FAILURE OF CLIMATE REGULATION IN A GEOPHYSIOLOGICAL MODEL

THERE has been much debate about how the Earth responds to changes in climate-specifically, how feedbacks involving the biota change with te mperature. There is in particular an urgent need to understand the ext ent of coupling and feedback between plant growth, global temperature and enhanced atmospheric concentrations of greenhouse gases. Here we p resent a simple, but we hope qualitatively realistic, analysis of the effects of temperature change on the feedbacks induced by changes in s urface distribution of marine algae and land plants. We assume that al gae affect climate primarily through their emission of dimethyl sulphi de(1-8) (which may influence cloud albedo), and that land plants do so by fixation of atmospheric CO2 (refs 9-12). When we consider how the planetary area occupied by these two ecosystems varies with temperatur e, we find that a simple model based on these ideas exhibits three fee dback regimes. In glacial conditions, both marine and terrestrial ecos ystems provide a negative feedback. As the temperature rises to presen t-day values, algae lose their strong climate influence, but terrestri al ecosystems continue to regulate the climate. But if global mean tem peratures rise above about 20 degrees C, both terrestrial and marine e cosystems are in positive feedback, amplifying any further increase of temperature. As the latter conditions have existed in the past, we pr opose that other climate-regulating mechanisms must operate in this wa rm regime.