Global climate models: past, present, and future.

Introduction Global climate is a result of the complex interactions between the atmosphere, cryosphere (ice), hydrosphere (oceans), lithosphere (land), and biosphere (life), fueled by the non-uniform spatial distribution of incoming solar radiation. We know from climate reconstructions using recorders such as ice cores, ocean and lake sediment cores, tree rings, corals, cave deposits, and ground water that the Earth’s climate has seen major changes over its history. An analysis of the temperature variations patched together from all these data reveals that climate change occurs in cycles with characteristic periods, for example 200 million, 100 thousand, or 4-7 years. For some of these cycles, particular mechanisms have been identified, for example forcing by changes in the Earth’s orbital parameters or internal oscillations of the coupled ocean-atmosphere system. However, major uncertainties remain in our understanding of the interplay of the components of the climate system. Paleoclimate reconstructions, in particular from ice cores (1) have also shown that climate can change (e.g. ∆T = 5C) over extremely short periods of time such as a few years. Over the last century, humans have altered the Earth’s surface and the composition of its atmosphere to the extent that these factors measurably affect current climate conditions. There is concern that perhaps even during one human generation we will gradually change climate conditions or even trigger a rapid and much more dramatic shift. We might be ‘poking an angry beast’ (2).