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The greenhouse-to-icehouse climate transition from the Eocene into the Oligocene is well documented by sea surface temperature records from the southwest Pacific and Antarctic margin, which show evidence of pronounced long-term cooling. However, identification of a driving mechanism depends on a better understanding of whether this cooling was also present in terrestrial settings. Here, we present a semi-continuous terrestrial temperature record spanning from the middle Eocene to the early Oligocene (~41–33 million years ago), using bacterial molecular fossils (biomarkers) preserved in a sequence of southeast Australian lignites. Our results show that mean annual temperatures in southeast Australia gradually declined from ~27 °C (±4.7 °C) during the middle Eocene to ~22–24 °C (±4.7 °C) during the late Eocene, followed by a ~2.4 °C-step cooling across the Eocene/Oligocene boundary. This trend is comparable to other temperature records in the Southern Hemisphere, suggesting a common driving mechanism, likely $$p{{\rm{CO}}_{2}}$$ . We corroborate these results with a suite of climate model simulations demonstrating that only simulations including a decline in $$p{{\rm{CO}}_{2}}$$ lead to a cooling in southeast Australia consistent with our proxy record. Our data form an important benchmark for testing climate model performance, sea–land interaction and climatic forcings at the onset of a major Antarctic glaciation. Terrestrial Southern Hemisphere cooling through the Eocene–Oligocene transition points to decreasing atmospheric CO2 dominantly driving global change, according to biomarker records from southeast Australian coals and palaeoclimate modelling.

This work deals with reconstructing firn layer thicknesses at the deposition time from the firn's observed thickness in ice cores, thus reconstructing the annual accumulation, yielding a timescale and an ice-core chronology. We employed a dynamic time warping algorithm to find an optimal, non-linear alignment between an H2O2 concentration data series from 98 m worth of ice cores of a borehole on the central ice divide of the Detroit Plateau, the Antarctic Peninsula, and an estimated local temperature time series. The viability and the physical reliability of the procedure are rooted in the robustness of the seasonal marker H2O2 in a high-accumulation context, which brought the entire borehole to within the operational life span of four Antarctic stations around the Antarctic Peninsula. The process was heavily based on numerical optimisation, producing a mathematically sound match between the two series to estimate the annual layering efficiently on the entire data section at once, being disposition-free. The results herein confirm a high annual accumulation rate of aN=2.8 m w.e./yr, which is of the same order of magnitude as and highly correlated with that of the Bruce Plateau and twice as large as that of the Gomez Plateau, 300 and 1200 km further south, respectively.

The sixth phase of the Coupled Model Intercomparison Project (CMIP6) is the latest modeling effort for general circulation models to simulate and project various aspects of climate change. Many of the general circulation models (GCMs) participating in CMIP6 provide archived output that can be used to calculate effective climate sensitivity (ECS) and forecast future temperature change based on emissions scenarios from several Shared Socioeconomic Pathways (SSPs). Here we use our multiple linear regression energy balance model, the Empirical Model of Global Climate (EM-GC), to simulate and project changes in global mean surface temperature (GMST), calculate ECS, and compare to results from the CMIP6 multi-model ensemble. An important aspect of our study is a comprehensive analysis of uncertainties due to radiative forcing of climate from tropospheric aerosols (AER RF) in the EM-GC framework. We quantify the attributable anthropogenic warming rate (AAWR) from the climate record using the EM-GC and use AAWR as a metric to determine how well CMIP6 GCMs replicate human-driven global warming over the last 40 years. The CMIP6 multi-model ensemble indicates a median value of AAWR over 1975–2014 of 0.221 ∘C per decade (range of 0.151 to 0.299 ∘C per decade; all ranges given here are for 5th and 95th confidence intervals), which is notably faster warming than our median estimate for AAWR of 0.157 ∘C per decade (range of 0.120 to 0.195 ∘C per decade) inferred from the analysis of the Hadley Centre Climatic Research Unit version 5 data record for GMST. Estimates of ECS found using the EM-GC assuming that climate feedback does not vary over time (best estimate 2.33 ∘C; range of 1.40 to 3.57 ∘C) are generally consistent with the range of ECS of 1.5 to 4.5 ∘C given by the IPCC's Fifth Assessment Report. The CMIP6 multi-model ensemble exhibits considerably larger values of ECS (median 3.74 ∘C; range of 2.19 to 5.65 ∘C). Our best estimate of ECS increases to 3.08 ∘C (range of 2.23 to 5.53 ∘C) if we allow climate feedback to vary over time. The dominant factor in the uncertainty for our empirical determinations of AAWR and ECS is imprecise knowledge of AER RF for the contemporary atmosphere, though the uncertainty due to time-dependent climate feedback is also important for estimates of ECS. We calculate the likelihood of achieving the Paris Agreement target (1.5 ∘C) and upper limit (2.0 ∘C) of global warming relative to pre-industrial for seven of the SSPs using both the EM-GC and the CMIP6 multi-model ensemble. In our model framework, SSP1-2.6 has a 53 % probability of limiting warming at or below the Paris target by the end of the century, and SSP4-3.4 has a 64 % probability of achieving the Paris upper limit. These estimates are based on the assumptions that climate feedback has been and will remain constant over time since the prior temperature record can be fit so well assuming constant climate feedback. In addition, we quantify the sensitivity of future warming to the curbing of the current rapid growth of atmospheric methane and show that major near-term limits on the future growth of methane are especially important for achievement of the 1.5 ∘C goal of future warming. We also quantify warming scenarios assuming climate feedback will rise over time, a feature common among many CMIP6 GCMs; under this assumption, it becomes more difficult to achieve any specific warming target. Finally, we assess warming projections in terms of future anthropogenic emissions of atmospheric carbon. In our model framework, humans can emit only another 150±79 Gt C after 2019 to have a 66 % likelihood of limiting warming to 1.5 ∘C and another 400±104 Gt C to have the same probability of limiting warming to 2.0 ∘C. Given the estimated emission of 11.7 Gt C per year for 2019 due to combustion of fossil fuels and deforestation, our EM-GC simulations suggest that the 1.5 ∘C warming target of the Paris Agreement will not be achieved unless carbon and methane emissions are severely curtailed in the next 10 years.

Time series with a significant trend, as is now being the case for the temperature in the course of climate change, need a careful approach for statistical evaluations. Climatological means and moments are usually taken from past data which means that the statistics does not fit to actual data anymore. Therefore, we need to determine the long-term trend before comparing actual data with the actual climate. This is not an easy task, because the determination of the signal—a climatic trend—is influenced by the random scatter of observed data. Different filter methods are tested upon their quality to obtain realistic smoothed trends of observed time series. A new method is proposed, which is based on a variational principle. It outperforms other conventional methods of smoothing, especially if periodic time series are processed. This new methodology is used to test, how extreme the temperature of 2018 in Vienna actually was. It is shown that the new annual temperature record of 2018 is not too extreme, if we consider the positive trend of the last decades. Also, the daily mean temperatures of 2018 are not found to be really extreme according to the present climate. The real extreme of the temperature record of Vienna—and many other places around the world—is the strongly increased positive temperature trend over the last years.

The variability of the East Asian monsoon (EAM) during the Holocene exhibits significant regional response, and its evolution needs further discussion. A well-dated, high-resolution sea-surface temperature record based on long-chain unsaturated alkenones, grain-size data, and clay mineral assemblages from the South Yellow Sea sediment is presented to investigate the sedimentary provenance and reconstruct the EAM over the past 9.5 ka. The results show that the sediments are most likely supplied by the Huanghe. The evolution of the East Asian summer monsoon (EASM) can be divided into three periods: strong and relatively stable conditions during 9.5–7.0 ka, weakened conditions during 7.0–1.5 ka, and strengthened conditions during 1.5–0 ka. The East Asian winter monsoon (EAWM) has experienced five periods: weakened conditions during 9.5–6.7 ka, weak and relatively stable conditions during 6.7–5.6 ka, strong and relatively stable conditions during 5.6–2.6 ka, strengthened conditions during 2.6–1.5 ka, and weak and stable conditions during 1.5–0 ka. Moreover, in-phase correlation was found between the EAWM and EASM at the orbital time scale in response to orbital-driven solar insolation, but out-of-phase correlation at a centennial time scale is predominantly associated with solar activity.

Radiative forcing provides an important basis for understanding and predicting global climate changes, but its quantification has historically been done independently for different forcing agents, has involved observations to varying degrees, and studies have not always included a detailed analysis of uncertainties. The Copernicus Atmosphere Monitoring Service reanalysis is an optimal combination of modelling and observations of atmospheric composition. It provides a unique opportunity to rely on observations to quantify the monthly and spatially resolved global distributions of radiative forcing consistently for six of the largest forcing agents: carbon dioxide, methane, tropospheric ozone, stratospheric ozone, aerosol–radiation interactions, and aerosol–cloud interactions. These radiative-forcing estimates account for adjustments in stratospheric temperatures but do not account for rapid adjustments in the troposphere. On a global average and over the period 2003–2017, stratospherically adjusted radiative forcing of carbon dioxide has averaged +1.89 W m−2 (5 %–95 % confidence interval: 1.50 to 2.29 W m−2) relative to 1750 and increased at a rate of 18 % per decade. The corresponding values for methane are +0.46 (0.36 to 0.56) W m−2 and 4 % per decade but with a clear acceleration since 2007. Ozone radiative-forcing averages +0.32 (0 to 0.64) W m−2, almost entirely contributed by tropospheric ozone since stratospheric ozone radiative forcing is only +0.003 W m−2. Aerosol radiative-forcing averages −1.25 (−1.98 to −0.52) W m−2, with aerosol–radiation interactions contributing −0.56 W m−2 and aerosol–cloud interactions contributing −0.69 W m−2 to the global average. Both have been relatively stable since 2003. Taking the six forcing agents together, there is no indication of a sustained slowdown or acceleration in the rate of increase in anthropogenic radiative forcing over the period. These ongoing radiative-forcing estimates will monitor the impact on the Earth's energy budget of the dramatic emission reductions towards net-zero that are needed to limit surface temperature warming to the Paris Agreement temperature targets. Indeed, such impacts should be clearly manifested in radiative forcing before being clear in the temperature record. In addition, this radiative-forcing dataset can provide the input distributions needed by researchers involved in monitoring of climate change, detection and attribution, interannual to decadal prediction, and integrated assessment modelling. The data generated by this work are available at https://doi.org/10.24380/ads.1hj3y896 (Bellouin et al., 2020b).

Individual glacier changes are still poorly documented in the Georgian Caucasus. In this paper, the change of Chalaati and Zopkhito glaciers in Georgian Caucasus has been studied between 1960 and 2014. Glacier geometries are reconstructed from archival topographic maps, Corona and Landsat images, along with modern field surveys. For the first time in the Georgian Caucasus aerial photogrammetric survey of both glacier termini was performed (2014) using a drone or Unmanned Aerial Vehicle, where high-resolution orthomosaics and digital elevation models were produced. We show that both glaciers have experienced area loss since 1960: 16.2±4.9 per cent for Chalaati Glacier and 14.6±5.1 per cent for Zopkhito Glacier with corresponding respective terminus retreat by ~675 m and ~720 m. These were accompanied by a rise in the equilibrium line altitudes of ~35 m and ~30 m, respectively. The glacier changes are a response to regional warming in surface air temperature over the last half century. We used a long-term temperature record from the town of Mestia and short-term meteorological observations at Chalaati and Zopkhito glaciers to estimate a longer-term air temperature record for both glaciers. This analysis suggests an increase in the duration of the melt season over the 54-year period, indicating the importance of summertime air temperature trends in controlling glacier loss in the Georgian Caucasus. We also observed supra-glacial debris cover increase for both glaciers over the last half century: from 6.16±6.9 per cent to 8.01±6.8 per cent for Chalaati Glacier and from 2.80±6.3 per cent to 8.53±5.7 per cent for Zopkhito Glacier.

The earliest temperature series in Paris, from May 1658 to September 1660, taken by Boulliau is made available at daily resolution after a careful work of homogenization, correction and calculation of the average values. New results are achieved concerning the Little Florentine Thermometer, building location, thermometer exposure, observation methodologies followed by Boulliau and weather in the mid-seventeenth century. Two methods are used and compared to calculate the daily average from readings taken at random sampling times. The first one is based on the reading needing the smallest correction to be transformed into a daily average; the second considers all the readings of the day and makes a bulk average of the individual results. The series is compared with the temperature record by the Grand Duke Ferdinand II in Florence, which was the primary station of the Medici Network (1654–70). In addition, the comparison of the earliest temperature series in Paris and Florence with their respective 1961–1990 reference period gives a clear image of the change of weather conditions in Europe in the middle of the seventeenth century. Key features were a strong variability for warm-air and cold-air outbreaks, severe winters and cold summers in Paris, not affecting Florence.

We employ JRA-55 (Japanese 55-year Reanalysis), a recent second-generation global reanalysis providing data of high quality in the stratosphere, to examine whether a distinguishable effect of geomagnetic activity on Northern Hemisphere stratospheric temperatures can be detected. We focus on how the statistical significance of stratospheric temperature differences may be robustly assessed during years with high and low geomagnetic activity. Two problems must be overcome. The first is the temporal autocorrelation of the data, which is addressed with a correction of the t statistics by means of the estimate of the number of independent values in the series of correlated values. The second is the problem of multiplicity due to strong spatial autocorrelations, which is addressed by means of a false discovery rate (FDR) procedure. We find that the statistical tests fail to formally reject the null hypothesis, i.e. no significant response to geomagnetic activity can be found in the seasonal-mean Northern Hemisphere stratospheric temperature record.

This paper presents a four year subannual isotope marine temperature record using modern Spisula sachalinensis specimens from Tomakomai (Hokkaido's Pacific coast, Japan). This species is commonly found in pre- and protohistoric shell middens and faunal assemblages from around the Seas of Japan and Okhotsk, so has significant potential as an indicator of past marine and, by inference, climatic conditions. However, previous sclerochronological research on the species’ growth has shown significant geographical variation in growth pattern, rendering palaeoclimatic interpretation difficult. To address this issue, this study applied sequential isotopic analysis to two sectioned modern valves, providing a proxy for sea surface temperature (SST) and allowing the direct identification of seasonality during shell growth. The sequences span four years of growth, as confirmed by visually identified growth patterns and the oxygen stable isotope results, which show clear annual cycles in δ18O. δ13C seasonality is less clear, but shows a weak inverse correlation with temperature potentially relating to primary productivity. Annual growth lines show that shell growth occurs during both warmer and cooler SSTs, but is more rapid during the cooler seasons. This is consistent with warm-season growth minima seen in shells from Hakodate Bay by Kato and Hamai (1975), but not with their suggestion that it is associated with shells growing at the southern limits of their distribution. Comparison to average local SST shows that δ18O-derived temperature falls within the expected range, but, contrary to expectations given preferential cool-season shell growth, appears biased towards warmer temperatures. Factors that could contribute to this are discussed. Overall, stable isotope analysis of Spisula sachalinensis is considered a useful complement to macro/microscopic sclerochronological research in building a holistic picture of shell growth, and has significant potential as a high resolution proxy for palaeoenvironmental studies of past SST.

The highest currently recognised air temperature (33.3 °C) ever recorded in the Republic of Ireland was logged at Kilkenny Castle in 1887. The original observational record however no longer exists. Given that Ireland is now the only country in Europe to have a national heat record set in the 19th century, a reassessment of the verity of this record is both timely and valuable. The present analysis undertakes a fundamental reassessment of the plausibility of the 1887 temperature record using methods similar to those used to assess various weather extremes under WMO auspices over recent years. Specifically, we undertake an inter-station reassessment using sparse available records and make recourse to the new and improved 20CRv3 sparse-input reanalysis product. Neither surrounding available stations nor the reanalysis offer substantive support for the Kilkenny record of 33.3 °C being correct. Moreover, recent data rescue efforts have uncovered several earlier extreme values, one of which exceeds the Kilkenny value (33.5 °C on 16th July 1876 recorded at the Phoenix Park). However, the sparsity of early observational networks, a distinct lack of synoptic support from 20CRv3 for many of the extreme heat values, and the fact that these measurements were obtained using non-standard exposures leads us to conclude that there is grossly insufficient evidence to support any of these 19th Century extremes as robust national heat record candidates. Data from the early 20th Century onwards benefits from a denser network of stations undertaking measurements in a more standardised manner, many under the direct auspices of Met Éireann and its predecessors, adhering to WMO guidance and protocols. This enables more robust cross-checking of records. We argue that the Met Éireann recognised 20th Century heat record from Boora in 1976 verifies as the most plausible robust national temperature record based upon the synoptic situation and comparisons with nearby neighbouring stations. This measurement of 32.5 °C thus likely constitutes the highest reliably recorded temperature measurement in the Republic of Ireland. Ultimately, the formal decision on any reassessment and reassignment of the national record rests with the national meteorological service, Met Éireann.

TEX86 is a paleothermometer based on Thaumarcheotal glycerol dialkyl glycerol tetraether (GDGT) lipids and is one of the most frequently used proxies for sea-surface temperature (SST) in warmer-than-present climates. However, the calibration of TEX86 to SST is controversial because its correlation to SST is not significantly stronger than that to depth-integrated surface to subsurface temperatures. Because GDGTs are not exclusively produced in and exported from the surface ocean, sedimentary GDGTs may contain a depth-integrated signal that is sensitive to local subsurface temperature variability, which can only be proved in downcore studies. Here, we present a 15 Myr TEX86 record from ODP Site 959 in the Gulf of Guinea and use additional proxies to elucidate the source of the recorded TEX86 variability. Relatively high GDGT[2/3] ratio values from 13.6 Ma indicate that sedimentary GDGTs were partly sourced from deeper (> 200 m) waters. Moreover, late Pliocene TEX86 variability is highly sensitive to glacial-interglacial cyclicity, as is also recorded by benthic δ18O, while the variability within dinoflagellate assemblages and surface/thermocline temperature records (Uk’37 and Mg/Ca), is not primarily explained by glacial-interglacial cyclicity. Combined, these observations are best explained by TEX86 sensitivity to sub-thermocline temperature variability. We conclude that the TEX86 record represents a depth-integrated signal that incorporates a SST and a deeper component, which is compatible the present-day depth distribution of Thaumarchaeota and with the GDGT[2/3] distribution in core tops. The depth-integrated TEX86 record can potentially be used to infer SST variability, because subsurface temperature variability is generally tightly linked to SST variability. Using a subsurface calibration with peak calibration weight between 100–350 m, we estimate that east equatorial Atlantic SST cooled by ~4.5 °C between the Late Miocene and Pleistocene. On shorter timescales, we use the TEX86 record as an Antarctic Intermediate Water (AAIW) proxy and evaluate climatological leads and lags around the Pliocene M2 glacial (~3.3 Ma). Our record, combined with published information, suggests that the M2 glacial was marked by AAIW cooling during an austral summer insolation minimum, and that decreasing CO2 levels were a feedback, not the initiator, of glacial expansion.

Previous studies on surface temperature reconstructions for the last 2000 years (2 k) revealed a long-term cooling trend for the last millennium in comparison to the previous millennium. However, knowledge on the decadal- to centennial-scale variability in sea surface temperature and the underlying governing mechanisms throughout the period is limited. We reconstructed high-resolution continuous sea surface temperature changes over the last 2 k in the northwest Pacific margin based on the alkenone unsaturation index. Our alkenone temperature record revealed enhanced and more rapidly changing climate variability during the last millennium (approximately 1200–1850 Common Era) than during the previous millennium. Cold and hot extremes also occurred more frequently during the last millennium. The enhanced and rapidly changing climate variability appears to be associated with frequent volcanic eruptions and grand solar minima. The reconstructed surface temperature variability tends to be associated with variations in the East Asia summer monsoon and the Pacific Decadal Oscillation, implying that these variations are also enhanced in the last millennium than in the previous millennium.

Multiple potential tipping points in the Earth system that involve alternative states have been identified that are susceptible to anthropogenic forcing. Past events—from millions of years ago to within the last century—have manifest as abrupt changes in climatic indicators such as the temperature record. Recent unprecedented heat waves in Australia, their associated devastation, and the considerations above provide motivation to ask whether the Australian daily maximum temperature record has been subject to such abrupt changes. Using a new diagnostic tool—the Lyapunov plot—here it is shown that multiple temperature transitions have occurred with respect to the maximum daily temperature record in widely separated locations in Australia over the last 150 years. All maximum Lyapunov exponents are positive in sign, indicating that the transitions are chaos-to-chaos transitions, and that the different climate modes identified are likely to be manifestations of distinct chaotic attractors. Many of these events occur simultaneously with transitions or extremes in the major natural cycles affecting Australia’s climate, but this observation is not universal. It is known that chaos-to-chaos transitions can result in changes in the value(s) of the state variable(s) that can range from subtle to severe. Although the identified transitions are not catastrophic, this observation does not rule out the possibility of severe, unprecedented, and discontinuous increases in average daily maximum temperatures occurring in Australia at any time within the next few decades.

Many mammalian populations conform spatially and temporally to Bergmann's rule. This ecogeographic pattern is driven by selection for larger body masses by cooler temperatures and smaller ones by warming temperatures. However, it is unclear whether the response to warming or cooling temperatures is (a)symmetrical. Studies of the evolutionary record suggest that mammals evolve smaller body sizes more rapidly than larger ones, suggesting that it may be “easier” to adapt to warming climates than cooling ones. Here, we examine the potential asymmetrical response of mammals to past temperature fluctuations. We use the fossil midden record of the bushy-tailed woodrat, Neotoma cinerea, a well-studied animal that generally conforms to Bergmann's rule, to test the ability of populations to respond to warming versus cooling climate throughout its modern range in western North America over the late Quaternary. We quantified the response to temperature change, as characterized by the Greenland Ice Sheet Project 2 temperature record, using N. cinerea presence/absence and “darwins.” Our results show that populations within the modern range of N. cinerea show little difference between warming and cooling events. However, northern, peripheral populations are absent during older, cooler periods, possibly due to climate or taphonomy. Our study suggests adaptation in situ may be an underestimated response to future climate change.

Long chain alkyl diols (LCDs) were studied to probe the ability of long chain diol index (LDI) as a sea surface temperature (SST) or temperature record at depth in the open northern South China Sea (SCS). The results showed that riverine LCDs, which may affect LDI-derived SSTs, was less significant due to the substantially low fractional abundance (generally

Despite a very strong El Nino Southern Oscillation in 2015/2016, no coral mortality associated with bleaching was observed at the northern Galapagos (Ecuador) Islands of Darwin and Wolf. From March 2016 to March 2018, coral cover and health as well as water chemistry (NO3− and PO43−) and temperature were recorded. A marked heat anomaly reached 30 °C at Wolf in February 2016, but peak temperatures were attenuated after 2 d by a 4 °C drop. Temperature patterns at three depths (10, 15, and 20 m) and a subsequent and persistent phytoplankton bloom suggest topographically driven upwelling as the source of colder water and dissolved inorganic nutrients—both of which helped corals endure the heating episode. Consequently, no mortality and only partial bleaching were recorded in March 2016. Partially bleached corals contained numerous healthy zooxanthellae in deeper tissue layers. A continuous temperature record from 2012 to 2014 suggests that such upwelling events are common, inducing temperature fluctuations of up to 6 °C within 24 h during the observation period. Events at Wolf in 2016 suggest local upwelling reduced coral stress by relieving heat and by delivering nutrients required by corals to retain their regular temperature tolerance.

Extreme cold weather events, such as the winter of 1962/63, the third coldest winter ever recorded in the Central England Temperature record, or more recently the winter of 2010/11, have significant consequences for the society and economy. This paper assesses the probability of such extreme cold weather across the United Kingdom (UK), as part of a probabilistic catastrophe model for insured losses caused by the bursting of pipes. A statistical model is developed in order to model the extremes of the Air Freezing Index (AFI), which is a common measure of the magnitude and duration of freezing temperatures. A novel approach in the modelling of the spatial dependence of the hazard has been followed which takes advantage of the vine copula methodology. The method allows complex dependencies to be modelled, especially between the tails of the AFI distributions, which is important to assess the extreme behaviour of such events. The influence of the North Atlantic Oscillation and of anthropogenic climate change on the frequency of UK cold winters has also been taken into account. According to the model, the occurrence of extreme cold events, such as the 1962/63 winter, has decreased approximately 2 times during the course of the 20th century as a result of anthropogenic climate change. Furthermore, the model predicts that such an event is expected to become more uncommon, about 2 times less frequent, by the year 2030. Extreme cold spells in the UK have been found to be heavily modulated by the North Atlantic Oscillation (NAO) as well. A cold event is estimated to be ≈3–4 times more likely to occur during its negative phase than its positive phase. However, considerable uncertainty exists in these results, owing mainly to the short record length and the large interannual variability of the AFI.

The timing of the Holocene summer monsoon maximum (HSMM) in northeastern China has been much debated and more quantitative precipitation records are needed to resolve the issue. In the present study, Holocene precipitation and temperature changes were quantitatively reconstructed from a pollen record from the sediments of Tianchi Crater Lake in northeastern China using a plant functional type-modern analogue technique (PFT-MAT). The reconstructed precipitation record indicates a gradual increase during the early to mid-Holocene and a HSMM at ~5500–3100 cal yr BP, while the temperature record exhibits a divergent pattern with a marked rise in the early Holocene and a decline thereafter. The trend of reconstructed precipitation is consistent with that from other pollen records in northeastern China, confirming the relatively late occurrence of the HSMM in the region. However, differences in the onset of the HSMM within northeastern China are also evident. No single factor appears to be responsible for the late occurrence of the HSMM in northeastern China, pointing to a potentially complex forcing mechanism of regional rainfall in the East Asian monsoon region. We suggest that further studies are needed to understand the spatiotemporal pattern of the HSMM in the region.

Since villages are usually regarded as part of the rural area, associated temperature records are assumed to be free of urban influences and might be used as unbiased reference data for city records. However, based on two years of data from a high temporal and spatial resolution sensor network, this study proves the development of a substantial UHI in the Spanish village Tivissa with intensities of >1.5 K in summer Tmin and Tmax compared to a rural reference. Hosting a meteorological station that has been relocated several times within Tivissa during its >100-year history, we here detail a method to remove UHI biases at past measurement sites to create a more reliable rural temperature record. Adjusting the time series results in a trend increase of up to 0.1 K per decade in Tmin, while Tmax trends are slightly reduced. Comparing the adjustments based on station history with adjustments produced by employing a commonly used statistical homogenization method reveals substantial differences of >3 K between these approaches. Applying the presented method to a greater number of stations is problematic though, as it is laborious and requires a suitable sensor network as well as detailed metadata.

We present a sub-centennial resolution summer temperature record from the SE Tibetan Plateau (Tiancai Lake, SW China, 3900 m a.s.l.) derived from a chironomid stratigraphy covering the last c.19.5 ka. The record highlights the interaction between tropical and high-latitude climate forcing through the changes in atmospheric circulation during the last deglaciation. The scale of the last glacial maximum (LGM) cooling is consistent with other tropical mountain regions at c.5 °C and a rapid recovery of temperatures at 19.0 ka is related to changes in adiabatic lapse rates at the end of the glaciation. The overall pattern of change shows that North Atlantic deglaciation climate events (the Heinrich 1 and Younger Dryas cooling events and the Bolling–Allerod warm period) are all recorded, but the influence of the events decline as the deglaciation progresses. We relate these patterns to North Atlantic Deep water ventilating the Southern Ocean and to the consequent movements of the Intertropical Convergence Zone (ITCZ) in the Indian Ocean Basin as transmitted through changes in the southern mid-latitude circulation.

The meteorological measurements in Brno, Czech Republic, is among the world’s oldest measurements, operating since 1799. Like many others, station was initially installed in the city center, relocated several times, and currently operates at an airport outside the city. These geographical changes potentially bias the temperature record due to different station surroundings and varying degrees of urban heat island effects. Here, we assess the influence of land cover on spatial temperature variations in Brno, capitol of Moravia and the second largest city of the Czech Republic. We therefore use a unique dataset of half-hourly resolved measurements from 11 stations spanning a period of more than 3.5 years and apply this information to reduce relocation biases in the long-term temperature record from 1799 to the present. Regression analysis reveals a significant warming influence from nearby buildings and a cooling influence from vegetation, explaining up to 80% of the spatial variability within our network. The influence is strongest during the warm season and for land cover changes between 300 and 500 m around stations. Relying on historical maps and recent satellite data, it was possible to capture the building densities surrounding the past locations of the meteorological station. Using the previously established land cover–temperature relation, the anthropogenic warming for each measurement site could be quantified and hence eliminated from the temperature record accordingly, thereby increasing the long-term warming trend.

Reduction in body size of organisms following mass extinctions is well‐known and often ascribed to the Lilliput effect. This phenomenon is expressed as a temporary body size reduction within surviving species. Despite its wide usage the term is often loosely applied to any small post‐extinction taxa. Here we assess the size of bivalves of the family Limidae (Rafineque) prior to, and in the aftermath of, the end‐Triassic mass extinction event. Of the species studied only one occurs prior to the extinction event, though is too scarce to test for the Lilliput effect. Instead, newly evolved species originate at small body sizes and undergo a within‐species size increase, most dramatically demonstrated by Plagiostoma giganteum (Sowerby) which, over two million years, increases in size by 179%. This trend is seen in both field and museum collections. We term this within‐species size increase of newly originated species in the aftermath of mass extinction, the Brobdingnag effect, after the giants that were contemporary with the Lilliputians in Swift's Gulliver's Travels. The size increase results from greater longevity and faster growth rates. The cause of the effect is unclear, although it probably relates to improved environmental conditions. Oxygen‐poor conditions in the Early Jurassic are associated with populations of smaller body size caused by elevated juvenile mortality but these are local/regional effects that do not alter the long‐term, size increase. Although temperature‐size relationships exist for many organisms (Temperature‐Size Rule and Bergmann's Rule), the importance of this is unclear here because of a poorly known Early Jurassic temperature record.

Aim: Catastrophic forest mortality due to more extreme rainfall deficits and higher temperatures under future climate scenarios has been predicted. The aim of this study is to explore the magnitude of historical drought-induced tree mortality under pre-warming conditions. Location: North-eastern Australia. Time period: 1845–2017. Major taxa studied: Trees. Methods: Field survey, historical analysis and climate analysis. Results: We present evidence of 18%–30% tree mortality from recent droughts across three regions of north-eastern Australia with rainfall deficits less severe than earlier historical droughts. The corrected temperature record represents modest warming at the stations with long records in the vicinity of the study areas. In terms of rainfall deficit the most severe drought on record occurred in the early 20th century, and historical evidence confirms that this drought and earlier droughts before the advent of pastoralism coincided with substantial tree mortality. Main conclusions: Dramatic declines in woody biomass in response to drought historically occurred more than once a century and are a natural phenomenon in semi-arid Australia. The magnitude of drought-induced tree mortality under natural climate fluctuations requires further investigation in other continents. Widespread drought-induced tree mortality is not just a recent global change phenomenon and has been underestimated as a natural ecological process. However, even more severe forest die-off events from more extreme hotter droughts are predicted if Earth’s warming proceeds as currently projected.

The long-term cooling or warming trend is one of the most important aspects of Holocene paleoclimatic research. However, there are currently significant discrepancies among climatic simulation results, sedimentary and synthesized Holocene temperature records. These discrepancies call for new Holocene temperature records with a robust chronology and unambiguous indicative significance. Here, we present a ca. 45-year-resolution alpine peat α-cellulose carbon isotopic (δ13C) record from the Altai Mountains in central Asia. The chronology of the record has been well-constrained by 22 AMS 14C dates of the peat α-cellulose samples. Based on detailed modern-process study results, the indicative significance of the record has been carefully determined as summer temperature indicator. The record reveals an overall trend of increasing summer temperature for the past ca. 11 ka with a warmer early (from ca. 8–6 ka BP) and late (from ca. 4–0 ka BP) Holocene and colder middle Holocene (from ca. 6–4 ka BP). The general pattern of a warmer early and late Holocene and colder middle Holocene is consistent with previously reported temperature records from central Asia, the Tibetan Plateau, and North China. The long-term warming trend in our summer temperature record is consistent with recently reported winter temperature records from high latitudes of continental Eurasia. All these results further support the reliability of our summer temperature record from central Asia. The better understandings about the Holocene temperature history and its possible driven mechanism(s) can be benefited potentially from more reliable Holocene temperature records, including our record reported in this paper.

The Mid-Pleistocene Transition (MPT) is a widely recognized global climate shift occurring between approximately 1,250 to 700 ka. At this time, Earth's climate underwent a major transition from dominant 40 kyr glacial-interglacial cycles to quasi-100 kyr cycles. The cause of the MPT remains a puzzling aspect of Pleistocene climate. Presently, there are few, if any, continuous MPT records from the Arctic yet understanding the role and response of the high latitudes to the MPT is required to better evaluate the causes of this climatic shift. Here, we present new continental biomarker records of temperature and vegetation spanning 1,142 to 752 ka from Lake El'gygytgyn (Far East Russia). We reconstruct warm-season temperature variations across the MPT based on branched glycerol dialkyl glycerol tetraethers (brGDGTs) using the MBTʹ5ME proxy. The new Arctic temperature record does not display an overall cooling trend during the MPT but does exhibit strong glacial-interglacial cyclicity. Spectral analysis demonstrates persistent obliquity and precession pacing over the study interval and reveals substantial sub-orbital temperature variations at ~900 kyr during the first “skipped” interglacial. Interestingly, Marine Isotope Stage (MIS) 31, which is widely recognized as a particularly warm interglacial, does not exhibit exceptional warmth at Lake El'gygytgyn. Instead, we find that MIS 29, 27 and 21 were as warm or warmer than MIS 31. In particular, MIS 21 (~870 to 820 ka) stands out as an especially warm and long interglacial in the continental Arctic while MIS 25 is a notably cold interglacial. Throughout the MPT, Lake El'gygytgyn pollen data exhibits a long-term drying trend, with a shift to an increasingly open landscape noted after around 900 ka (Zhao et al., 2018), which is also reflected in our higher plant leaf wax (n-alkane) distributions. Although the mechanisms driving the MPT remain a matter of debate, our new climate records from the continental Arctic exhibit some similarities to changes noted around the North Pacific region. Overall, the new organic geochemical data from Lake El'gygytgyn contribute to expanding our knowledge of the high-latitude response to the MPT.

To help understanding the potential relationship between chemical weathering and Indian summer monsoon (ISM) since the last glacial period a gravity core (BoB-56) was retrieved from the central Bay of Bengal (BoB). The data of chemical weathering indexes (CIA, WIP, and αAlNa) used in this study showed general synchronicity with the regional monsoon precipitation and temperature record on precessional scale, indicating existence of control from the ISM on weathering. Corresponding to alteration of warm/cold period during the last deglaciation, obvious simultaneously alteration of higher/lower values of the chemical weathering and terrestrial input proxies’ record support our hypothesis that the ISM driving chemical weathering on the millennial scale. However, a contradiction occurred during the Holocene period, when the ISM precipitation and temperature rose to a higher level, while the alternative indexes unanimously reflected a weaker chemical weathering conditions. In this study, we discussed the applicability of chemical weathering indexes in the BoB during the Holocene period. Besides the possible weakened monsoon during 6–3 ka, recorded by the stalagmite δ18O and Sea Surface Temperature (SST) reconstruction results in the northeastern Indian Ocean, other factors were responsible for this phenomenon, including the grain size effect and distinction between the mountain high land and floodplain low land. The chemical weathering records, during the last glaciation, indicated the presence of control from the ISM on weathering at precessional and millennial scales. While, during the Holocene, they failed to reflect the actual chemical weathering dynamics of the source area. Indeed, a mixture of physical erosion and chemical weathering seems to be representative of the chemical weathering dynamics in the area. Our findings emphasized on the tight connections between the chemical weathering evolution and global-regional climate conditions around the BoB, implying possible ISM-controlled mechanisms during different time scales.

The Miocene experienced both, ice-house periods with continental ice-sheets covering both poles and warm greenhouse conditions with strongly increased global temperature, glacier retreat and sea-level rise. The Mid-Miocene Climatic Optimum (MMCO) is the most pronounced warming event in the last 24 Ma, standing out in a time of protracted cooling. The MMCO is marked by a period of intensive global warming between ca. 17 and 15 Ma. The subsequent Mid-Miocene Climate Transition (MMCT), in contrast, was affected by global temperature decline, growth of Antarctic ice sheets, sea level fall and marine biota overturn.Miocene climate conditions were intensely studied on both, global and regional scales, based on i.a. marine isotope records and continental paleobotanical and mammalian fossil data sets. Despite the dense data sets continental Miocene temperature evolution still remains unclear owing to a large range of inferred temperatures and/or poor age constraints of the associated records.Here, we present a long-term terrestrial climate record that covers the time interval between ~20 and ~13 Ma and is based on stable (δ18O) and clumped isotope (Δ47) geochemical data. We apply Δ47 thermometry on terrestrial foreland basin sediments to reconstruct the Middle Miocene continental temperature evolution for central Europe. Pedogenic carbonates from well dated fossil soils from several sites in the Northern Alpine Foreland Basin (Switzerland) reveal warm and stable temperatures for the early Miocene (20 – 19 Ma), followed by overall strongly enhanced variability in temperatures with maximum values attained between ca. 17 and 14 Ma. We observe a highly dynamic transition to cooler climates at the end of the MMCO and a subsequent rapid temperature decline of approximately 20°C after 14 Ma during the MMCT. The highly variable temperature patterns during the cooling period coincide with phases of high seasonality in the precipitation pattern as derived from oxygen isotope compositions of soil water.

The Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) concluded that warming of the climate system since the start of the instrumental record was unequivocal. While this conclusion does not solely rest upon the surface temperature record, this record is a key aspect underpinning this finding. In this chapter, the basis for the observational evidence for increases in global surface temperatures, since the start of the instrumental record, is discussed. The chapter begins with a discussion on how the measurements have been taken over time. This is followed by a description of the necessary efforts required to adjust the records to account for time varying biases. The resulting global estimates and attempts at quantifying uncertainties are then discussed. The changes in temperature extremes are briefly highlighted and the chapter closes with a view to future research directions and innovations.

Summary In order to improve our understanding of Earth’s climate history, proxies that accurately reconstruct past temperatures are needed. The distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs), the membrane lipids of certain bacteria, is correlated to temperature in modern settings, and therefore is at the basis of several paleothermometers. Here we apply several brGDGT based temperature calibrations developed specifically for lakes to the sediment sequence from Lake Chala, in equatorial East African, to generate temperature records which cover the last 170 kyr in unprecedented high temporal resolution (210 years on average). Surprisingly, application of the recently developed East African lake calibration leads to ambiguous results. By contrast applying a calibration created before the discovery of 5- and 6-Me brGDGT isomers produces a temperature record strongly reminiscent of established climate records such as the isotope records from Antarctica and contains periodicities relating to orbital precession and obliquity. Therefore it appears that 6-Me brGDGTs are key to the temperature signal archived in Lake Chala. In Lake Chala today, 5- and 6-Me brGDGTs generally occupy different parts of the water column, and thus their relative abundance may be indirectly related to temperature through changes in lake depth or seasonal mixing.

The shell δ18O of young modern Aequipecten opercularis from the southern North Sea provides an essentially faithful record of seasonal variation in seafloor temperature. In this well-mixed setting, A. opercularis shell δ18O also serves as a proxy for seasonal variation in surface temperature. Individuals from less agitated (e.g. deeper) settings in a warm climate would not be expected to record the full seasonal range in surface temperature because of thermal stratification in summer. Such circumstances have been invoked to explain cool isotopic summer temperatures from early Pliocene A. opercularis of eastern England. Support for a sub-thermocline setting derives from high-amplitude variation in microgrowth-increment size, which resembles the pattern in sub-thermocline A. opercularis from the southern Mediterranean Sea. Here, we present isotope and increment profiles from further sub-thermocline individuals, live-collected from a location in the Adriatic Sea for which we provide modelled values of expected shell δ18O. We also present data from supra-thermocline shells from the English Channel and French Mediterranean coast. The great majority of sub-thermocline A. opercularis show high-amplitude variation in increment size, and winter and summer δ18O values are generally quite close to expectation. However, the relatively warm summer conditions of 2015 are not recorded, in most cases due to a break in growth, perhaps caused by hypoxia. The supra-thermocline shells show subdued increment variation and yield isotopic winter and summer temperatures quite close to the local directly measured values. A. opercularis shells therefore provide a fairly good isotopic record of ambient temperature (if not always of relatively warm summer conditions below the thermocline) and their hydrographic setting can be determined from increment data. Early Pliocene examples from eastern England can be interpreted as having lived in a setting below the thermocline, with a higher seasonal range in surface temperature than now in the adjacent southern North Sea.

The 0.6 °C warming observed in global temperature datasets from 1940 to 1960 to 2000–2020 can be partially due to urban heat island (UHI) and other non-climatic biases in the underlying data, although several previous studies have argued to the contrary. Here we identify land regions where such biases could be present by locally evaluating their diurnal temperature range (DTR = TMax − TMin trends between the decades 1945–1954 and 2005–2014 and between the decades 1951–1960 and 1991–2000 versus their synthetic hindcasts produced by the CMIP5 models. Vast regions of Asia (in particular Russia and China) and North America, a significant part of Europe, part of Oceania, and relatively small parts of South America (in particular Colombia and Venezuela) and Africa show DTR reductions up to 0.5–1.5 °C larger than the hindcasted ones, mostly where fast urbanization has occurred, such as in central-east China. Besides, it is found: (1) from May to October, TMax globally warmed 40% less than the hindcast; (2) in Greenland, which appears nearly free of any non-climatic contamination, TMean warmed about 50% less than the hindcast; (3) the world macro-regions with, on average, the lowest DTR reductions and with low urbanization (60S-30N:120 W–90 E and 60 S–10 N:90 E–180 E: Central and South America, Africa, and Oceania) warmed about 20–30% less than the models’ hindcast. Yet, the world macro-region with, on average, the largest DTR reductions and with high urbanization (30 N–80 N:180 W–180 E: most of North America, Europe, and Central Asia) warmed just a little bit more (5%) than the hindcast, which indicates that the models well agree only with potentially problematic temperature records. Indeed, also tree-based proxy temperature reconstructions covering the 30°N–70°N land area produce significantly less warming than the correspondent instrumentally-based temperature record since 1980. Finally, we compare land and sea surface temperature data versus their CMIP5 simulations and find that 25–45% of the 1 °C land warming from 1940–1960 to 2000–2020 could be due to non-climatic biases. By merging the sea surface temperature record (assumed to be correct) and an adjusted land temperature record based on the model prediction, the global warming during the same period is found to be 15–25% lower than reported. The corrected warming is compatible with that shown by the satellite UAH MSU v6.0 low troposphere global temperature record since 1979. Implications for climate model evaluation and future global warming estimates are briefly addressed.

The thermometer, a device for measuring temperature, was developed in the seventeenth century but proper calibration and the development of systematic records did not occur until the nineteenth century. The World Meteorological Organization set up a system for collecting temperature measurements on a global scale early in the twentieth century. Data collection and homogenization have become ever more sophisticated. Records from single sites reflect the chaos of the weather, but when combined into regional analyses show distinct trends. North America appears to be a special case, showing a rise in the first half of the twentieth century, a plateau until about 1970, and a subsequent rise. The same patterns can be seen, but not so distinctly, in European records. It is thought that the rises reflect the increasing levels of atmospheric CO2, and the plateau the industrial pollution of the middle of the century. Records from the tropics show the slowest upward trend, but higher latitude records show more pronounced upward trends.

The correlation between the averaged reconstructed March temperature record for Kyoto, Japan, and the reconstructed Total Solar Irradiance (TSI) over 660 years from 1230 to 1890 gives evidence with 98% probability that the Little Ice Age with four cold periods is forced by variations of TSI. If the correlation is restricted to the period 1650–1890, with two cold periods in the 17th and 19th century and for which two independent reconstructed March temperature records are available, the probability of solar forcing increases to 99.99%. As solar irradiance variations have a global effect there has to be a global climatic solar forcing impact. However, by how much global temperature were lower during these minima and with what amplitude TSI was varying is not accurately known. The two quantities, global temperature and TSI, are linked by the energy equilibrium equation for the Earth system. The derivation of this equation with respect to a variation of the solar irradiance has two terms: A direct forcing term, which can be derived analytically and quantified accurately from the Stefan-Boltzmann law, and a second term, describing indirect influences on the surface temperature. If a small TSI variation should force a large temperature variation, then it has to be the second indirect term that strongly amplifies the effect of the direct forcing. The current knowledge is summarized by three statements:During the minima periods in the 13th, 15/16th, 17th, and 19th centuries the terrestrial climate was colder by 0.5–1.5 °C;Indirect Top-down and Bottom-up mechanisms do not amplify direct forcing by a large amount, i.e. indirect solar forcing is of the same magnitude (or smaller) as direct solar forcing;The radiative output of the Sun cannot be lower by more than 2 Wm−2 below the measured present-day TSI value during solar cycle minimum.These three statements contradict each other and it is concluded that at least one is not correct. Which one is a wrong statement is presently not known conclusively. It is argued that it is the third statement and it is speculated that over centennial time scales the Sun might vary its radiance significantly more than observed so far during the last 40 years of space TSI measurements. To produce Maunder minimum type cold climate excursions, a TSI decrease of the order of 10 Wm−2 is advocated.

Climate changes are due to anthropogenic factors, volcano eruptions and the natural variability of the Earth&rsquo, s system. Herein the natural variability of the global surface temperature is modeled using a set of harmonics spanning from the inter-annual to the millennial scales. The model is supported by the following considerations: (1) power spectrum evaluations show 11 spectral peaks (from the sub-decadal to the multi-decadal scales) above the 99% confidence level of the known temperature uncertainty, (2) spectral coherence analysis between the independent global surface temperature periods 1861&ndash, 1937 and 1937&ndash, 2013 highlights at least eight common frequencies between 2- and 20-year periods, (3) paleoclimatic temperature reconstructions during the Holocene present secular to millennial oscillations. The millennial oscillation was responsible for the cooling observed from the Medieval Warm Period (900&ndash, 1400) to the Little Ice Age (1400&ndash, 1800) and, on average, could have caused about 50% of the warming observed since 1850. The finding implies an equilibrium climate sensitivity of 1.0&ndash, 2.3 &deg, C for CO2 doubling likely centered around 1.5 &deg, C. This low sensitivity to radiative forcing agrees with the conclusions of recent studies. Semi-empirical models since 1000 A.D. are developed using 13 identified harmonics (representing the natural variability of the climate system) and a climatic function derived from the Coupled Model Intercomparison Project 5 (CMIP5) model ensemble mean simulation (representing the mean greenhouse gas&mdash, GHG, aerosol, and volcano temperature contributions) scaled under the assumption of an equilibrium climate sensitivity of 1.5 &deg, C. The harmonic model is evaluated using temperature data from 1850 to 2013 to test its ability to predict the major temperature patterns observed in the record from 2014 to 2020. In the short, medium, and long time scales the semi-empirical models predict: (1) temperature maxima in 2015&ndash, 2016 and 2020, which is confirmed by the 2014&ndash, 2020 global temperature record, (2) a relatively steady global temperature from 2000 to 2030&ndash, 2040, (3) a 2000&ndash, 2100 mean projected global warming of about 1 &deg, C. The semi-empirical model reconstructs accurately the historical surface temperature record since 1850 and hindcasts mean surface temperature proxy reconstructions since the medieval period better than the model simulation that is unable to simulate the Medieval Warm Period.

Alkenones are biomarkers produced solely by algae in the order Isochrysidales that have been used to reconstruct sea surface temperature (SST) since the 1980s. However, alkenone-based SST reconstructions in the northern high latitude oceans show significant bias towards warmer temperatures in core-tops, diverge from other SST proxies in down core records, and are often accompanied by anomalously high relative abundance of the C37 tetra-unsaturated methyl alkenone (%C37:4). Elevated %C37:4 is widely interpreted as an indicator of low sea surface salinity from polar water masses, but its biological source has thus far remained elusive. Here we identify a lineage of Isochrysidales that is responsible for elevated C37:4 methyl alkenone in the northern high latitude oceans through next-generation sequencing and lab-culture experiments. This Isochrysidales lineage co-occurs widely with sea ice in marine environments and is distinct from other known marine alkenone-producers, namely Emiliania huxleyi and Gephyrocapsa oceanica. More importantly, the %C37:4 in seawater filtered particulate organic matter and surface sediments is significantly correlated with annual mean sea ice concentrations. In sediment cores from the Svalbard region, the %C37:4 concentration aligns with the Greenland temperature record and other qualitative regional sea ice records spanning the past 14 kyrs, reflecting sea ice concentrations quantitatively. Our findings imply that %C37:4 is a powerful proxy for reconstructing sea ice conditions in the high latitude oceans on thousand- and, potentially, on million-year timescales., Some algae produce compounds called alkenones that can reconstruct sea surface temperature through geological time, but in high latitudes unknown species complicate use of this proxy. Here the authors find a lineage of sea ice algae that produces alkenones and can be used as a paleo-sensor for sea ice abundance.

Using new and previously published CO2 data from the EPICA Dome C ice core (EDC), we reconstruct a new high-resolution record of atmospheric CO2 during Marine Isotope Stage (MIS) 6 (190 to 135 ka) the penultimate glacial period. Similar to the last glacial cycle, where high-resolution data already exists, our record shows that during longer North Atlantic (NA) stadials, millennial CO2 variations during MIS 6 are clearly coincident with the bipolar seesaw signal in the Antarctic temperature record. However, during one short stadial in the NA, atmospheric CO2 variation is small ( ∼5 ppm) and the relationship between temperature variations in EDC and atmospheric CO2 is unclear. The magnitude of CO2 increase during Carbon Dioxide Maxima (CDM) is closely related to the NA stadial duration in both MIS 6 and MIS 3 (60–27 ka). This observation implies that during the last two glacials the overall bipolar seesaw coupling of climate and atmospheric CO2 operated similarly. In addition, similar to the last glacial period, CDM during the earliest MIS 6 show different lags with respect to the corresponding abrupt CH4 rises, the latter reflecting rapid warming in the Northern Hemisphere (NH). During MIS 6i at around 181.5± 0.3 ka, CDM 6i lags the abrupt warming in the NH by only 240± 320 years. However, during CDM 6iv ( 171.1± 0.2 ka) and CDM 6iii ( 175.4± 0.4 ka) the lag is much longer: 1290± 540 years on average. We speculate that the size of this lag may be related to a larger expansion of carbon-rich, southern-sourced waters into the Northern Hemisphere in MIS 6, providing a larger carbon reservoir that requires more time to be depleted.

The Tibetan Plateau (TP) has numerous glaciers that provide water for more than one-third of the world’s population. Reconstructing past temperature change on the TP provides a valuable context for assessing the current and possible future status of glaciers. However, the quantitative paleotemperature records since the last deglaciation on the TP are sparse. Moreover, existing records have revealed a conflicting Holocene temperature variation patterns on the northeastern and western TP. Quantitative temperature records on the central TP would be essential for a better understanding of the spatiotemporal complexity of temperature variation. In this study, we report the temperature record from the sedimentary record of lake Linggo Co on the central TP since the last deglaciation using branched glycerol dialkyl glycerol tetraethers (bGDGTs) based proxy. Our results indicate that the paleoclimate during the last deglaciation on the central TP was characterized by large fluctuations in temperature. The mean annual temperature of lake Linggo Co remained low during the early Holocene (11.7–10 ka BP) and gradually increased to 4 oC at 8.3 ka, rapidly declining to -2 oC on average towards the present day. Solar radiation, continental glacier feedback, as well as atmosphere circulation play a major role in the distribution of sensitive and latent heat, thus affecting the Holocene temperature variability of the TP. Discrepancies in published records on the TP can result from a seasonal bias of the proxies and spatial differences due to topography - boundary effect. Our results suggest that the seasonal bias of proxies, the spatiotemporal difference should be taken into consideration before regional or global synthesis of paleotemperature records.

International audience; The end-Triassic mass extinction coincided with the eruption of the Central Atlantic Magmatic Province, a large igneous province responsible for the massive atmospheric input of potentially climate-altering volatile compounds that is associated with a sharp rise in atmospheric CO2. The extinction mechanism is debated, but both short-term cooling (similar to 10s of years) related to sulfur aerosols and longer-term warming (10,000 yrs) related to CO2 emissions-essentially opposite hypotheses-are suggested triggers. Until now, no temperature records spanning this crucial interval were available to provide a baseline or to differentiate between hypothesized mechanisms. Here, we use clumped-isotope paleothermometry of shallow marine microbialites coupled with climate modeling to reconstruct ocean temperature at the extinction horizon. We find mild to warm ocean temperatures during the extinction event and evidence for repeated temperature swings of similar to 16 degrees C, which we interpret as a signature of strong seasonality. These results constitute the oldest non-biomineralized marine seasonal temperature record. We resolve no apparent evidence for short-term cooling or initial warming across the 1-80kyr of the extinction event our record captures, implying that the initial onset of the biodiversity crisis may necessitate another mechanism.

Mountains of the arid Great Basin region of Nevada are home to critical water resources and numerous species of plants and animals. Understanding the nature of climatic variability in these environments, especially in the face of unfolding climate change, is a challenge for resource planning and adaptation. Here, we utilize an Embedded Sensor Network (ESN) to investigate landscape-scale temperature variability in Great Basin National Park (GBNP). The ESN was installed in 2006 and has been maintained during uninterrupted annual student research training expeditions. The ESN is comprised of 29 Lascar sensors that record hourly near-surface air temperature and relative humidity at locations spanning 2000 m and multiple ecoregions within the park. From a maximum elevation near 4000 m a.s.l. atop Wheeler Peak, the sensor locations are distributed: (1) along a multi-mountain ridgeline to the valley floor, located ~2000 m lower; (2) along two streams in adjoining eastern-draining watersheds; and (3) within multiple ecological zones including sub-alpine forests, alpine lakes, sagebrush meadows, and a rock glacier. After quality checking all available hourly observations, we analyze a twelve-year distributed temperature record for GBNP and report on key patterns of variability. From 2006 to 2018, there were significantly increasing trends in daily maximum, minimum and mean temperatures for all elevations. The average daily minimum temperatures increased by 2.1°C. The trend in daily maximum temperatures above 3500 m was significantly greater than the increasing trends at lower elevations, suggesting that daytime forcings may be driving enhanced warming at GBNP’s highest elevations. These results indicate that existing weather stations, such as the Wheeler Peak SNOTEL site, alone cannot account for small-scale variability found in GBNP. This study offers an alternative, low-cost methodology for sustaining long-term, distributed observations of conditions in heterogeneous mountainous environments at finer spatial resolutions. In arid mountainous regions with vulnerable water resources and fragile ecosystems, it is imperative to maintain and extend existing networks and observations as climate change continues to alter conditions.

The aims of the study were to assess the severity of temperature conditions in Europe, in June 2019, using a newly developed extremes index, as well as to evaluate circulation conditions that favored the occurrence of extremely hot days in June 2019, as seen over the long term. The main focus of this work was on two European regions particularly affected by high temperatures in June 2019, namely Central Europe and Iberia. To comprehensively characterize heat events in terms of their spatial extent and intensity, we proposed the extremity index (EI) and used it to compare hot days occurring in areas of different sizes and with different climatic conditions. The role of atmospheric circulation in the occurrence of hot days was evaluated using the Grosswetterlagen (GWL) circulation types catalog, as well as composite maps created with the bootstrap resampling technique. Our results reveal that June 2019 was unusually hot, and in terms of the magnitude of the anomaly, it has no analogue in the 70-year-long temperature record for Europe. However, the properties of heat events in the two considered regions were substantially different. The occurrence of hot days in June 2019, in Europe, was mainly associated with the GWL types forcing advection from the southern sector and co-occurrence of high-pressure systems which was significantly proven by the results of bootstrap resampling. In terms of the applicability of the new approach, the EI proved to be a useful tool for the analysis and evaluation of the severity of hot days based on their intensity and spatial range.

As European heatwaves become more severe, summers in the United Kingdom (UK) are also getting warmer. The UK record temperature of 38.7 °C set in Cambridge in July 2019 prompts the question of whether exceeding 40 °C is now within reach. Here, we show how human influence is increasing the likelihood of exceeding 30, 35 and 40 °C locally. We utilise observations to relate local to UK mean extremes and apply the resulting relationships to climate model data in a risk-based attribution methodology. We find that temperatures above 35 °C are becoming increasingly common in the southeast, while by 2100 many areas in the north are likely to exceed 30 °C at least once per decade. Summers which see days above 40 °C somewhere in the UK have a return time of 100-300 years at present, but, without mitigating greenhouse gas emissions, this can decrease to 3.5 years by 2100., The United Kingdom has experienced a strong heat wave in 2019 that set a new temperature record for the country of 38.7 °C. In this study the authors show that under climate change, local temperatures are increasingly likely to exceed 35 °C and 40 °C in the next decades and, hence, summers like the one of 2019 become more frequent.

We examine what can be learnt about climate sensitivity from variability in the surface air temperature record over the instrumental period, from around 1880 to the present. While many previous studies have used trends in observational time series to constrain equilibrium climate sensitivity, it has also been argued that temporal variability may also be a powerful constraint. We explore this question in the context of a simple widely used energy balance model of the climate system. We consider two recently proposed summary measures of variability and also show how the full information content can be optimally used in this idealised scenario. We find that the constraint provided by variability is inherently skewed, and its power is inversely related to the sensitivity itself, discriminating most strongly between low sensitivity values and weakening substantially for higher values. It is only when the sensitivity is very low that the variability can provide a tight constraint. Our investigations take the form of “perfect model” experiments, in which we make the optimistic assumption that the model is structurally perfect and all uncertainties (including the true parameter values and nature of internal variability noise) are correctly characterised. Therefore the results might be interpreted as a best-case scenario for what we can learn from variability, rather than a realistic estimate of this. In these experiments, we find that for a moderate sensitivity of 2.5 ∘C, a 150-year time series of pure internal variability will typically support an estimate with a 5 %–95% range of around 5 ∘C (e.g. 1.9–6.8 ∘C). Total variability including that due to the forced response, as inferred from the detrended observational record, can provide a stronger constraint with an equivalent 5 %–95 % posterior range of around 4 ∘C (e.g. 1.8–6.0 ∘C) even when uncertainty in aerosol forcing is considered. Using a statistical summary of variability based on autocorrelation and the magnitude of residuals after detrending proves somewhat less powerful as a constraint than the full time series in both situations. Our results support the analysis of variability as a potentially useful tool in helping to constrain equilibrium climate sensitivity but suggest caution in the interpretation of precise results.

[EN]The geothermal resources in Spain have been a source of deep research in recent years and are, in general, well-defined. However, there are some areas where the records from the National Institute for Geology and Mining show thermal activity from different sources despite no geothermal resources being registered there. This is the case of the area in the south of the Duero basin where this research was carried out. Seizing the opportunity of a deep borehole being drilled in the location, some geophysical resources were used to gather information about the geothermal properties of the area. The employed geophysical methods were time-domain electromagnetics (TDEM) and borehole logging; the first provided information about the depth of the bedrock and the general geological structure, whereas the second one gave more detail on the geological composition of the different layers and a temperature record across the whole sounding. The results allowed us to establish the geothermal gradient of the area and to discern the depth of the bedrock. Using the first 200 m of the borehole logging, the thermal conductivity of the ground for shallow geothermal systems was estimated.