QUEST Deglaciation: Climate and

During the past twenty thousand years the Earth's climate has changed from a full ice age to a warm period. During ice ages there are large ice sheets in Europe and North America, the oceans are cold with large areas of sea-ice in winter, the continents are cold and generally dry, forests worldwide are much reduced, and the climate is kept cold by very low concentrations of carbon dioxide and other 'greenhouse gases' and by enormous amounts of dust in the air. The climate of the last ice age was also unstable, punctuated by large coolings and sudden warmings lasting tens to hundreds of years; and the warming after the ice age was suddenly interrupted by a cold period lasting a thousand years. The climate has been warmer and rather more stable since then, but there have still been changes in climate, vegetation and the atmosphere. For example, there was a slow rise in carbon dioxide concentration during some eight thousand years before the dramatic rise that began with the Industrial Revolution. Some climate changes were dramatic, like the formation of the Sahara desert. These facts are well established, but to understand the mechanisms behind them is still a challenge to science. The causes of changes in climate, and of pre-industrial changes in greenhouse gases and dust (which in turn affect the climate), are not fully understood. The only known external cause is the variation of the Earth's orbit, which gradually alters the amount of the Sun's energy received at different latitudes and seasons. The Earth apparently responds to this variation in a very complex way, and sometimes abruptly. The changes in the atmosphere, especially, give a clue that climate changes are not just a matter of physics; they also involve changes in 'biogeochemical cycles' / the exchanges of carbon and other elements between the atmosphere, ocean and land, which are regulated by living organisms. Understanding these processes is particularly important because human activities are now changing the atmosphere and climate, and we need to be able to predict the long-term consequences. Making predictions 'in reverse' is one way to test the models we use. The aim of this project is a fuller understanding of what has driven changes in climate, atmospheric composition and biogeochemical cycles during the period from the peak of the last ice age until recent times. We will combine two ways of working, which have usually been separate. On the one hand, we will try to predict the past. We will use computer climate models with extra features, including a dynamic global vegetation model that can predict changes in wetlands, deserts and forest fires. This will enable us to us simulate the land-atmosphere exchanges of many important substances that affect climate, such as carbon dioxide, methane, volatile hydrocarbons, dust and soot. We will use versions of these models which are very efficient because they only simulate large-scale patterns. In this way we can do many more and longer simulations than is usually done. On the other hand, we will use existing data bases and new data-analysis methods to make a major synthesis of the data from sediment cores around the world, including pollen counts (an indicator of past vegetation), charcoal counts (an indicator of past fires) and carbon isotope measurements. This work will provide, for the first time, a continuous picture of the state of the Earth's land surface from the last ice age up to recent times. We will assess how well the climate models are working, and get insights into how the vegetation changes are interacting with the climate and the composition of the atmosphere, by making a detailed comparison of this reconstruction with our computer-generated 'virtual history' of the Earth.