The ka BP climate simulations

It is important for us to understand a similar impact of climate warming in response to the increased atmospheric carbon dioxide in the future, and climate warming in response to one likely consequence of the changes in radiation forcing in the mid-Holocene (6 kaBP), which has become one of hot topics in international community of Global Changes.

An international project for paleoclimate modeling at 6 kaBP (PMIP: Paleaoclimate Modeling Intercomparison Project) was launched in 1990s and has made great progresses. However, some important issues have not resolved so far. The PMIP designed a program for AGCM (Atmospheric General Circulation Model) simulations in which variations of solar radiation is considered as a major dynamic force for 6 kaBP climate. As the solar radiation variation played a unique role in the simulations, consequently most of the AGCMs have simulated the paleoclimate of 6ka BP with a general pattern of a warm summer and a cold winter over the major continents of the world.

However, synthesized studies from Quaternary data indicate that winter temperature at 6 kaBP was 2.5°C warmer in the east and 3-4°C warmer in the west of China than that at the present. This warm climate condition was recorded not only in China, but also in northern America. These facts do not agree with the modeling results that were only driven by solar radiation. Therefore, there would be other control factors that contributed to the warm climate at 6 ka BP, especially to the winter warming. It is necessary for us to explore the causes of winter warming in the mid-Holocene in the East Asia.

Studies from TEMPO Program have revealed that, after introducing the feedback of surface albedo with a coupled climate-biome model, a northward-shift climate zone in northern Africa can be simulated. Chinese scientists also did modeling experiments to test the impact of vegetation on the monsoon precipitation. Claussen et al., (1996) found that the influence of changed orbital geometry of the Earth has been amplified in the climate change in subtropical climate zone because of the feedback of atmosphere and vegetation. It is likely believed that changes in the regional and global vegetation have strong feedbacks on climates of the mid-Holocene.

Although the above studies have suggested that the climate at 6kaBP was directly or indirectly controlled by a feedback between climate and land surface, the simulations of PMIP before 1999 did not include the forcing from an active land surface in the boundary conditions of the modeling experiments. This caused that modeling results disagreed with observed data from the East Asia, especially they produced a contrary result for winter temperature in China.

In this section, an AGCM plus SSiB was used to simulate climate at 6 ka BP. As an improved dynamical factor, land surface conditions of vegetation at 6 ka BP were reconstructed from geological records and input into the climate model in order to simulate more realistic features of paleocli-mate (Chen et al., 2002).

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