High-resolution simulations of two cold palaeo climates in Europe : MIS 3 and LGM

Abstract: The study of past climate is important because it increases our understanding of how the climate system works. Past climate is often reconstructed by using proxies (that is observations of things that tell something about past climate, for example tree rings, pollen in lake sediments and fossils). Model simulations of past climate further increases the knowledge since it has the possibility to gap the space and time between the sparse and scattered proxy observations, since a model simulation gives relatively continuous information about the whole simulated area. Model simulations can also give internally coherent information about parameters that is not easily reconstructed from proxies (for example heat fluxes).  In this thesis two periods in the past are simulated by climate models: the Marine Isotope Stage 3 (MIS 3), 44 000 years ago, and the Last Glacial Maximum (LGM), 21 000 years ago. Both periods are characterised by low temperature, low sea level and low level of carbon dioxide. The topography in northern Europe is dominated by ice sheets covering Iceland, Norway and parts of Sweden at MIS3; and more extensive ice sheets covering Iceland, Scandinavia, the British Isles and Northern Germany at LGM. These periods are firstly simulated by a global climate model. Those simulations are subsequently used in a regional climate model to increase the level of detail over Europe. To make the regional climate model simulation more realistic vegetation simulated by a dynamical vegetation model is used in the regional climate model.   The climate models simulate European climates much colder than today, especially at LGM. The temperature differences ranges from 5 to 45 °C colder than today; the largest differences being at the ice sheets where the perennial ice cover and the high altitude keep temperatures low. Precipitation is reduced with as much as almost 100 % in northern Europe due to reduced evaporation. Precipitation is increased with as much as 100 % in parts of southern Europe due to changes in atmospheric circulation. The simulations are in broad agreement with proxies, although there are differences.  The vegetation model simulates tundra like vegetation (herbs and shrubs) in the ice-free parts of central and southern Europe. The eastern parts of Europe are dominated by needle-leaved trees. The short and cool summers limit vegetation. The simulated vegetation is in broad agreement with reconstructions. Sensitivity studies of vegetation show that changed vegetation can change the monthly mean temperature with 1-3 °C in some seasons and regions. The response depends on regional surface characteristics. Sensitivity studies of ice sheets show that the simulated climate is consistent with the assumptions about the ice sheet extent made in the simulation. The simulated climate is cold enough in northern Europe to support the ice sheet, and warm enough in southern Europe to prevent the ice sheet from expanding in this direction. A removal of the ice sheet would only have an effect on the local scale in the vicinity of the ice sheet, but this experiment did not include changes in the large-scale global atmospheric circulation.  Although the regional climate model simulations are to a large degree depending on the global climate model simulations they provide new information. When comparing proxies with model data or studying local/regional climatic features (such as the interplay between climate and vegetation) high horizontal resolution, as in the regional climate model, is important.