Temperature Records Research Articles

East Asian summer monsoon variations across the Miocene−Pliocene boundary recorded by sediments from the Guide Basin, northeastern Tibetan Plateau

Records of atmospheric carbon dioxide concentration, sea-surface temperature, and global vegetation show that Earth’s climate and environment changed significantly during the late Miocene−early Pliocene. Understanding the environmental response to insolation forcing during this transitional period may provide insights into future environmental variations resulting from the perturbation of the global carbon cycle caused by fossil fuel combustion. However, terrestrial paleoclimate records capable of resolving orbital time-scale environmental variations are mostly from Europe, especially from the region around the Mediterranean Sea. Here, we present high-resolution records of grain size, black carbon, and geochemistry from a sedimentary sequence from the northeastern margin of the Tibetan Plateau, where precipitation is mainly via the East Asian summer monsoon. We observed increases in sediment accumulation rate and black carbon mass accumulation rate at ca. 5.3 Ma, which we interpret as the result of intensified seasonal precipitation associated with the strengthening of the East Asian summer monsoon; concurrently, precessional and obliquity cycles became more prominent during the early Pliocene. Our results suggest that, in response to current and future high atmospheric carbon dioxide concentrations, changes in the East Asian summer monsoon are likely to result in increased precipitation and seasonality within its region of influence.

Evidence of Urban Blending in Homogenized Temperature Records in Japan and in the United States: Implications for the Reliability of Global Land Surface Air Temperature Data

Abstract To reduce the amount of nonclimatic biases of air temperature in each weather station’s record by comparing it with neighboring stations, global land surface air temperature datasets are routinely adjusted using statistical homogenization to minimize such biases. However, homogenization can unintentionally introduce new nonclimatic biases due to an often-overlooked statistical problem known as “urban blending” or “aliasing of trend biases.” This issue arises when the homogenization process inadvertently mixes urbanization biases of neighboring stations into the adjustments applied to each station record. As a result, urbanization biases of the original unhomogenized temperature records are spread throughout the homogenized data. To evaluate the extent of this phenomenon, the homogenized temperature data for two countries (Japan and the United States) are analyzed. Using the Japanese stations in the widely used Global Historical Climatology Network (GHCN) dataset, it is first confirmed that the unhomogenized Japanese temperature data are strongly affected by urbanization bias (possibly ∼60% of the long-term warming). The U.S. Historical Climatology Network (USHCN) dataset contains a relatively large amount of long, rural station records and therefore is less affected by urbanization bias. Nonetheless, even for this relatively rural dataset, urbanization bias could account for ∼20% of the long-term warming. It is then shown that urban blending is a major problem for the homogenized data for both countries. The IPCC’s estimate of urbanization bias in the global temperature data based on homogenized temperature records may have been low as a result of urban blending. Recommendations on how future homogenization efforts could be modified to reduce urban blending are discussed. Significance Statement Most weather station–based global land temperature datasets currently use a process called “statistical homogenization” to reduce the amount of nonclimatic biases. However, using temperature data from two countries (Japan and the United States), we show that the homogenization process unintentionally introduces new nonclimatic biases into the data as a result of an “urban blending” problem. Urban blending arises when the homogenization process inadvertently mixes the urbanization (warming) bias of the neighboring stations into the adjustments applied to each station record. As a result, the urbanization biases of the unhomogenized temperature records are spread throughout all of the homogenized data. The net effect tends to artificially add warming to rural stations and subtract warming from urban stations until all stations have about the same amount of urbanization bias.

Disparate energy sources for slow and fast Dansgaard–Oeschger cycles

Abstract. During the Late Pleistocene, Dansgaard–Oeschger (DO) cycles triggered warming events that were as abrupt as the present-day human-induced warming. However, in the absence of a periodic forcing operating on millennial timescales, the main energy sources of DO cycles remain debated. Here, we identify the energy sources of DO cycles by applying a bispectral analysis to the North Greenland Ice Core Project (NGRIP) oxygen isotope (δ18Oice) record; a 123 kyr long proxy record of air temperatures (Tair) over Greenland. For both modes of DO cyclicity – slow and fast – we detect disparate energy sources. Slow DO cycles, marked by multi-millennial periodicities in the 12.5 to 2.5 kyr bandwidth, receive energy from astronomical periodicities. Fast DO cycles, characterized by millennial periodicities in the 1.5 ± 0.5 kyr range, receive energy from centennial periodicities. We propose cryospheric and oceanic mechanisms that facilitate the transfer of energy from known sources to slow and fast DO cycles, respectively. Our findings stress the importance of understanding energy-transfer mechanisms across a broad range of timescales to explain the origins of climate cycles without primary periodic energy sources.

Minimizing the impact of geographical basis risk on weather derivatives

AbstractIn the last decade, the index-based weather products (also called weather derivatives) have been gaining attention in the climate resilience discussion. Weather derivatives are designed to help companies hedging against climate variability. These products, that can be market-traded or over-the-counter, compensate individuals based on a pre-defined weather index. Thus, pay-offs of a weather derivative depend on a weather index and not, as with traditional types of insurance, on the actual amount of money lost due to adverse weather. One of the major drawbacks that may prevent weather derivatives to catch on is the impact of the Geographical Basis Risk (GBR), that is the deviation of weather conditions at different locations. In fact, when the reference weather station is not located in the immediate vicinity of the site of interest the hedging effectiveness may be reduced. In this paper, we contribute to the existing literature on GBR by proposing an optimization method that may help in offering a tailored solution, while at the same time keeping a standardized instrument as a reference. Using a historical record of Italian temperatures, strikes for temperatures are the choice variables of a penalty function containing pay-offs of a reference station and all other stations. Further, altitude and latitude of meteorological stations are shown to be relevant predictors to explain GBR. This can be an interesting starting point for the design of weather derivatives, since, from a unique station where the “reference” derivative is priced, all the other stations may be easily settled.

Future climate change effects on US forest composition may offset benefits of reduced atmospheric deposition of N and S.

Climate change and atmospheric deposition of nitrogen (N) and sulfur (S) are important drivers of forest demography. Here we apply previously derived growth and survival responses for 94 tree species, representing >90% of the contiguous US forest basal area, to project how changes in mean annual temperature, precipitation, and N and S deposition from 20 different future scenarios may affect forest composition to 2100. We find that under the low climate change scenario (RCP 4.5), reductions in aboveground tree biomass from higher temperatures are roughly offset by increases in aboveground tree biomass from reductions in N and S deposition. However, under the higher climate change scenario (RCP 8.5) the decreases from climate change overwhelm increases from reductions in N and S deposition. These broad trends underlie wide variation among species. We found averaged across temperature scenarios the relative abundance of 60 species were projected to decrease more than 5% and 20 species were projected to increase more than 5%; and reductions of N and S deposition led to a decrease for 13 species and an increase for 40 species. This suggests large shifts in the composition of US forests in the future. Negative climate effects were mostly from elevated temperature and were not offset by scenarios with wetter conditions. We found that by 2100 an estimated 1 billion trees under the RCP 4.5 scenario and 20 billion trees under the RCP 8.5 scenario may be pushed outside the temperature record upon which these relationships were derived. These results may not fully capture future changes in forest composition as several other factors were not included. Overall efforts to reduce atmospheric deposition of N and S will likely be insufficient to overcome climate change impacts on forest demography across much of the United States unless we adhere to the low climate change scenario.

Holocene temperature variability in China

The patterns and mechanisms of Holocene temperature variability in China remain highly controversial, possibly due to the lack of sufficient high-resolution records and few systematic compilations and reviews of published records. Here, we used branched glycerol dialkyl glycerol tetraethers (brGDGTs) to generate a Holocene mean annual air temperature (MAAT) record from Lake Yangzonghai (YZH) in southwest (SW) China. Our record, largely ruling out the effect of seasonal bias, displays a long-term cooling of ∼1.5 °C since 10 cal ka BP, indicating that the discrepancy of Holocene temperature records (such as long-term cooling vs warming trends) in China cannot be attributed to the seasonal bias in temperature proxies. We compiled 67 published Holocene temperature records from China. These temperature records exhibit a mathematically consistent pattern with a relatively warmer mid-Holocene and colder late Holocene in the Asian summer monsoon (ASM) domain, whilst records show a relatively colder mid-Holocene and warmer late Holocene in the westerlies domain. This spatial heterogeneity of Holocene temperature variability is attributed to the modulation of monsoon systems, including the warming effect of ASM and the cooling effect of the East Asian winter monsoon (EAWM). In the ASM domain, the dominant pattern with the relatively warmer mid-Holocene and colder late Holocene is primarily controlled by the persistent weakening of the warming effect of the ASM and the concurrent strengthening of the cooling effect of the EAWM during the mid-to-late Holocene. In the westerlies domain, the warming effect of the ASM disappears, the cooling effect of the EAWM is confined to the north of the Tibetan Plateau due to the blocking effect of the Tibetan Plateau. This dominant pattern with the relatively colder mid-Holocene and warmer late Holocene is driven by the increases in atmospheric greenhouse gases (GHG) radiative forcing and Northern Hemisphere Annual Insolation (NHAI) forcing during the mid-to-late Holocene.

Empirical assessment of the role of the Sun in climate change using balanced multi-proxy solar records

The role of the Sun in climate change is hotly debated. Some studies suggest its impact is significant, while others suggest it is minimal. The Intergovernmental Panel on Climate Change (IPCC) supports the latter view and suggests that nearly 100% of the observed surface warming from 1850–1900 to 2020 is due to anthropogenic emissions. However, the IPCC’s conclusions are based solely on computer simulations made with global climate models (GCMs) forced with a total solar irradiance (TSI) record showing a low multi-decadal and secular variability. The same models also assume that the Sun affects the climate system only through radiative forcing – such as TSI – even though the climate could also be affected by other solar processes. In this paper I propose three “balanced” multi-proxy models of total solar activity (TSA) that consider all main solar proxies proposed in scientific literature. Their optimal signature on global and sea surface temperature records is assessed together with those produced by the anthropogenic and volcanic radiative forcing functions adopted by the CMIP6 GCMs. This is done by using a basic energy balance model calibrated with a differential multi-linear regression methodology, which allows the climate system to respond to the solar input differently than to radiative forcings alone, and to evaluate the climate’s characteristic time-response as well. The proposed methodology reproduces the results of the CMIP6 GCMs when their original forcing functions are applied under similar physical conditions, indicating that, in such a scenario, the likely range of the equilibrium climate sensitivity (ECS) could be 1.4 °C to 2.8 °C, with a mean of 2.1 °C (using the HadCRUT5 temperature record), which is compatible with the low-ECS CMIP6 GCM group. However, if the proposed solar records are used as TSA proxies and the climatic sensitivity to them is allowed to differ from the climatic sensitivity to radiative forcings, a much greater solar impact on climate change is found, along with a significantly reduced radiative effect. In this case, the ECS is found to be 0.9–1.8 °C, with a mean of around 1.3 °C. Lower ECS ranges (up to 20%) are found using HadSST4, HadCRUT4, and HadSST3. The result also suggests that at least about 80% of the solar influence on the climate may not be induced by TSI forcing alone, but rather by other Sun-climate processes (e.g., by a solar magnetic modulation of cosmic ray and other particle fluxes, and/or others), which must be thoroughly investigated and physically understood before trustworthy GCMs can be created. This result explains why empirical studies often found that the solar contribution to climate changes throughout the Holocene has been significant, whereas GCM-based studies, which only adopt radiative forcings, suggest that the Sun plays a relatively modest role.

Temperature variability over Dokriani glacier region, Western Himalaya, India

Long-term climate records which help decipher past climate variability and its impact are scarce in the tough terrain of the Himalayan region. Therefore, in order to fill the climate data gap and understand the glacier climate linkage, we developed a 231 year long (1785–2015 CE) March–June temperature record using ring-width chronology of Himalayan fir (Abies pindrow (Royle ex D.Don) Royle) for the Din Gad valley, Dokriani glacier region, Uttarkashi, Uttarakhand, in the Western Himalaya. The Din Gad, originating from the Dokriani glacier, is a meltwater river contributing to Bhagirathi catchment in the headwaters of the socio-economically vital Ganga River. The 21-year running mean of the temperature record showed 1978–1998 CE as the coldest period followed by 1925–1945 CE, and 1890–1910 CE as the warmest period followed by 1946–1966 CE over the entire time series. The reconstruction matches well with tree-ring based temperature records available from the Garhwal Himalaya. It also shows similarity to tree-ring based temperature reconstructions from the Western Himalaya, Nepal, Tibetan Plateau and Bhutan, thus displaying a regional scale climate signal. The low frequency fluctuation patterns of the March–June temperature also matches with Asia and Northern hemisphere temperature records. Reconstructed March–June temperature record showed a statistically negligible warming temperature trend during 1901–1989 CE in the 20th century. It, however, captured a warming spike from 1990s CE which continues rising into the 21st century, which is also evident in the Northern hemisphere temperature record. Moreover, temperature rise is not anomalous in the past 231 years and well within range of the rest of the series. The present temperature record exclusively from the glacier region revealed a strong linkage with the benchmark Dokriani glacier's winter mass balance (November–April) revealing mass loss (gain) episodes occurred in warm (cool) phases. This first such record from the glacier valleys in Ganga headwaters would be of great value at providing insight into past climate variability and glacier behaviour with respect to climate change in long term perspective, and thus would provide valuable information for water resource management in light of climate change.

Keep calm and stay warm: a QI project to optimise perioperative temperature control in paediatric anaesthetics at the Evelina London Children’s Hospital

Background: Hypothermia is defined as a core body temperature <36°C.1 Complications of anaesthetic-related heat loss include delayed recovery and increased risk of surgical wound infection.2 Predominantly owing to a smaller weight-to-surface-area ratio,3 this is more common in paediatrics. With ∼45.1%4 of cases involving at least one hypothermic episode, it is vital that body temperature is regularly monitored and corrected throughout the perioperative period. We aimed to increase the percentage of anaesthetic charts with temperatures recorded at the correct time intervals according to the Evelina guidelines for perioperative temperature control by 20% by mid-February 2023. Methods: After an initial shadowing exercise observing temperature recording in practice, we created driver diagrams and completed two PDSA (plan-do-study-act) cycles: 1. a 30-min presentation to key stakeholders on the importance of perioperative temperature control and current local guidelines; 2. we displayed posters containing cycle 1 results and a summarised guideline flowchart. Data were collected over 18 weeks and analysed using descriptive statistics. Results: At baseline, 8.4% of temperature measurements were recorded correctly. After intervention 1, this increased to 8.8% and 13.6% after intervention 2. A 5.2% increase was noted overall. Conclusion: Our results indicate that reminder-based interventions can improve guideline compliance within a short timeframe and at low cost. However, we were unable to reach our aim. A questionnaire distributed to the team revealed that 75% (n=3) felt the interventions provided useful reminders of forgotten information while 56% (n=5) were absent from the presentation. Staff engagement should hence be prioritised in future. Further PDSA cycles may also consider reiterating the guidelines by other means (e.g. attaching reminders to anaesthetic desks). As many commented on paper charts being a hindrance for timely documentation, electronic prompts that indicate when recordings are to be made may also be considered. 1.Hart SR, Bordes B, Hart J, Corsino D, Harmon D. Ochsner J 2011; 11: 259–702.Diaz M, Becker DE. Anesth Prog 2010; 57: 25–333.Nemeth M, Miller C, Bräuer A. Int J Environ Res Public Health 2021; 18: 75414.Görges M, Afshar K, West N, Pi S, Bedford J, Whyte SD, et al. Pediatr Anesth 2019; 29: 27–37

Seasonal Mg/Ca-inferred temperatures of brackish water ostracods

The common brackish water ostracods Cyprideis torosa and Loxoconcha elliptica frequently occur together in high abundances in marginal marine environments. Seasonality of calcification differs between species and can have important implications for palaeotemperature reconstructions. There are existing palaeotemperature calibrations for both genera. However, the Loxoconcha spp. Mg/Ca temperature calibration has not thus far been applied to L. elliptica. The equation for Loxoconcha spp. does not rely on a known Mg/Cawater value, unlike the calibration for C. torosa, suggesting it may be possible to reconstruct temperatures without an estimation of Mg/Cawater, which is potentially particularly beneficial in environments that have highly dynamic Mg/Cawater. However, the calibration has only been applied in environments with marine-like Mg/Cawater. Demonstrating the applicability of the equation in marginal marine environments (with Mg/Cawater 3–5 mol/mol) and tracking the seasonal calcification of L. elliptica alongside C. torosa, therefore, has the potential to improve uncertainty in seasonal palaeotemperature reconstructions. Here, we compare previous monitoring of C. torosa with L. elliptica from the same collections. We demonstrate that the Mg/Ca temperature calibration for Loxoconcha spp. is appropriate to use with L. elliptica. Mg/Ca-inferred temperatures broadly track spring temperatures and suggest spring calcification. Cyprideis torosa Mg/Ca-inferred temperatures record the range of expected temperatures between spring and autumn. When analysing multiple single valves of L. elliptica and C. torosa simultaneously, the maximum Mg/CaC.torosa can, therefore, be used to reconstruct maximum summer temperatures, the minimum Mg/CaC.torosa to reconstruct autumn temperatures, and the Mg/CaL.elliptica to reconstruct the range in spring temperatures.

Temporal resolution of climate pressures on façades in Oxford 1815–2021

Changes in climate will exert increasing pressure on heritage, so standard climate metrics need to be tuned to heritage threats. Historical meteorological records are commonly available as monthly summaries, with few offering daily observations as daily readings may not have been taken or yet digitised. As data averaged over longer intervals misses short weather events, we investigate the extent to which temporal resolution is important for assessing climate pressures on façades. The Radcliffe Meteorological Station, Oxford, UK, provides the longest continual record of daily temperature and precipitation measurements in the UK. We use this record to assess the role of temporal scale in heritage climate parameters relating to (i) sunshine and warmth, (ii) rainy days and (iii) freezing events. Where there is a linear relationship between daily and monthly scale data, monthly observations can be interpolated as heritage climate parameters. However, for the majority of parameters, daily data was required to capture the variability in the datasets. We argue for the increased availability of daily observations to help assess the threat of climate to heritage.

Unexpected cold season warming during the Little Ice Age on the northeastern Tibetan Plateau

There is a general agreement that Northern Hemisphere temperatures have cooled over the past two millennia, culminating in the Little Ice Age. However, this understanding partly relies on the compilation of existing proxy records, the majority of which carry a warm season bias such that there is an underrepresentation of cold-season temperatures. Here we report a unique cold-season temperature record based on the alkenone paleothermometer from the northeastern Tibetan Plateau that spans the last two millennia. In contrast to the regional- and hemisphere-scale summer cooling, our reconstruction shows a long-term warming through the Medieval Climate Anomaly into Little Ice Age. We attribute these opposing temperature trends to combined effects of seasonally divergent insolation and North Atlantic subpolar gyre circulation. Our study indicates that the cold season during the Little Ice Age was not the coldest period of the last two millennia at least on the northeastern Tibetan Plateau.

Extracting quartz deformation fabrics from polymineralic rocks

Quartz dynamic recrystallization fabrics are the record of deformation temperatures, strain rates, and fluids, and can be qualitatively and quantitatively evaluated using optical microscopy and electron backscatter diffraction (EBSD). Characterization of quartz deformation fabrics have been largely guided by textures in homogenous monomineralic samples. However, in nature such lithological homogeneity is uncommon, and mica and feldspar that interrupt the interconnectedness of quartz inhibit the exchange of material across quartz grain boundaries, effectively pinning these grains and preventing dislocation creep.We present two EBSD methods of extracting quartz domains within polyphase mylonites to investigate the effects of discontinuous quartz distribution and remove isolated quartz grains pinned by other phases. These samples provide textural evidence for grain boundary migration in quartz at temperatures exceeding 500 °C. Quartz domains vary in size from cm thick foliation parallel continuous ribbons, to isolated quartz grains separated by networks of mica and feldspar. We quantify contrasting textural responses to deformation between connected and isolated quartz grains using existing measures of EBSD data analysis, including M-index, grain orientation spread, kernel averaged misorientation, and grain size. We find that removing isolated quartz grains yields pole figures that are more representative of the regional deformation.

Long-term daily stream temperature record for Scotland reveals spatio-temporal patterns in warming of rivers in the past and further warming in the future

Stream temperature is directly and indirectly affected by climate change. To be able to project future changes in stream temperature, historic trends and factors influencing these trends need to be understood. There is a demand for daily data to analyse historical trends and future changes in stream temperature. However, long-term daily stream temperature data are rare and observations of coarse temporal resolution (e.g. once-a-month) do not allow for robust trend analyses. Here, we present a methodology to reconstruct a national long-term daily stream temperature record (1960–2080) from 40 years of once-a-month observations (for 45 Scottish catchments). This involved implementing climatic and hydrological variables in generalized additive models. These models were then used in combination with regional climate projections (UKCP18 Strand 3 - RCP8.5) to predict future spatio-temporal temperature patterns.The results indicate that for the Scottish dataset (i) in addition to air temperature, the dominant environmental controls on stream temperature are unique combinations for each catchment; (ii) a general increase of up to 0.06 °C/year in historic stream temperature over all catchments resulted mainly from increases in spring and summer stream temperatures; (iii) future spatial patterns in stream temperature are more homogenous and differ therefore from the past where temperatures in N Scotland were relatively lower; (iv) future changes of up to +4.0 °C in annual stream temperature are strongest in those catchments which show lower stream temperature in the past (NW and W Scotland). These results are important in the context of water quality and stream temperature management. The methodology can be applied to smaller scale sites or to other national/global datasets enabling the analysis of historic trends and future changes at a high temporal resolution.

Last glacial maximum cooling of 9 °C in continental Europe from a 40 kyr-long noble gas paleothermometry record

The Last Glacial Maximum (LGM; ∼26–18 kyr ago) is a time interval of great climatic interest characterized by substantial global cooling driven by radiative forcings and feedbacks associated with orbital changes, lower atmospheric CO2, and large ice sheets. However, reliable proxies of continental paleotemperatures are scarce and often qualitative, which has limited our understanding of the spatial structure of past climate changes. Here, we present a quantitative noble gas temperature (NGT) record of the last ∼40 kyr from the Albian aquifer in Eastern Paris Basin (France, ∼48°N). Our NGT data indicate that the mean annual surface temperature was ∼5 °C during the Marine Isotope Stage 3 (MIS3; ∼40–30 kyr ago), before cooling to ∼2 °C during the LGM, and warming to ∼11 °C in the Holocene, which closely matches modern ground surface temperatures in Eastern France. Combined with water stable isotope analyses, NGT data indicate δD/NGT and δ18O/NGT transfer functions of +1.6 ± 0.4‰/°C and +0.18 ± 0.04‰/°C, respectively. Our noble-gas derived LGM cooling of ∼9 °C (relative to the Holocene) is consistent with previous studies of noble gas paleothermometry in European groundwaters but larger than the low-to-mid latitude estimate of 5.8 ± 0.6 °C derived from a compilation of noble gas records, which supports the notion that continental LGM cooling was more extreme at higher latitudes. While an LGM cooling of ∼9 °C in Eastern France appears compatible with recent data assimilation studies, this value is greater than most estimates from current-generation climate model simulations of the LGM. Comparing our estimate for the temperature in Eastern France during MIS3 (6.4 ± 0.5 °C) with GCM outputs presents a promising avenue to further evaluate climate model simulations and constrain European climate evolution over the last glacial cycle.

In the Pacific Northwest, 2021 Was the Hottest Year in a Millennium

A 1,000-year temperature record shows unprecedented warming in the Pacific Northwest, and new modeling predicts the likelihood of future heat waves in the decades to come.

Rapid grain boundary diffusion in foraminifera tests biases paleotemperature records

The oxygen isotopic compositions of fossil foraminifera tests constitute a continuous proxy record of deep-ocean and sea-surface temperatures spanning the last 120 million years. Here, by incubating foraminifera tests in 18O-enriched artificial seawater analogues, we demonstrate that the oxygen isotopic composition of optically translucent, i.e., glassy, fossil foraminifera calcite tests can be measurably altered at low temperatures through rapid oxygen grain-boundary diffusion without any visible ultrastructural changes. Oxygen grain boundary diffusion occurs sufficiently fast in foraminifera tests that, under normal upper oceanic sediment conditions, their grain boundaries will be in oxygen isotopic equilibrium with the surrounding pore fluids on a time scale of <100 years, resulting in a notable but correctable bias of the paleotemperature record. When applied to paleotemperatures from 38,400 foraminifera tests used in paleoclimate reconstructions, grain boundary diffusion can be shown to bias prior paleotemperature estimates by as much as +0.86 to −0.46 °C. The process is general and grain boundary diffusion corrections can be applied to other polycrystalline biocarbonates composed of small nanocrystallites (<100 nm), such as those produced by corals, brachiopods, belemnites, and molluscs, the fossils of which are all highly susceptible to the effects of grain boundary diffusion.

The most at-risk regions in the world for high-impact heatwaves

Heatwaves are becoming more frequent under climate change and can lead to thousands of excess deaths. Adaptation to extreme weather events often occurs in response to an event, with communities learning fast following unexpectedly impactful events. Using extreme value statistics, here we show where regional temperature records are statistically likely to be exceeded, and therefore communities might be more at-risk. In 31% of regions examined, the observed daily maximum temperature record is exceptional. Climate models suggest that similar behaviour can occur in any region. In some regions, such as Afghanistan and parts of Central America, this is a particular problem - not only have they the potential for far more extreme heatwaves than experienced, but their population is growing and increasingly exposed because of limited healthcare and energy resources. We urge policy makers in vulnerable regions to consider if heat action plans are sufficient for what might come.

A novel Bayesian multilevel regression approach to the reconstruction of an eastern Mediterranean temperature record for the last 10,000 years

Climate reconstructions derived from proxy records for individual sites often fail to incorporate existing regional information, which may help to determine uncertainties and express variability within specific reconstructions or commonalities between datasets. Such an understanding is crucial when examining past human-environment interactions. Taking the eastern Mediterranean as our case study, we present here a new air temperature model for the last 10,000 years that utilises data from 33 previously published proxy-based independent reconstructions, using a novel fully Bayesian approach that applies multilevel regression models of individual temperature datasets grouped together into 300 year long consecutive sub-intervals. A Bayesian multilevel approach allows the model to share information between each regression model from the individual datasets and the 300 year grouped regression models. The results demonstrate commonalities between individual datasets derived from different sources, and embed the uncertainties within the model. Our results establish that the eastern Mediterranean region was consistently warmer than the 20th century, except for two short intervals at the end of the Early Holocene (between 8400 and 8250 yrs BP) and the start of the mid-Holocene (between 7800 and 7650 yrs BP). We also identify changes within temperature associated with both the 8.2 ka and 4.2 ka BP events, however our findings identify regional warming in the eastern Mediterranean, rather than cooling often associated with the 4.2 ka BP event. Our results are comparable with previous large scale hemispheric reconstructions, demonstrating that our model is a robust candidate for temperature reconstructions within a confined region, which can range from mesoscale up to macroscale.

Relationship between Holocene lake water temperature and glacier meltwater on the northwestern Tibetan Plateau

Understanding the relationship between glacier meltwater and lake water temperature changes on the Tibetan Plateau (TP) is important for water resources management, environmental protection, and geological risk analysis. Summer temperature is one of the main factors affecting glacier evolution, yet we lack summer temperature records for high-altitude regions where glaciers are widely developed. In this study, we established a calibration between TEX86, which measures the abundance of aquatic isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) of Group I.1a aquatic Thaumarchaeota, and mean annual lake water temperature (MLWT) for 17 surface sediment samples from 10 lakes on the TP (MLWT = 4.1× TEX86 + 2.6 °C, R2 = 0.77). Given that lakes freeze during the cold season, MLWT is a record of the warm season temperature, especially the summer-biased temperature. We applied this calibration to reconstruct a Holocene MLWT record for Bangdag Co, a high-altitude hydrologically-closed lake (4909 m a.s.l.) on the northwestern TP, where glaciers are widely developed. The record shows low temperatures during the early Holocene, high temperatures during the middle Holocene, and a cooling trend during the middle to late Holocene. The MLWT is negatively correlated with the glacial meltwater volume during the early Holocene, indicating a large glacial meltwater influx to the lake at this time which resulted in low lake-water temperatures. Following the decrease in summer insolation during the middle to late Holocene, summer temperatures decreased and thus glacier meltwater production decreased. The MLWT shows a significant positive correlation with the glacial meltwater volume during the middle to late Holocene, which indicates that glacier meltwater barely influenced the MLWT, which was mainly controlled by the summer air temperature. Overall, when the air temperature decreased, the MLWT was low and the glacier meltwater production decreased. Our MLWT record for Bangdag Co agrees well with the cooling trend evident in summer temperature records and model simulations for the TP during the middle to late Holocene, which can be primarily attributed to decreasing summer insolation.