temperature gradient changes.

To stabilize global mean temperature change within the range of 1.5–2.0°C in accordance with the Paris Agreement, countries worldwide submitted their Intended Nationally Determined Contributions with their proposed emission reductions. However, it remains unclear what the resulting climate change in terms of temperature and precipitation would be in response to the Intended Nationally Determined Contribution emission efforts. This study quantifies the global and regional temperature and precipitation changes in response to the updated Intended Nationally Determined Contribution scenarios, using simulations of 14 Fifth Coupled Climate Model Intercomparison Project models. Our results show that Intended Nationally Determined Contribution emissions would lead to a global mean warming of 1.4°C (1.3–1.7°C) in 2030 and 3.2°C (2.6–4.3°C) in 2100, above the preindustrial level (the 1850–1900 average). Spatially, the Arctic is projected to have the largest warming, 2.5 and 3 times the global average for 2030 and 2100, respectively, with strongest positive trends at 70–85°N over Asia, Europe and North America (6.5–9.0°C). The excessive warming under Intended Nationally Determined Contribution scenarios is substantially above the 1.5°C or 2.0°C long-term stabilization level. Global mean precipitation is projected to be similar to preindustrial levels in 2030, and an increase of 6% (4–9%) by 2100 compared with the preindustrial level. Regional precipitation changes will be heterogeneous, with significant increases over the equatorial Pacific (about +120%) and strong decreases over the Mediterranean, North Africa and Central America (−15% – −30%). These results clearly show that it is necessary to adjust and strengthen national mitigation efforts on current Intended Nationally Determined Contributions to meet the long-term temperature target.

If emissions continue at the present-day rate, about 22 years are left until global mean warming reaches the 1.5 °C Paris Agreement target, suggests a new metric based on the observed level and rate of anthropogenic warming.

The previous ‘Journal peer review information’ for this Letter was incorrect. The correct statement is “Nature Climate Change thanks Jie He, Hanh Nguyen, Caroline Ummenhofer and other anonymous reviewer(s) for their contribution to the peer review of this work.” This statement has now been amended.