Holocene global mean surface temperature, a multi-method reconstruction approach

Millennial changes and cooling trends in land surface warm-season temperatures during the Holocene

The scarcity of proxies and calibration models for quantitatively reconstructing millennial timescale seasonal temperature tremendously constraints our understanding of the Holocene thermal variation and its driven mechanisms. Here, we established two global warm-season temperature models by applying deep learning neural network analysis to the branched tetraether membrane lipids originating from surface soil and lacustrine sediment bacteria. We utilized these optimal models in global well-dated lacustrine, peatland, and loess profiles covering the Holocene. All reconstructions of warm-season temperatures, consistent with climate model simulations, indicate cooling trends since the early Holocene, primarily induced by decreased solar radiation in the Northern Hemisphere due to the precession peak at the early. We further demonstrated that the membrane lipids can effectively enhance the future millennial seasonal temperature research, including winter temperatures, without being restricted by geographical location and sedimentary carrier.

Reconciling East Asia's mid-Holocene temperature discrepancy through vegetation-climate feedback

The term "Holocene temperature conundrum" refers to the inconsistencies between proxy-based reconstructions and transient model simulations, and it challenges our understanding of global temperature evolution during the Holocene. Climate reconstructions indicate a cooling trend following the Holocene Thermal Maximum, while model simulations indicate a consistent warming trend due to ice-sheet retreat and rising greenhouse gas concentrations. Various factors, such as seasonal biases and overlooked feedback processes, have been proposed as potential causes for this discrepancy. In this study, we examined the impact of vegetation-climate feedback on the temperature anomaly patterns in East Asia during the mid-Holocene (∼6 ka). By utilizing the fully coupled Earth system model EC-Earth and performing simulations with and without coupled dynamic vegetation, our objective was to isolate the influence of vegetation changes on regional temperature patterns. Our findings reveal that vegetation-climate feedback contributed to warming across most of East Asia, resulting in spatially diverse temperature changes during the mid-Holocene and significantly improved model-data agreement. These results highlight the crucial role of vegetation-climate feedback in addressing the Holocene temperature conundrum and emphasize its importance for simulating accurate climate scenarios.

Recent tropical Andean glacier retreat is unprecedented in the Holocene

Tropical glaciers have retreated over recent decades, but whether the magnitude of this retreat exceeds the bounds of Holocene fluctuations is unclear. We measured cosmogenic beryllium-10 and carbon-14 concentrations in recently exposed bedrock at the margin of four glaciers spanning the tropical Andes to reconstruct their past extents relative to today. Nuclide concentrations are near zero in almost all samples, suggesting that these locations were never exposed during the Holocene. Our data imply that many glaciers in the tropics are probably now smaller than they have been in at least 11,700 years, making the tropics the first large region where this milestone has been documented.

Carbon and oxygen isotopes in mummified wood reveal warmer and wetter winters in the Siberian Arctic 3000 years ago

Paleoclimate reconstructions from the Holocene are important for defining baseline conditions in order to interpret and contextualize the effects of modern climate change. Such records are particularly lacking for Siberia, a region that represents ~ 50% of the Arctic. In addition, the majority of proxy-based paleoclimate reconstructions for the Holocene represent mean annual conditions, and few quantify winter temperature, which is particularly important for predicting the effects of global warming in Arctic environments. Here we provide the first quantitative proxy reconstruction of precipitation and temperature for both summer and winter for 3000 years ago via novel high-resolution intra-annual carbon and oxygen isotope measurements across annual growth rings of fossil wood mummified within the permafrost of far northeastern Siberia. We found that the site experienced greater precipitation year-round (~ 10% increase in summer and ~ 30% increase in winter), cooler summer temperatures, and warmer winter temperatures, compared with today. Our findings indicate that warmer winter temperatures (+ 3.0 °C above early twentieth century values) in the Arctic 3000 years ago drove higher mean annual temperature by up to 1 °C, despite the existence of cooler summers, a similar phenomenon to what is observed within today's Arctic environments, and past intervals of extreme global warmth.

Spatial patterns of Holocene temperature changes over mid-latitude Eurasia

The Holocene temperature conundrum, the discrepancy between proxy-based Holocene global cooling and simulated global annual warming trends, remains controversial. Meanwhile, reconstructions and simulations show inconsistent spatial patterns of terrestrial temperature changes. Here we report Holocene alkenone records to address spatial patterns over mid-latitude Eurasia. In contrast with long-term cooling trends in warm season temperatures in northeastern China, records from southwestern Siberia are characterized by colder conditions before ~6,000 years ago, thus long-term warming trends. Together with existing records from surrounding regions, we infer that colder airmass might have prevailed in the interior of mid-latitude Eurasian continent during the early to mid-Holocene, perhaps associated with atmospheric response to remnant ice sheets. Our results challenge the proposed seasonality bias in proxies and modeled spatial patterns in study region, highlighting that spatial patterns of Holocene temperature changes should be re-considered in record integrations and model simulations, with important implications for terrestrial hydroclimate changes.

Response of Arctic benthic foraminiferal traits to past environmental changes

The Arctic is subjected to all-encompassing disruptions in marine ecosystems caused by anthropogenic warming. To provide reliable estimates of how future changes will affect the ecosystems, knowledge of Arctic marine ecosystem responses to past environmental variability beyond the instrumental era is essential. Here, we present a novel approach on how to evaluate the state of benthic marine biotic conditions during the deglacial and Holocene period on the Northeast Greenland shelf. Benthic foraminiferal species were assigned traits (e.g., oxygen tolerance, food preferences) aiming to identify past faunal changes as a response to external forcing mechanisms. This approach was applied on sediment cores from offshore Northeast Greenland. We performed numerical rate-of-change detection to determine significant changes in the benthic foraminiferal traits. That way, the significant abrupt trait changes can be assessed across sites, providing a better understanding of the impact of climate drivers on the traits. Our results demonstrate that during the last ~ 14,000 years, bottom water oxygen is the main factor affecting the variability in the benthic foraminiferal faunas in this area. Our results show that significant changes in the traits correspond to drastic climate perturbations. Specifically, the deglacial-Holocene transition and mid-Holocene warm period exhibited significant change, with several trait turnovers.

Earth beyond six of nine planetary boundaries

This planetary boundaries framework update finds that six of the nine boundaries are transgressed, suggesting that Earth is now well outside of the safe operating space for humanity. Ocean acidification is close to being breached, while aerosol loading regionally exceeds the boundary. Stratospheric ozone levels have slightly recovered. The transgression level has increased for all boundaries earlier identified as overstepped. As primary production drives Earth system biosphere functions, human appropriation of net primary production is proposed as a control variable for functional biosphere integrity. This boundary is also transgressed. Earth system modeling of different levels of the transgression of the climate and land system change boundaries illustrates that these anthropogenic impacts on Earth system must be considered in a systemic context.

Past climate-driven range shifts structuring intraspecific biodiversity levels of the giant kelp (Macrocystis pyrifera) at global scales

The paradigm of past climate-driven range shifts structuring the distribution of marine intraspecific biodiversity lacks replication in biological models exposed to comparable limiting conditions in independent regions. This may lead to confounding effects unlinked to climate drivers. We aim to fill in this gap by asking whether the global distribution of intraspecific biodiversity of giant kelp (Macrocystis pyrifera) is explained by past climate changes occurring across the two hemispheres. We compared the species' population genetic diversity and structure inferred with microsatellite markers, with range shifts and long-term refugial regions predicted with species distribution modelling (SDM) from the last glacial maximum (LGM) to the present. The broad antitropical distribution of Macrocystis pyrifera is composed by six significantly differentiated genetic groups, for which current genetic diversity levels match the expectations of past climate changes. Range shifts from the LGM to the present structured low latitude refugial regions where genetic relics with higher and unique diversity were found (particularly in the Channel Islands of California and in Peru), while post-glacial expansions following ~ 40% range contraction explained extensive regions with homogenous reduced diversity. The estimated effect of past climate-driven range shifts was comparable between hemispheres, largely demonstrating that the distribution of intraspecific marine biodiversity can be structured by comparable evolutionary forces across the global ocean. Additionally, the differentiation and endemicity of regional genetic groups, confers high conservation value to these localized intraspecific biodiversity hotspots of giant kelp forests.

Safe and just Earth system boundaries

The stability and resilience of the Earth system and human well-being are inseparably linked^1-3, yet their interdependencies are generally under-recognized; consequently, they are often treated independently^4,5. Here, we use modelling and literature assessment to quantify safe and just Earth system boundaries (ESBs) for climate, the biosphere, water and nutrient cycles, and aerosols at global and subglobal scales. We propose ESBs for maintaining the resilience and stability of the Earth system (safe ESBs) and minimizing exposure to significant harm to humans from Earth system change (a necessary but not sufficient condition for justice)⁴. The stricter of the safe or just boundaries sets the integrated safe and just ESB. Our findings show that justice considerations constrain the integrated ESBs more than safety considerations for climate and atmospheric aerosol loading. Seven of eight globally quantified safe and just ESBs and at least two regional safe and just ESBs in over half of global land area are already exceeded. We propose that our assessment provides a quantitative foundation for safeguarding the global commons for all people now and into the future.

Shed female caribou antlers extend records of calving activity on the Arctic National Wildlife Refuge by millennia

Caribou (Rangifer tarandus) have among the longest annual migrations of any terrestrial mammal as they move from winter ranges to spring calving grounds. Biomonitoring records indicate broad consistencies in calving geography across the last several decades, but how long have herds used particular calving grounds? Furthermore, how representative are modern patterns of calving geography to periods that pre-date recent climatic perturbations and increased anthropogenic stresses? While modern ecological datasets are not long enough to address these questions, bones from past generations of caribou lying on the tundra provide unique opportunities to study historical calving geography. This is possible because female caribou shed their antlers within days of giving birth, releasing a skeletal indicator of calving. Today, the Coastal Plain of the Arctic National Wildlife Refuge (Alaska) is a key calving ground for the Porcupine Caribou Herd (PCH). To test the duration across which caribou have used this area as a calving ground, we radiocarbon dated three highly weathered female antlers collected from tundra surfaces on the Coastal Plain. Calibrated radiocarbon dates indicate that these antlers were shed between ~1,600 and more than 3,000 calendar years ago. The antiquity of these shed antlers provides the first physical evidence of calving activity on the PCH calving grounds from previous millennia, substantiating the long ecological legacy of the Coastal Plain as a caribou calving ground. Comparisons to published lake core records also reveal that dates of two of the antlers correspond to periods with average summer temperatures that were warmer than has been typical during the last several decades of biomonitoring. This finding expands the range of climatic settings in which caribou are known to use the current PCH calving grounds and suggests that the Coastal Plain of the Arctic Refuge may remain an important caribou calving ground during at least portions of predicted future warming. Discarded skeletal materials provide opportunities to assess the historical states of living populations, including aspects of reproductive biology and migration. Particularly in high-latitude settings, these insights can extend across millennia and offer rare glimpses into the past that can inform current and future management policies.

Revisiting the Holocene global temperature conundrum

Recent global temperature reconstructions for the current interglacial period (the Holocene, beginning 11,700 years ago) have generated contrasting trends. This Review examines evidence from indicators and drivers of global change, as inferred from proxy records and simulated by climate models, to evaluate whether anthropogenic global warming was preceded by a long-term warming trend or by global cooling. Multimillennial-scale cooling before industrialization requires extra climate forcing and major climate feedbacks that are not well represented in most climate models at present. Conversely, global warming before industrialization challenges proxy-based reconstructions of past climate. The resolution of this conundrum has implications for contextualizing post-industrial warming and for understanding climate sensitivity to several forcings and their attendant feedbacks, including greenhouse gases. From a large variety of available evidence, we find support for a relatively mild millennial-scale global thermal maximum during the mid-Holocene, but more research is needed to firmly resolve the conundrum and to advance our understanding of slow-moving climate variability.

Seasonal temperatures in West Antarctica during the Holocene

The recovery of long-term climate proxy records with seasonal resolution is rare because of natural smoothing processes, discontinuities and limitations in measurement resolution. Yet insolation forcing, a primary driver of multimillennial-scale climate change, acts through seasonal variations with direct impacts on seasonal climate¹. Whether the sensitivity of seasonal climate to insolation matches theoretical predictions has not been assessed over long timescales. Here, we analyse a continuous record of water-isotope ratios from the West Antarctic Ice Sheet Divide ice core to reveal summer and winter temperature changes through the last 11,000 years. Summer temperatures in West Antarctica increased through the early-to-mid-Holocene, reached a peak 4,100 years ago and then decreased to the present. Climate model simulations show that these variations primarily reflect changes in maximum summer insolation, confirming the general connection between seasonal insolation and warming and demonstrating the importance of insolation intensity rather than seasonally integrated insolation or season duration^2,3. Winter temperatures varied less overall, consistent with predictions from insolation forcing, but also fluctuated in the early Holocene, probably owing to changes in meridional heat transport. The magnitudes of summer and winter temperature changes constrain the lowering of the West Antarctic Ice Sheet surface since the early Holocene to less than 162 m and probably less than 58 m, consistent with geological constraints elsewhere in West Antarctica^4-7.

Agriculture and food security under a changing climate: An underestimated challenge

Pathways to eradicate global hunger while bending the curve of biodiversity loss unanimously suggest changing to less energy-rich diets, closing yield gaps through agroecological principles, adopting modern breeding technologies to foster stress resilience and yields, as well as minimizing harvest losses and food waste. Against the background of a brief history of global agriculture, we review the available evidence on how the global food system might look given a global temperature increase by 3°. We show that a moderate gain in the area suitable for agriculture is confronted with substantial yield losses through strains on crop physiology, multitrophic interactions, and more frequent extreme events. Self-amplifying feedback are unresolved and might lead to further losses. In light of these uncertainties, we see that complexity is underestimated and more systemic research is needed. Efficiency gains in agriculture, albeit indispensable, will not be enough to achieve food security under severe climate change.

Emerging biological archives can reveal ecological and climatic change in Antarctica

Anthropogenic climate change is causing observable changes in Antarctica and the Southern Ocean including increased air and ocean temperatures, glacial melt leading to sea-level rise and a reduction in salinity, and changes to freshwater water availability on land. These changes impact local Antarctic ecosystems and the Earth's climate system. The Antarctic has experienced significant past environmental change, including cycles of glaciation over the Quaternary Period (the past ~2.6 million years). Understanding Antarctica's paleoecosystems, and the corresponding paleoenvironments and climates that have shaped them, provides insight into present day ecosystem change, and importantly, helps constrain model projections of future change. Biological archives such as extant moss beds and peat profiles, biological proxies in lake and marine sediments, vertebrate animal colonies, and extant terrestrial and benthic marine invertebrates, complement other Antarctic paleoclimate archives by recording the nature and rate of past ecological change, the paleoenvironmental drivers of that change, and constrain current ecosystem and climate models. These archives provide invaluable information about terrestrial ice-free areas, a key location for Antarctic biodiversity, and the continental margin which is important for understanding ice sheet dynamics. Recent significant advances in analytical techniques (e.g., genomics, biogeochemical analyses) have led to new applications and greater power in elucidating the environmental records contained within biological archives. Paleoecological and paleoclimate discoveries derived from biological archives, and integration with existing data from other paleoclimate data sources, will significantly expand our understanding of past, present, and future ecological change, alongside climate change, in a unique, globally significant region.

Complex spatio-temporal structure of the Holocene Thermal Maximum

Inconsistencies between Holocene climate reconstructions and numerical model simulations question the robustness of climate models and proxy temperature records. Climate reconstructions suggest an early-middle Holocene Thermal Maximum (HTM) followed by gradual cooling, whereas climate models indicate continuous warming. This discrepancy either implies seasonal biases in proxy-based climate reconstructions, or that the climate model sensitivity to forcings and feedbacks needs to be reevaluated. Here, we analyze a global database of Holocene paleotemperature records to investigate the spatiotemporal structure of the HTM. Continental proxy records at mid and high latitudes of the Northern Hemisphere portray a “classic” HTM (8–4 ka). In contrast, marine proxy records from the same latitudes reveal an earlier HTM (11–7ka), while a clear temperature anomaly is missing in the tropics. The results indicate a heterogeneous response to climate forcing and highlight the lack of globally synchronous HTM.

Analysis of the largest available database of Holocene temperature time series covering past 12,000 years reveals complex spatio-temporal trends and challenges the paradigm of a globally synchronous Holocene Thermal Maximum.

Estimation of the Mixed Layer Depth in the Indian Ocean from Surface Parameters: A Clustering-Neural Network Method

The effective estimation of mixed-layer depth (MLD) plays a significant role in the study of ocean dynamics and global climate change. However, the methods of estimating MLD still have limitations due to the sparse resolution of the observed data. In this study, a hybrid estimation method that combines the K-means clustering algorithm and an artificial neural network (ANN) model was developed using sea-surface parameter data in the Indian Ocean as a case study. The oceanic datasets from January 2012 to December 2019 were obtained via satellite observations, Argo in situ data, and reanalysis data. These datasets were unified to the same spatial and temporal resolution (1° × 1°, monthly). Based on the processed datasets, the K-means classifier was applied to divide the Indian Ocean into four regions with different characteristics. For ANN training and testing in each region, the gridded data of 84 months were used for training, and 12-month data were used for testing. The ANN results show that the optimized NN architecture comprises five input variables, one output variable, and four hidden layers, each of which has 40 neurons. Compared with the multiple linear regression model (MLR) with a root-mean-square error (RMSE) of 5.2248 m and the HYbrid-Coordinate Ocean Model (HYCOM) with an RMSE of 4.8422 m, the RMSE of the model proposed in this study was reduced by 27% and 22%, respectively. Three typical regions with high variability in their MLDs were selected to further evaluate the performance of the ANN model. Our results showed that the model could reveal the seasonal variation trend in each of the selected regions, but the estimation accuracy showed room for improvement. Furthermore, a correlation analysis between the MLD and input variables showed that the surface temperature and salinity were the main influencing factors of the model. The results of this study suggest that the pre-clustering ANN method proposed could be used to estimate and analyze the MLD in the Indian Ocean. Moreover, this method can be further expanded to estimate other internal parameters for typical ocean regions and to provide effective technical support for ocean researchers when studying the variability of these parameters.

Pollen-based reconstruction reveals the impact of the onset of agriculture on plant functional trait composition

The onset of agriculture improved the capacity of ecosystems to produce food, but inadvertently altered other vital ecosystem functions. Plant traits play a central role in determining ecosystem properties, therefore we investigated how the onset of agriculture in Europe changed plant trait composition using 78 pollen records. Using a novel Bayesian approach for reconstructing plant trait composition from pollen records, we provide a robust method that can account for trait variability within pollen types. We estimate an overall four-fold decrease in plant size through agriculture and associated decreases in leaf and seed size. We show an increase in niche space towards the resource-acquisitive end of the leaf economic spectrum. Decreases in leaf phosphorus might have been caused by nutrient depletion through grazing and burning. Our results show that agriculture, from its start, has likely been gradually impacting biogeochemical cycles through altered vegetation composition.

Elevational Gradients Impose Dispersal Limitation on Streptomyces

Dispersal governs microbial biogeography, but the rates and mechanisms of dispersal remain poorly characterized for most microbial taxa. Dispersal limitation is driven by limits on dissemination and establishment, respectively. Elevation gradients create striking patterns of biogeography because they produce steep environmental gradients at small spatial scales, and these gradients offer a powerful tool to examine mechanisms of dispersal limitation. We focus on Streptomyces, a bacterial genus common to soil, by using a taxon-specific phylogenetic marker, the RNA polymerase-encoding rpoB gene. By targeting Streptomyces, we assess dispersal limitation at finer phylogenetic resolution than is possible using whole community analyses. We characterized Streptomyces diversity at local spatial scales (100 to 3,000 m) in two temperate forest sites located in the Adirondacks region of New York State: Woods Lake (<100 m elevation change), and Whiteface Mountain (>1,000 m elevation change). Beta diversity varied considerably at both locations, indicative of dispersal limitation acting at local spatial scales, but beta diversity was significantly higher at Whiteface Mountain. Beta diversity varied across elevation at Whiteface Mountain, being lowest at the mountain’s base. We show that Streptomyces taxa exhibit elevational preferences, and these preferences are phylogenetically conserved. These results indicate that habitat preferences influence Streptomyces biogeography and suggest that barriers to establishment structure Streptomyces communities at higher elevations. These data illustrate that Streptomyces biogeography is governed by dispersal limitation resulting from a complex mixture of stochastic and deterministic processes.

Emergence patterns of locally novel plant communities driven by past climate change and modern anthropogenic impacts

Anthropogenic disturbance and climate change can result in dramatic increases in the emergence of new, ecologically novel, communities of organisms. We used a standardised framework to detect local novel communities in 2135 pollen time series over the last 25,000 years. Eight thousand years of post‐glacial warming coincided with a threefold increase in local novel community emergence relative to glacial estimates. Novel communities emerged predominantly at high latitudes and were linked to global and local temperature change across multi‐millennial time intervals. In contrast, emergence of locally novel communities in the last 200 years, although already on par with glacial retreat estimates, occurred at midlatitudes and near high human population densities. Anthropogenic warming does not appear to be strongly associated with modern local novel communities, but may drive widespread emergence in the future, with legacy effects for millennia after warming abates.

How to Account for Changes in Carbon Storage from Coal Mining and Reclamation in Eastern China? Taking Yanzhou Coalfield as an Example to Simulate and Estimate

Carbon sequestration in terrestrial ecosystems plays an essential role in coping with global climate change and achieving regional carbon neutrality. In mining areas with high groundwater levels in eastern China, underground coal mining has caused severe damage to surface ecology. It is of practical significance to evaluate and predict the positive and negative effects of coal mining and land reclamation on carbon pools. This study set up three scenarios for the development of the Yanzhou coalfield (YZC) in 2030, including: (1) no mining activities (NMA); (2) no reclamation after mining (NRM); (3) mining and reclamation (MR). The probability integral model (PIM) was used to predict the subsidence caused by mining in YZC in 2030, and land use and land cover (LULC) of 2010 and 2020 were interpreted by remote sensing images. Based on the classification of land damage, the LULC of different scenarios in the future was simulated by integrating various social and natural factors. Under different scenarios, the InVEST model evaluated carbon storage and its temporal and spatial distribution characteristics. The results indicated that: (1) By 2030, YZC would have 4341.13 ha of land disturbed by coal mining activities. (2) Carbon storage in the NRM scenario would be 37,647.11 Mg lower than that in the NMA scenario, while carbon storage in the MR scenario would be 18,151.03 Mg higher than that in the NRM scenario. Significantly, the Nantun mine would reduce carbon sequestration loss by 72.29% due to reclamation measures. (3) Carbon storage has a significant positive spatial correlation, and coal mining would lead to the fragmentation of the carbon sink. The method of accounting for and predicting carbon storage proposed in this study can provide data support for mining and reclamation planning of coal mine enterprises and carbon-neutral planning of government departments.

Northern Hemisphere vegetation change drives a Holocene thermal maximum

The Holocene thermal maximum, a period of global warmth evident in early to mid-Holocene proxy reconstructions, is controversial. Most model simulations of the Holocene have not reproduced this warming, leading to a disagreement known as the Holocene Temperature Conundrum. Pollen records document the expansion of vegetation in the early and mid-Holocene African Sahara and Northern Hemisphere mid- and high latitudes, which has been overlooked in previous modeling studies. Here, we use time slice simulations of the Community Earth System Model to assess the impact of Northern Hemisphere vegetation change on Holocene annual mean temperatures. Our simulations indicate that expansion of Northern Hemisphere vegetation 9000 and 6000 years ago warms Earth's surface by ~0.8° and 0.7°C, respectively, producing a better match with proxy-based reconstructions. Our results suggest that vegetation change is critical for modeling Holocene temperature evolution and highlight its role in driving a mid-Holocene temperature maximum.

Contemporary mass balance on a cold Eastern Alpine ice cap as a potential link to the Holocene climate

Alpine cold ice caps are sensitive indicators of local climate. The adequate interpretation of this information in an ice core requires detailed in situ glaciological and meteorological records, of which there are few. The Weißseespitze summit ice cap (3499 m) presents an ideal case to compare past and present climate and mass balance, with limited ice flow, but close to 6000 years locked into about 10 m of ice. First-ever meteorological observations at the ice dome have revealed that over 3 years of observation most of the accumulation took place between October and December and from April to June. In the colder winter months, between January and March, wind erosion prevents accumulation. Melt occurred between June and September, ice was only affected during short periods, mainly in August, which caused ice losses of up to 0.6 m (i.e. ~ 5% of the total ice thickness). Historical data points at a loss of of 34.9 ± 10.0 m between 1893 and 2018 and almost balanced conditions between 1893 and 1914. The local evidence of ice loss lays the basis for the interpretation of past gaps in the ice core records as past warm/melt events.

Impact of paleoclimate on present and future evolution of the Greenland Ice Sheet

Using transient climate forcing based on simulations from the Alfred Wegener Institute Earth System Model (AWI-ESM), we simulate the evolution of the Greenland Ice Sheet (GrIS) from the last interglacial (125 ka, kiloyear before present) to 2100 AD with the Parallel Ice Sheet Model (PISM). The impact of paleoclimate, especially Holocene climate, on the present and future evolution of the GrIS is explored. Our simulations of the past show close agreement with reconstructions with respect to the recent timing of the peaks in ice volume and the climate of Greenland. The maximum and minimum ice volume at around 18-17 ka and 6-5 ka lag the respective extremes in climate by several thousand years, implying that the ice volume response of the GrIS strongly lags climatic changes. Given that Greenland's climate was getting colder from the Holocene Thermal Maximum (i.e., 8 ka) to the Pre-Industrial era, our simulation implies that the GrIS experienced growth from the mid-Holocene to the industrial era. Due to this background trend, the GrIS still gains mass until the second half of the 20th century, even though anthropogenic warming begins around 1850 AD. This is also in agreement with observational evidence showing mass loss of the GrIS does not begin earlier than the late 20th century. Our results highlight that the present evolution of the GrIS is not only controlled by the recent climate changes, but is also affected by paleoclimate, especially the relatively warm Holocene climate. We propose that the GrIS was not in equilibrium throughout the entire Holocene and that the slow response to Holocene climate needs to be represented in ice sheet simulations in order to predict ice mass loss, and therefore sea level rise, accurately.

Glacial Ice Age Shapes Microbiome Composition in a Receding Southern European Glacier

Glaciers and their microbiomes are exceptional witnesses of the environmental conditions from remote times. Climate change is threatening mountain glaciers, and especially those found in southern Europe, such as the Monte Perdido Glacier (northern Spain, Central Pyrenees). This study focuses on the reconstruction of the history of microbial communities over time. The microorganisms that inhabit the Monte Perdido Glacier were identified using high-throughput sequencing, and the microbial communities were compared along an altitudinal transect covering most of the preserved ice sequence in the glacier. The results showed that the glacial ice age gradient did shape the diversity of microbial populations, which presented large differences throughout the last 2000 years. Variations in microbial community diversity were influenced by glacial conditions over time (nutrient concentration, chemical composition, and ice age). Some groups were exclusively identified in the oldest samples as the bacterial phyla Fusobacteria and Calditrichaeota, or the eukaryotic class Rhodophyceae. Among groups only found in modern samples, the green sulfur bacteria (phylum Chlorobi) stood out, as well as the bacterial phylum Gemmatimonadetes and the eukaryotic class Tubulinea. A patent impact of human contamination was also observed on the glacier microbiome. The oldest samples, corresponding to the Roman Empire times, were influenced by the beginning of mining exploitation in the Pyrenean area, with the presence of metal-tolerant microorganisms. The most recent samples comprise 600-year-old ancient ice in which current communities are living.

Globally resolved surface temperatures since the Last Glacial Maximum

Climate changes across the past 24,000 years provide key insights into Earth system responses to external forcing. Climate model simulations^1,2 and proxy data^3-8 have independently allowed for study of this crucial interval; however, they have at times yielded disparate conclusions. Here, we leverage both types of information using paleoclimate data assimilation^9,10 to produce the first proxy-constrained, full-field reanalysis of surface temperature change spanning the Last Glacial Maximum to present at 200-year resolution. We demonstrate that temperature variability across the past 24 thousand years was linked to two primary climatic mechanisms: radiative forcing from ice sheets and greenhouse gases; and a superposition of changes in the ocean overturning circulation and seasonal insolation. In contrast with previous proxy-based reconstructions^6,7 our results show that global mean temperature has slightly but steadily warmed, by ~0.5 °C, since the early Holocene (around 9 thousand years ago). When compared with recent temperature changes¹¹, our reanalysis indicates that both the rate and magnitude of modern warming are unusual relative to the changes of the past 24 thousand years.

Enhanced North Pacific subtropical gyre circulation during the late Holocene

The North Pacific Subtropical Gyre circulation redistributes heat from the Western Pacific Warm Pool towards the mid- to high-latitude North Pacific. However, the driving mechanisms of this circulation and how it changed over the Holocene remain poorly understood. Here, we present alkenone-based sea surface temperature reconstructions along the Kuroshio, California and Alaska currents that cover the past ~7,000 years. These and other paleorecords collectively demonstrate a coherent intensification of the boundary currents, and thereby the basin-scale subtropical gyre circulation, since ~3,000-4,000 years ago. Such enhanced circulation during the late Holocene appears to have resulted from a long-term southward migration of the Intertropical Convergence Zone, associated with Holocene ocean cooling. Our results imply that the North Pacific Subtropical Gyre circulation could be weakened under future global warming.

Reconstructing Ocean Heat Content for Revisiting Global Ocean Warming from Remote Sensing Perspectives

Global ocean heat content (OHC) is generally estimated using gridded, model and reanalysis data; its change is crucial to understanding climate anomalies and ocean warming phenomena. However, Argo gridded data have short temporal coverage (from 2005 to the present), inhibiting understanding of long-term OHC variabilities at decadal to multidecadal scales. In this study, we utilized multisource remote sensing and Argo gridded data based on the long short-term memory (LSTM) neural network method, which considers long temporal dependence to reconstruct a new long time-series OHC dataset (1993–2020) and fill the pre-Argo data gaps. Moreover, we adopted a new machine learning method, i.e., the Light Gradient Boosting Machine (LightGBM), and applied the well-known Random Forests (RFs) method for comparison. The model performance was measured using determination coefficients (R2) and root-mean-square error (RMSE). The results showed that LSTM can effectively improve the OHC prediction accuracy compared with the LightGBM and RFs methods, especially in long-term and deep-sea predictions. The LSTM-estimated result also outperformed the Ocean Projection and Extension neural Network (OPEN) dataset, with an R2 of 0.9590 and an RMSE of 4.45 × 1019 in general in the upper 2000 m for 28 years (1993–2020). The new reconstructed dataset (named OPEN-LSTM) correlated reasonably well with other validated products, showing consistency with similar time-series trends and spatial patterns. The spatiotemporal error distribution between the OPEN-LSTM and IAP datasets was smaller on the global scale, especially in the Atlantic, Southern and Pacific Oceans. The relative error for OPEN-LSTM was the smallest for all ocean basins compared with Argo gridded data. The average global warming trends are 3.26 × 108 J/m2/decade for the pre-Argo (1993–2004) period and 2.67 × 108 J/m2/decade for the time-series (1993–2020) period. This study demonstrates the advantages of LSTM in the time-series reconstruction of OHC, and provides a new dataset for a deeper understanding of ocean and climate events.

Archaeogenetics and Landscape Dynamics in Sicily during the Holocene: A Review

The Mediterranean islands and their population history are of considerable importance to the interpretation of the population history of Europe as a whole. In this context, Sicily, because of its geographic position, represents a bridge between Africa, the Near East, and Europe that led to the stratification of settlements and admixture events. The genetic analysis of extant and ancient human samples has tried to reconstruct the population dynamics associated with the cultural and demographic changes that took place during the prehistory and history of Sicily. In turn, genetic, demographic and cultural changes need to be understood in the context of the environmental changes that took place over the Holocene. Based on this framework, this paper aims to discuss the cultural and demographic dimension of the island by reviewing archaeogenetic studies, and lastly, we discuss the ecological constraints related to human peopling in times of change in landscapes that occurred on the island in various periods. Finally, possible directions for future archaeogenetic studies of Sicily are discussed. Despite its long human history, Sicily is still one of the world’s biodiversity hotspots. The lessons we learn from the past use of landscape provide models for sustainable future management of the Mediterranean’s landscapes.

The archaeology of climate change: The case for cultural diversity

Anthropogenic climate change is currently driving environmental transformation on a scale and at a pace that exceeds historical records. This represents an undeniably serious challenge to existing social, political, and economic systems. Humans have successfully faced similar challenges in the past, however. The archaeological record and Earth archives offer rare opportunities to observe the complex interaction between environmental and human systems under different climate regimes and at different spatial and temporal scales. The archaeology of climate change offers opportunities to identify the factors that promoted human resilience in the past and apply the knowledge gained to the present, contributing a much-needed, long-term perspective to climate research. One of the strengths of the archaeological record is the cultural diversity it encompasses, which offers alternatives to the solutions proposed from within the Western agro-industrial complex, which might not be viable cross-culturally. While contemporary climate discourse focuses on the importance of biodiversity, we highlight the importance of cultural diversity as a source of resilience.

Middle Holocene Climate Oscillations Recorded in the Western Dvina Lakeland

Although extensive archeological research works have been conducted in the Serteya region in recent years, the Holocene climate history in the Western Dvina Lakeland in Western Russia is still poorly understood. The Neolithic human occupation of the Serteyka lake–river system responded to climate oscillations, resulting in the development of a pile-dwelling settlement between 5.9 and 4.2 ka cal BP. In this paper, we present the quantitative paleoclimatic reconstructions of the Northgrippian stage (8.2–4.2 ka cal BP) from the Great Serteya Palaeolake Basin. The reconstructions were created based on a multiproxy (Chironomidae, pollen and Cladocera) approach. The mean July air temperature remained at 17–20 °C, which is similar to the present temperature in the Smolensk Upland. The summer temperature revealed only weak oscillations during 5.9 and 4.2 ka cal BP. A more remarkable feature during those events was an increase in continentality, manifested by a lower winter temperature and lower annual precipitation. During the third, intermediate oscillation in 5.0–4.7 ka cal BP, a rise in summer temperature and stronger shifts in continental air masses were recorded. It is still unclear if the above-described climate fluctuations are linked to the North Atlantic Oscillation and can be interpreted as an indication of Bond events because only a few high-resolution paleoclimatic reconstructions from the region have been presented and these reconstructions do not demonstrate explicit oscillations in the period of 5.9 and 4.2 ka cal BP.

Economic damages from Hurricane Sandy attributable to sea level rise caused by anthropogenic climate change

In 2012, Hurricane Sandy hit the East Coast of the United States, creating widespread coastal flooding and over $60 billion in reported economic damage. The potential influence of climate change on the storm itself has been debated, but sea level rise driven by anthropogenic climate change more clearly contributed to damages. To quantify this effect, here we simulate water levels and damage both as they occurred and as they would have occurred across a range of lower sea levels corresponding to different estimates of attributable sea level rise. We find that approximately $8.1B ($4.7B-$14.0B, 5th-95th percentiles) of Sandy's damages are attributable to climate-mediated anthropogenic sea level rise, as is extension of the flood area to affect 71 (40-131) thousand additional people. The same general approach demonstrated here may be applied to impact assessments for other past and future coastal storms.

Climate impacts on organisms, ecosystems and human societies: integrating OCLTT into a wider context

Physiological studies contribute to a cause and effect understanding of ecological patterns under climate change and identify the scope and limits of adaptation. Across most habitats, this requires analyzing organism responses to warming, which can be modified by other drivers such as acidification and oxygen loss in aquatic environments or excess humidity or drought on land. Experimental findings support the hypothesis that the width and temperature range of thermal performance curves relate to biogeographical range. Current warming causes range shifts, hypothesized to include constraints in aerobic power budget which in turn are elicited by limitations in oxygen supply capacity in relation to demand. Different metabolic scopes involved may set the borders of both the fundamental niche (at standard metabolic rate) and the realized niche (at routine rate). Relative scopes for aerobic performance also set the capacity of species to interact with others at the ecosystem level. Niche limits and widths are shifting and probably interdependent across life stages, with young adults being least thermally vulnerable. The principles of thermal tolerance and performance may also apply to endotherms including humans, their habitat and human society. Overall, phylogenetically based comparisons would need to consider the life cycle of species as well as organism functional properties across climate zones and time scales. This Review concludes with a perspective on how mechanism-based understanding allows scrutinizing often simplified modeling approaches projecting future climate impacts and risks for aquatic and terrestrial ecosystems. It also emphasizes the usefulness of a consensus-building process among experimentalists for better recognition in the climate debate.

Holocene Millennial-Scale Solar Variability and the Climatic Responses on Earth

The solar impact on Earth’s climate is both a rich and open-ended topic with intense debates. In this study, we use the reconstructed data available to investigate periodicities of solar variability (i.e., variations of sunspot numbers) and temperature changes (10 sites spread all over the Earth) as well as the statistical inter-relations between them on the millennial scale during the past 8640 years (BC 6755–AD 1885) before the modern industrial era. We find that the variations of the Earth’s temperatures show evidence for the Eddy cycle component, i.e., the 1000-year cyclicity, which was discovered in variations of sunspot numbers and believed to be an intrinsic periodicity of solar variability. Further wavelet time-frequency analysis demonstrates that the co-variation between the millennium cycle components of solar variability and the temperature change held stable and statistically strong for five out of these 10 sites during our study interval. In addition, the Earth’s climatic response to solar forcing could be different region-by-region, and the temperatures in the southern hemisphere seemed to have an opposite changing trend compared to those in the northern hemisphere on this millennial scale. These findings reveal not only a pronounced but also a complex relationship between solar variability and climatic change on Earth on the millennial timescale. More data are needed to further verify these preliminary results in the future.

Seasonal origin of the thermal maxima at the Holocene and the last interglacial

Proxy reconstructions from marine sediment cores indicate peak temperatures in the first half of the last and current interglacial periods (the thermal maxima of the Holocene epoch, 10,000 to 6,000 years ago, and the last interglacial period, 128,000 to 123,000 years ago) that arguably exceed modern warmth^1-3. By contrast, climate models simulate monotonic warming throughout both periods^4-7. This substantial model-data discrepancy undermines confidence in both proxy reconstructions and climate models, and inhibits a mechanistic understanding of recent climate change. Here we show that previous global reconstructions of temperature in the Holocene^1-3 and the last interglacial period⁸ reflect the evolution of seasonal, rather than annual, temperatures and we develop a method of transforming them to mean annual temperatures. We further demonstrate that global mean annual sea surface temperatures have been steadily increasing since the start of the Holocene (about 12,000 years ago), first in response to retreating ice sheets (12 to 6.5 thousand years ago), and then as a result of rising greenhouse gas concentrations (0.25 ± 0.21 degrees Celsius over the past 6,500 years or so). However, mean annual temperatures during the last interglacial period were stable and warmer than estimates of temperatures during the Holocene, and we attribute this to the near-constant greenhouse gas levels and the reduced extent of ice sheets. We therefore argue that the climate of the Holocene differed from that of the last interglacial period in two ways: first, larger remnant glacial ice sheets acted to cool the early Holocene, and second, rising greenhouse gas levels in the late Holocene warmed the planet. Furthermore, our reconstructions demonstrate that the modern global temperature has exceeded annual levels over the past 12,000 years and probably approaches the warmth of the last interglacial period (128,000 to 115,000 years ago).

Dynamic of Tridacna spp. population variability in northern SCS over past 4500 years derived from AMS 14C dating

The marine bivalve, Tridacna spp. is an iconic invertebrate of the Indo-Pacific coral reef communities from Eocene to present. However, field observations found that the population of Tridacna spp. has declined in recent decades and some species are now endangered in the northern South China Sea (SCS) of western Pacific, which are speculated to be connected with the human overfishing and/or climate changes. Thus distinguishing the impacts of human activities and climate changes on Tridacna spp. populations is essential for understanding the dynamic of Tridacna spp. population variability. Such effort will be important in launching conservation policies and restoring population. Here, extensive sampling was applied on sub-fossil Tridacna spp. shells at the North Reef of the northern SCS, and the long-lived (with a lifespan more than 30 years) Tridacna spp. population index (LTPI) over the past 4500 years was obtained based on the AMS¹⁴C dating method. The results show that LTPI has experienced several short-term collapses (shorter than 200 years) over the past 4500 years, which may be associated with excessive cold winter temperatures. Remarkably, LTPI usually recovered rapidly after the rewarming of temperatures, indicating a robust self-recruitment mechanisms in response to natural climate changes. However, the last catastrophic collapse of LTPI that occurred at around ~1820 CE - ~1900 CE didn't rebound despite the significant rise in temperature over the recent 100 years. The decoupling between LTPI and climate changes in recent hundred years was probably induced by the increased commercial fishing in the SCS, which has overwhelmed and exacerbated the self-recruitment mechanisms between Tridacna spp. population and climate changes.

Peatland Development, Vegetation History, Climate Change and Human Activity in the Valdai Uplands (Central European Russia) during the Holocene: A Multi-Proxy Palaeoecological Study

Peatlands are remarkable for their specific biodiversity, crucial role in carbon cycling and climate change. Their deposits preserve organism remains that can be used to reconstruct long-term ecosystem and environmental changes as well as human impact in the prehistorical and historical past. This study presents a new multi-proxy reconstruction of the peatland and vegetation development investigating climate dynamics and human impact at the border between mixed and boreal forests in the Valdai Uplands (the East European Plain, Russia) during most of the Holocene. We performed plant macrofossil, pollen, testate amoeba, Cladocera, diatom, peat humification, loss on ignition, carbon and nitrogen content, δ13C and δ15N analyses supported by radiocarbon dating of the peat deposits from the Krivetskiy Mokh mire. The results of the study indicate that the wetland ecosystem underwent a classic hydroserial succession from a lake (8300 BC–900 BC) terrestrialized through a fen (900 BC–630 AD) to an ombrotrophic bog (630 AD–until present) and responded to climate changes documented over the Holocene. Each stage was associated with clear changes in local diversity of organisms responding mostly to autogenic successional changes during the lake stage and to allogenic factors at the fen-bog stage. The latter can be related to increased human impact and greater sensitivity of peatland ecosystems to external, especially climatic, drivers as compared to lakes.

Speaking to the past

“Speak to the past and it shall teach thee”. I first read those words on a dedication tablet within the John Carter Brown library at Brown University where I was a graduate student. Little did I know the phrase would accurately describe the next three and a half decades of my career. Paleoclimate data are the language we use to look into the past to understand ourselves and ultimately our future.