Land‐surface characterization in greenhouse climate simulations

A simplified Holdridge‐type vegetation prediction scheme has been coupled to a version of the NCAR community climate model (CCM1‐Oz) that includes the biosphere‐atmosphere transfer scheme (BATS) and a mixed‐layer ocean. This interactive vegetation climate model has been used to conduct two complementary CO₂‐doubling experiments: an instantaneous 2 x CO₂ simulation (15 years in total) and a fast, transiently increasing CO₂ simulation (45 years in total). There are some differences in the predicted vegetation distributions and areas. However, there is agreement that in a warmed world the vegetation type termed ‘agriculture’ increases in area at the expense of deciduous needle‐leaf trees and short grass; and the tundra extent, already underestimated, decreases further whereas deserts and the deciduous broadleaf tree areas expand. The overall vegetation areas predicted are not particularly sensitive to initialization, although effects of different initialization can be monitored for 1–2 years. On the other hand, when the sensitivity of the modelled climate to the inclusion of some aspects of an interactive biosphere is examined, it is found that annually updated continental characteristics do not disrupt the climate simulation but do modify zonal temperatures and precipitation and increase continental evaporation. The latter intensifies the Hadley circulation, especially in July, and, thus, leads to increased evaporation globally. These results, if corroborated by other similar studies, indicate that simple, post facto application of vegetation diagnostic schemes once climatic equilibrium is achieved may be diagnosing vegetation from an incorrect climatic state.