Modelling studies of glacial-interglacial transitions.

摘要

Glaciation/deglaciation is one of the most extreme and fundamental climatic events in Earth's history. The origin of the glacial-interglacial cycles has been explored for more than a century and the astronomical theory is now well established. However, the mechanism that links the astronomical forcing to the geological record in the Earth's climate system is poorly understood. In this thesis, aspects of the last glacial termination and the last glacial inception, are studied.; First, the response of ocean's thermohaline circulation to changes in orbital geometry and atmospheric CO₂ concentration in the last glacial termination is investigated using a coupled climate (atmosphere-ocean-sea ice) model. It is shown that the thermohaline circulation is affected by both orbital and CO₂ forcing and the details of the mechanisms involved are explored. The climatic impact of changes in the thermohaline circulation is then investigated. It is revealed that the influence of changes in the thermohaline circulation on surface air temperature is concentrated in the North Atlantic and adjacent continents. It is also shown that this influence has its peak in winter rather than in summer. A dynamic ice sheet model is then globally and asynchronously coupled to the climate model. The relative importance of orbital and CO₂ forcing in the mass balance of ice sheets is investigated using the coupled climate-ice sheet model. It is shown that CO₂ forcing is of secondary importance to orbital forcing as the warming in eastern North America and Scandinavia due to CO₂ forcing has its peak in winter, whereas that due to orbital forcing has its peak in summer. It is, nevertheless, concluded that the last glacial termination was initiated through increasing summer insolation and accelerated by a subsequent increase in atmospheric CO₂ concentration.; Second, the importance of subgrid topography in simulating the last glacial inception is investigated using the coupled climate model. The effects of subgrid elevation and subgrid ice-flow are incorporated in the model. Despite the use of high subgrid resolution, the coupled climate model fails to capture the last glacial inception. An atmospheric general circulation model is then used to explore the reasons for the failure, as well as the importance of changes in sea surface conditions and vegetation in simulating the last glacial inception. A realistic, geographic distribution of perennial snow cover and global net accumulation rate are successfully simulated when colder sea surface conditions than those of the present-day are specified. It is also shown that the effect of the vegetation feedback is large.; It is revealed that changes in ocean circulation and vegetation are at least partly responsible for the complicated link between astronomical forcing and climate states during the glacial-interglacial cycles. As these two components play important roles, it is suggested that both components as well as ice sheet dynamics should be included in realistic paleoclimate simulations.