THERE has been much debate about how the Earth responds to changes in climate-specifically, how feedbacks involving the biota change with temperature. There is in particular an urgent need to understand the extent of coupling and feedback between plant growth, global temperature and enhanced atmospheric concentrations of greenhouse gases. Here we present a simple, but we hope qualitatively realistic, analysis of the effects of temperature change on the feedbacks induced by changes in surface distribution of marine algae and land plants. We assume that algae affect climate primarily through their emission of dimethyl sulphide(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 temperature, we find that a simple model based on these ideas exhibits three feedback regimes. In glacial conditions, both marine and terrestrial ecosystems provide a negative feedback. As the temperature rises to present-day values, algae lose their strong climate influence, but terrestrial ecosystems continue to regulate the climate. But if global mean temperatures rise above about 20 degrees C, both terrestrial and marine ecosystems are in positive feedback, amplifying any further increase of temperature. As the latter conditions have existed in the past, we propose that other climate-regulating mechanisms must operate in this warm regime.