A two‐dimensional (2‐D) dynamic ice sheet model coupled to the McGill Paleoclimate Model (MPM) under Milankovitch forcing and Vostok‐derived atmospheric CO2 levels is used to investigate the last glacial inception and subsequent rapid ice sheet growth in the Northern Hemisphere (NH). The impacts on ice sheet growth of the elevation effect of orography and the freezing of rain/refreezing of meltwater are evaluated. The results show that while Milankovitch forcing only is sufficient to initiate the formation of permanent North American and Eurasian ice at around 120 kyr BP, rapid ice sheet growth during the next 10 kyr only occurs when the above two processes and an active ocean component are included. The modelled ice volume‐equivalent drop in sea level during this growth period is estimated to be about two‐thirds of that found from sea level reconstructions. Finally, the ice sheet‐thermohaline circulation interactions and ice sheet thickness distribution are also investigated.

[1] There are three fundamental features which characterize large glacial millennial (Dansgaard-Oeschger) oscillations: (1) the climatic transitions were abrupt and large; (2) the lengths of both interstadials and stadials and the period of Dansgaard-Oeschger oscillations were not uniform; and (3) there were no large millennial oscillations during an early stage of a glacial period and a peak glacial period. In this modeling study we offer a consistent explanation for these three features by employing an Earth system Model of Intermediate Complexity. We demonstrate that a moderate global cooling forces the Atlantic meridional overturning circulation (MOC) into an unstable state and hence causes the flip-flop of the Atlantic MOC between a strong mode and a weak mode. The durations of both interstadials and stadials associated with these millennial oscillations are modulated by the changing background climate in qualitative agreement with the observations. In a warm climate the Atlantic MOC is strong and stable, with the deep water formed mainly by intense heat loss to the atmosphere. In a cold climate the Atlantic MOC is weak and stable, and this mode is largely maintained by the process of sea ice brine rejection. Since the Dansgaard-Oeschger oscillations result from an alternation between these two MOC states during an intermediate phase climate, we conclude that brine rejection plays a necessary role in the oscillations, confirming a hypothesis suggested in some proxy data studies.

[1] A coupled atmosphere-ocean-sea ice-land surface-ice sheet model of intermediate complexity, the so-called McGill Paleoclimate Model, is employed to study the response of the thermohaline circulation (THC) to various global climate coolings, which are realized by increasing the present-day planetary emissivity to various values. Generally, it is found that the response of the THC to global cooling is nonlinear: For a slightly cold climate the THC in the North Atlantic and the Pacific upwelling become intensified. For a very cold climate the THC in the North Atlantic may be weakened or even collapsed. The associated Pacific upwelling for a very cold climate also becomes weak when the THC is weakened, and intermediate deep water may form in the Pacific when the THC is collapsed. Some support for this nonlinear response is found in recent paleoceanographic data. The reduced atmospheric poleward moisture transport due to the global cooling is mainly responsible for the intensification of the THC in the North Atlantic for a slightly cold climate. For a very cold climate the global cooling may lead to a decrease of the meridional surface density gradient and an increase of the vertical density difference (lower layer density minus upper layer density) in the deep water formation region, which can weaken or shut down the THC. It is the temperature-dependent part of the density differences that is mainly responsible for the weakening or shutting down of the THC. The potential influence of surface temperature changes must be taken into account for a full understanding of the role of the THC in the climate system.

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In order to explore the influence of anthropogenic land use on the climate system during the last millennium, a set of experiments is performed with an Earth system model of intermediate complexity--the McGill Paleoclimate Model (MPM-2). The present paper mainly focuses on biogeophysical effects of historical land cover changes. A dynamic scenario of deforestation is described based on changes in cropland fraction (RF99). The model simulates a decrease in global mean annual temperature in the range of 0.09-0.16℃, especially 0.14-0.22℃ in Northern Hemisphere during the last 300 years. The responses of climate system to GHGs concentration changes are also calculated for comparisons. Now, afforestation is becoming an important choice for the enhancement of terrestrial carbon sequestration and adjustment of regional climate. The results indicate that biogeophysical effects of land cover changes cannot be neglected in the assessments of climate change.climate change, radiative forcing, land cover changes, deforestation, climate-biosphere interactions With the development of our society, especially after the Industrial Evolution, the effects of anthropogenic activities on climate are becoming more and more important, and now the climate system would be influenced by both nature and humankind. Compared to the natural factors, such as insolation and volcanic activities, humankind affects the climate system in many ways, particularly by modifying atmospheric gas composition and by changing land surface properties. Till now, global warming induced by increasing greenhouse gases (GHGs) has been paid close attention to, however, the influence of land cover changes has not been considered enough and few researches have been focused on these. In fact, human-induced land cover changes began probably as early as 8000 years ago [1] , and at present, about one third of global vegetation cover has being modified by agricultural and forestry activities [2] . Therefore, it is necessary for us to evaluate the effects of land cover changes.Changes in land cover have affected the climate system through emissions of GHGs (biogeochemical effects) and modification of land surface albedo and roughness (biogeophysical effects). Biogeophysical mechanisms of land cover changes are considered quite complex and could affect not only regional but also global climate. Hansen et al. [3] emphasized the radiative effects of vegetation cover changes in the review of climate forcings, pointing out that their radiative forcing was in the range of about (−0.2 ± 0.2) W/m 2 and maybe leads to a global cooling by 0.14℃. The mechanism of this forcing is mainly that land surface albedo increases a lot due to the replacement of forests by croplands and pastures, and it could be more notable in the high northern latitudes, where snow-masking effect of vegetations is very remarkable. Bonan et al. [4] revealed a cooling effect of boreal deforestation and except for direct influences of deforestation, the sea ice-albedo feedback also played an important role i...