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Geothermal temperature‐depth measurements can be used to reconstruct past climate without the need for calibration against meteorological data. The global geothermal database is one of opportunity and is therefore subject to variations in measurement protocol, quality control and potential non‐climatic influences. As the climatic history recovery is sensitive to these factors, we developed a Bayesian hierarchical model that allows us to treat errors and uncertainty formally. This way we may better isolate the climate signal. For the Northern Hemisphere extra‐tropics our reconstruction shows a warming beginning in CE 1750 and it captures the observed two‐phase warming over the past century. We clearly identify the northern tropics as a region of greatest ground warming and hypothesize that this reflects land‐use change. For the Southern Hemisphere, the inclusion of newer data leads to a modest cooling until CE 1750. Outside the tropics, agreement with multi‐proxy reconstructions has improved relative to earlier studies. Plain Language Summary Understanding past climate is invaluable for evaluating the natural context of man‐made warming. Long term surface‐air temperature records only exist at a few locations. To reconstruct global trends further back in time proxies must then be used. Measurements from such systems are then calibrated against observed climate variations. Temperatures measured in the ground can provide more direct information on past variations because sustained trends at the surface drive thermal perturbations that penetrate into the subsurface and which can be measured today. These geothermal data have the advantage that they do not require calibration and so are independent of meteorological observations. However, recovering the climate signal is not trivial. For this reason we developed a new statistical approach to infer past temperature variations from a database of 1012 temperature‐depth profiles distributed near‐globally. The results show excellent agreement with observed temperatures and also demonstrate improved agreement with proxy‐based records. One exception is noted over equatorial regions in the northern hemisphere where a potential influence of historical land‐use may be significant. Key Points Reconstructed warming only since CE 1750 over both the northern extra‐tropics and southern hemisphere Longer‐term trends in these two regions agree well with multiproxy results Ground warming is greatest in the northern tropics where land‐use change may be important

The Sahara region has experienced periodic wet periods over the Quaternary and beyond. These North African Humid Periods (NAHPs) are astronomically paced by precession which controls the intensity of the African monsoon system. However, most climate models cannot reconcile the magnitude of these events and so the driving mechanisms remain poorly constrained. Here, we utilise a recently developed version of the HadCM3B coupled climate model that simulates 20 NAHPs over the past 800 kyr which have good agreement with NAHPs identified in proxy data. Our results show that precession determines NAHP pacing, but we identify that their amplitude is strongly linked to eccentricity via its control over ice sheet extent. During glacial periods, enhanced ice-albedo driven cooling suppresses NAHP amplitude at precession minima, when humid conditions would otherwise be expected. This highlights the importance of both precession and eccentricity, and the role of high latitude processes in determining the timing and amplitude of the NAHPs. This may have implications for the out of Africa dispersal of plants and animals throughout the Quaternary. A climate model identifies that periodic wet phases in the Sahara, termed North African Humid Periods, were driven by Earths orbital variations and were suppressed during glacial periods due to the influence of extensive ice sheets.

While paleoclimate records show that the Earth System is characterized by several different tipping points, their representation within Earth System models (ESMs) remains poorly constrained. This is because historical observations do not encompass variations large enough to provoke such regime changes, and paleoclimate conditions are rarely used to help develop and tune ESMs, which potentially ignores a rich source of information on abrupt climate change. A critical example is the early to mid-Holocene “greening” and subsequent rapid desertification of the Sahara, which most ESMs fail to reproduce, casting doubt on the representation of land–atmosphere coupling and monsoon dynamics. Here, we show that this greening and abrupt termination can be successfully simulated with one ESM after optimizing uncertain model components using both present-day observations and crucially mid-Holocene (6,000 y before present) reconstructions. The optimized model displays abrupt threshold behavior, which shows excellent agreement with long paleoclimate records that were not used in the original optimization. These results suggest that in order to realistically capture climate-system thresholds, ESMs first need to be conditioned with appropriate paleoclimate information.

Greenland ice cores provide excellent evidence of past abrupt climate changes. However, there is no universally accepted theory of how and why these Dansgaard–Oeschger (DO) events occur. Several mechanisms have been proposed to explain DO events, including sea ice, ice shelf buildup, ice sheets, atmospheric circulation, and meltwater changes. DO event temperature reconstructions depend on the stable water isotope (δ 18O) and nitrogen isotope measurements from Greenland ice cores: interpretation of these measurements holds the key to understanding the nature of DO events. Here, we demonstrate the primary importance of sea ice as a control on Greenland ice core δ 18O: 95% of the variability in δ 18O in southern Greenland is explained by DO event sea ice changes. Our suite of DO events, simulated using a general circulation model, accurately captures the amplitude of δ 18O enrichment during the abrupt DO event onsets. Simulated geographical variability is broadly consistent with available ice core evidence. We find an hitherto unknown sensitivity of the δ 18O paleothermometer to the magnitude of DO event temperature increase: the change in δ 18O per Kelvin temperature increase reduces with DO event amplitude. We show that this effect is controlled by precipitation seasonality.

Warm periods in Earth’s history offer opportunities to understand the dynamics of the Earth system under conditions that are similar to those expected in the near future. The Middle Pliocene warm period (MPWP), from 3.3 to 3.0 My B.P, is the most recent time when atmospheric CO₂ levels were as high as today. However, climate model simulations of the Pliocene underestimate high-latitude warming that has been reconstructed from fossil pollen samples and other geological archives. One possible reason for this is that enhanced non-CO₂ trace gas radiative forcing during the Pliocene, including from methane (CH₄), has not been included in modeling. We use a suite of terrestrial biogeochemistry models forced with MPWP climate model simulations from four different climate models to produce a comprehensive reconstruction of the MPWP CH₄ cycle, including uncertainty. We simulate an atmospheric CH₄ mixing ratio of 1,000 to 1,200 ppbv, which in combination with estimates of radiative forcing from N₂O and O₃, contributes a non-CO₂ radiative forcing of 0.9 W·m−2 (range 0.6 to 1.1), which is 43% (range 36 to 56%) of the CO₂ radiative forcing used in MPWP climate simulations. This additional forcing would cause a global surface temperature increase of 0.6 to 1.0 °C, with amplified changes at high latitudes, improving agreement with geological evidence of Middle Pliocene climate. We conclude that natural trace gas feedbacks are critical for interpreting climate warmth during the Pliocene and potentially many other warm phases of the Cenezoic. These results also imply that using Pliocene CO₂ and temperature reconstructions alone may lead to overestimates of the fast or Charney climate sensitivity.

The ‘Greening’ and subsequent desertification of the Sahara during the early to mid-Holocene is a dramatic example of natural climate change. We analyse a suite of simulations with a newly palaeo-conditioned configuration of the HadCM3 coupled model that is able to capture an abrupt desertification of North Africa during this time. We find that this model crosses a threshold of moisture availability for vegetation at around 6000 years before present. The resultant rapid reduction in vegetation cover acts to reduce precipitation through moisture recycling and surface albedo feedbacks. Precursor drying events which are not directly forced also indicate that the model is close to a critical moisture level. Similar precursor-like events appear in a Holocene record from the East of the continent, hinting that the natural system may resemble some of the properties of this model simulation. The overall response is not fundamentally altered by the inclusion of solar irradiance variations or volcanic eruptions. The simulated timing of the abrupt transition is mostly controlled by orbital forcing and local positive feedbacks, but it is also modulated to some extent by the state of the atmosphere and ocean. Comparisons with proxy records across North Africa show good agreement with the model simulations, although the simulations remain overly dry in the East. Overall, a threshold response may present a useful model of the real transition, but more high-resolution palaeoclimate records would help to discriminate among the predictions of climate models.

During abrupt climate changes of the last glacial period paleorecords show large amplitude changes in the dust cycle. We use Earth System model simulations to evaluate processes operating across these events. Idealized Heinrich stadial‐like simulations show a southwards migration of tropical rainfall that dries the Sahel and reduces wet deposition causing a widespread enhancement of tropical dust loading. However, several discrepancies with marine core dust deposition reconstructions are evident. Simulations with a more limited freshwater forcing (0.4 Sv instead of 1.0 Sv) and weaker cooling over the North Atlantic (less than 3°C) show a switch in sign of the stadial dust deposition anomaly in several regions, improving agreement with paleorecords. The simulated dust cycle therefore displays in places a non‐linear response to abrupt change. The global‐mean stadial dust radiative forcing in the more realistic simulations is around −0.2 to −0.6 W m−2 and so could represent an amplifying feedback during these events. Plain Language Summary Mineral dust in the atmosphere is mostly sourced from arid regions like the Sahara desert. The amount and geographical spread of this dust in the atmosphere is sensitive to environmental conditions such as drought. Paleoclimate records show that rapid cooling events centered on the North Atlantic during the last ice‐age also saw large increases in the rate of dust deposited over the ocean. Earth System models are a computational tool that can be used to understand the links between climate and the dust cycle. Using a series of such simulations we found that abrupt cooling events in the North Atlantic can lead to a massive increase in dust over the equatorial (tropical) regions. However, the model's response is in many places non‐linear, meaning that the change in dust is very sensitive to the magnitude of the climate changes occurring. This non‐linearity is mostly due to interactions between changes in the water cycle and the dust. We show that a less severe cooling produces a more realistic dust response when evaluated against paleo‐dust records. Since dust scatters incoming sunlight, increased atmospheric dust during these events may have amplified and even extended the cold phases. Key Points Freshwater hosing forcing increases tropical dust loading in Earth System model simulations of the last glacial maximum Simulated response is non‐linear with forcing in some regions, particularly North Africa Weaker events improve agreement with data and emitted dust produces a self‐amplifying feedback

The impacts of volcanic eruptions on climate are increasingly well understood, but the mirror question of how climate changes affect volcanic systems and processes, which we term “climate-volcano impacts”, remains understudied. Accelerating research on this topic is critical in view of rapid climate change driven by anthropogenic activities. Over the last two decades, we have improved our understanding of how mass distribution on the Earth’s surface, in particular changes in ice and water distribution linked to glacial cycles, affects mantle melting, crustal magmatic processing and eruption rates. New hypotheses on the impacts of climate change on eruption processes have also emerged, including how eruption style and volcanic plume rise are affected by changing surface and atmospheric conditions, and how volcanic sulfate aerosol lifecycle, radiative forcing and climate impacts are modulated by background climate conditions. Future improvements in past climate reconstructions and current climate observations, volcanic eruption records and volcano monitoring, and numerical models all have a role in advancing our understanding of climate-volcano impacts. Important mechanisms remain to be explored, such as how changes in atmospheric circulation and precipitation will affect the volcanic ash life cycle. Fostering a holistic and interdisciplinary approach to climate-volcano impacts is critical to gain a full picture of how ongoing climate changes may affect the environmental and societal impacts of volcanic activity.

We present a continuous land-based climate reconstruction dataset extending back 60 kyr from 0 BP (1950) at 0.5° resolution on a monthly timestep for 0°N to 90°N. It has been generated from 42 discrete snapshot simulations using the HadCM3B-M2.1 coupled general circulation model. We incorporate Dansgaard-Oeschger (DO) and Heinrich events to represent millennial scale variability, based on a temperature reconstruction from Greenland ice-cores, with a spatial fingerprint based on a freshwater hosing simulation with HadCM3B-M2.1. Interannual variability is also added and derived from the initial snapshot simulations. Model output has been downscaled to 0.5° resolution (using simple bilinear interpolation) and bias corrected. Here we present surface air temperature, precipitation, incoming shortwave energy, minimum monthly temperature, snow depth, wind chill and number of rainy days per month. This is one of the first open access climate datasets of this kind and can be used to study the impact of millennial to orbital-scale climate change on terrestrial greenhouse gas cycling, northern extra-tropical vegetation, and megaflora and megafauna population dynamics.Measurement(s)climate change • temperature of air • precipitation process • radiation • snow • water-based rainfallTechnology Type(s)computational modeling technique • digital curationFactor Type(s)YearSample Characteristic - Environmentclimate systemSample Characteristic - LocationNorthern hemisphereMachine-accessible metadata file describing the reported data: 10.6084/m9.figshare.10012859

Reconstructions and simulations disagree on whether the Holocene exhibited a long‐term cooling or warming signal. Anthropogenic land‐use could be an important forcing regionally, but available population‐based estimates differ widely. We examine transient Holocene climate model simulations forced with three population‐based disturbed‐land reconstructions and compare this with a fourth scenario derived entirely from fossil pollen records. The direct biophysical temperature effects are broadly similar across the scenarios but the pollen‐based product suggests an earlier onset of disturbance, particularly in China and accounting for its limited spatial coverage, falls closer to the upper limit of the existing uncertainty range. Impacts in many areas begin during the mid‐Holocene but emergence of a signal varies spatially with earliest impacts over Europe, China and the North Atlantic. Significant uncertainties remain, and these could be tackled by improving the representation of land‐use effects in climate models or by merging different information sources related to Holocene land‐use.