Into the Holocene, anatomy of the Younger Dryas cold reversal and preboreal oscillation.

During the most recent deglaciation, the upwards trend of warmer Northern Hemisphere (NH) temperatures was punctuated by a rapid and intense return to glacial conditions: the Younger Dryas (YD). The end of this event marks the beginning of the Holocene. Using the University of Toronto version of CCSM4, a model of the climate prior to the YD was created with correct boundary conditions. Various amounts of freshwater forcing were then applied to the Beaufort Gyre for forcing intervals ranging from 1 to 125 years. In several cases, this was sufficient to collapse the Atlantic Meridional Overturning Circulation (AMOC) and cause significant cooling over the NH. Crucially, after the forcing was ceased, the AMOC stayed in an off state for approximately a millennium before mounting a rapid recover to pre-YD levels. This recovery, which permanently reduced the extent of NH sea ice, occurred through the mechanism of a Polynya opening in the Irminger Sea during winter and led to a pronounced "overshoot" of the AMOC, during which NH temperatures were higher than before the YD.

Deciphering local and regional hydroclimate resolves contradicting evidence on the Asian monsoon evolution.

The winter and summer monsoons in Southeast Asia are important but highly variable sources of rainfall. Current understanding of the winter monsoon is limited by conflicting proxy observations, resulting from the decoupling of regional atmospheric circulation patterns and local rainfall dynamics. These signals are difficult to decipher in paleoclimate reconstructions. Here, we present a winter monsoon speleothem record from Southeast Asia covering the Holocene and find that winter and summer rainfall changed synchronously, forced by changes in the Pacific and Indian Oceans. In contrast, regional atmospheric circulation shows an inverse relation between winter and summer controlled by seasonal insolation over the Northern Hemisphere. We show that disentangling the local and regional signal in paleoclimate reconstructions is crucial in understanding and projecting winter and summer monsoon variability in Southeast Asia.

Highly restricted near-surface permafrost extent during the mid-Pliocene warm period.

Accurate understanding of permafrost dynamics is critical for evaluating and mitigating impacts that may arise as permafrost degrades in the future; however, existing projections have large uncertainties. Studies of how permafrost responded historically during Earth's past warm periods are helpful in exploring potential future permafrost behavior and to evaluate the uncertainty of future permafrost change projections. Here, we combine a surface frost index model with outputs from the second phase of the Pliocene Model Intercomparison Project to simulate the near-surface (~3 to 4 m depth) permafrost state in the Northern Hemisphere during the mid-Pliocene warm period (mPWP, ~3.264 to 3.025 Ma). This period shares similarities with the projected future climate. Constrained by proxy-based surface air temperature records, our simulations demonstrate that near-surface permafrost was highly spatially restricted during the mPWP and was 93 ± 3% smaller than the preindustrial extent. Near-surface permafrost was present only in the eastern Siberian uplands, Canadian high Arctic Archipelago, and northernmost Greenland. The simulations are similar to near-surface permafrost changes projected for the end of this century under the SSP5-8.5 scenario and provide a perspective on the potential permafrost behavior that may be expected in a warmer world.

Prehistoric human migration between Sundaland and South Asia was driven by sea-level rise.

Rapid sea-level rise between the Last Glacial Maximum (LGM) and the mid-Holocene transformed the Southeast Asian coastal landscape, but the impact on human demography remains unclear. Here, we create a paleogeographic map, focusing on sea-level changes during the period spanning the LGM to the present-day and infer the human population history in Southeast and South Asia using 763 high-coverage whole-genome sequencing datasets from 59 ethnic groups. We show that sea-level rise, in particular meltwater pulses 1 A (MWP1A, ~14,500-14,000 years ago) and 1B (MWP1B, ~11,500-11,000 years ago), reduced land area by over 50% since the LGM, resulting in segregation of local human populations. Following periods of rapid sea-level rises, population pressure drove the migration of Malaysian Negritos into South Asia. Integrated paleogeographic and population genomic analysis demonstrates the earliest documented instance of forced human migration driven by sea-level rise.

Glacial isostatic adjustment: physical models and observational constraints.

By far the most prescient insights into the interior structure of the planet have been provided on the basis of elastic wave seismology. Analysis of the travel times of shear or compression wave phases excited by individual earthquakes, or through analysis of the elastic gravitational free oscillations that individual earthquakes of sufficiently large magnitude may excite, has been the central focus of Earth physics research for more than a century. Unfortunately, data provide no information that is directly relevant to understanding the solid state 'flow' of the polycrystalline outer 'mantle' shell of the planet that is involved in the thermally driven convective circulation that is responsible for powering the 'drift' of the continents and which controls the rate of planetary cooling on long timescales. For this reason, there has been an increasing focus on the understanding of physical phenomenology that is unambiguously associated with mantle flow processes that are distinct from those directly associated with the convective circulation itself. This paper reviews the past many decades of work that has been invested in understanding the most important of such processes, namely that which has come to be referred to as 'glacial isostatic adjustment' (GIA). This process concerns the response of the planet to the loading and unloading of the high latitude continents by the massive accumulations of glacial ice that have occurred with almost metronomic regularity over the most recent million years of Earth history. Forced by the impact of gravitationaln-body effects on the geometry of Earth's orbit around the Sun through the impact upon the terrestrial regime of received solar insolation, these surface mass loads on the continents have left indelible records of their occurrence in the 'Earth system' consisting of the oceans, continents, and the great polar ice sheets on Greenland and Antarctica themselves. Although this ice-age phenomenology has been clearly recognized since early in the last century, it was for over 50 years considered to be no more than an interesting curiosity, the understanding of which remained on the periphery of the theoretical physics of the Earth. This was the case in part because no globally applicable theory was available that could be applied to rigorously interpret the observations. Equally important to understanding the scientific lethargy that held back the understanding of this phenomenon involving mantle flow processes was the lack of appreciation of the wide range of observations that were in fact related to GIA physics. This paper is devoted to a review of the global theories of the GIA process that have since been developed as a means of interpreting the extensive variety of observations that are now recognized as being involved in the response of the planet to the loading and unloading of its surface by glacial ice. The paper will also provide examples of the further analyses of Earth physics and climate related processes that applications of the modern theoretical structures have enabled.

Past terrestrial hydroclimate sensitivity controlled by Earth system feedbacks.

Despite tectonic conditions and atmospheric CO2 levels (pCO2) similar to those of present-day, geological reconstructions from the mid-Pliocene (3.3-3.0 Ma) document high lake levels in the Sahel and mesic conditions in subtropical Eurasia, suggesting drastic reorganizations of subtropical terrestrial hydroclimate during this interval. Here, using a compilation of proxy data and multi-model paleoclimate simulations, we show that the mid-Pliocene hydroclimate state is not driven by direct CO2 radiative forcing but by a loss of northern high-latitude ice sheets and continental greening. These ice sheet and vegetation changes are long-term Earth system feedbacks to elevated pCO2. Further, the moist conditions in the Sahel and subtropical Eurasia during the mid-Pliocene are a product of enhanced tropospheric humidity and a stationary wave response to the surface warming pattern, which varies strongly with land cover changes. These findings highlight the potential for amplified terrestrial hydroclimate responses over long timescales to a sustained CO2 forcing.

Drier tropical and subtropical Southern Hemisphere in the mid-Pliocene Warm Period.

Thermodynamic arguments imply that global mean rainfall increases in a warmer atmosphere; however, dynamical effects may result in more significant diversity of regional precipitation change. Here we investigate rainfall changes in the mid-Pliocene Warm Period (~ 3 Ma), a time when temperatures were 2-3ºC warmer than the pre-industrial era, using output from the Pliocene Model Intercomparison Projects phases 1 and 2 and sensitivity climate model experiments. In the Mid-Pliocene simulations, the higher rates of warming in the northern hemisphere create an interhemispheric temperature gradient that enhances the southward cross-equatorial energy flux by up to 48%. This intensified energy flux reorganizes the atmospheric circulation leading to a northward shift of the Inter-Tropical Convergence Zone and a weakened and poleward displaced Southern Hemisphere Subtropical Convergences Zones. These changes result in drier-than-normal Southern Hemisphere tropics and subtropics. The evaluation of the mid-Pliocene adds a constraint to possible future warmer scenarios associated with differing rates of warming between hemispheres.

Snowball Earth prevention by dissolved organic carbon remineralization.

The 'snowball Earth' hypothesis posits the occurrence of a sequence of glaciations in the Earth's history sufficiently deep that photosynthetic activity was essentially arrested. Because the time interval during which these events are believed to have occurred immediately preceded the Cambrian explosion of life, the issue as to whether such snowball states actually developed has important implications for our understanding of evolutionary biology. Here we couple an explicit model of the Neoproterozoic carbon cycle to a model of the physical climate system. We show that the drawdown of atmospheric oxygen into the ocean, as surface temperatures decline, operates so as to increase the rate of remineralization of a massive pool of dissolved organic carbon. This leads directly to an increase of atmospheric carbon dioxide, enhanced greenhouse warming of the surface of the Earth, and the prevention of a snowball state.