Simulations of the Climate of the Holocene: Perspectives Gained with Models of Different Complexity

The Earth's climate has changed significantly in the past 10,000 years. These climate changes were associated with changes of incoming solar radiation caused by orbital changes. The climatic response to the change in external forcing involved major adjustments of the atmosphere, ocean, cryosphere, and biosphere. Because these Earth System changes are well documented in time and space, there are opportunities to compare the observed behavior of the system to the results of simulations by numerical models. This chapter reviews progress in climate simulations over a two-decade interval as models have been developed to increase the coupling among system components: first atmosphere models, then atmosphere-ocean-cryosphere models; and, recently, atmosphere-ocean-cryosphere-biosphere models. The scope of studies has also expanded from an initial emphasis on simulating the average climatic conditions of different epochs to a current emphasis on the simulation of climate variability at interannual to century time scales, and abrupt climatic changes, as a function of changing external forcing. In most instances the simulations have shown that ocean, cryosphere, and biosphere feedbacks can amplify the climate's response to changes of external forcing. Including more system components has generally led to improved agreement between observations and simulations. However, so far models have been unable to simulate the full magnitude, and the spatial and temporal structure, of Holocene climate change. These results also help underscore the importance of including all Earth System components in models being used to assess possible changes of climate in the future, changes that could be caused by human-related activities such as the burning of fossil fuels and deforestation.