Exploring multiple steady states in Earth's long-term carbon cycle

The long-term carbon cycle regulates Earth's climate and atmospheric CO2 levels over multimillion-year timescales, but it is not clear that this system has a single steady state for a given input rate of CO2. In this paper we explore the possibility for multiple steady states in the long-term climate system. Using a simple carbon cycle box model, we show that the location of precipitation bands around the tropics and high mid-latitudes, coupled with the response of the terrestrial biosphere to local surface temperature, can result in system bi-stability. Here, maximum CO2 drawdown can occur when either the tropics or high mid-latitudes are at the photosynthetic optimum temperature of around 25°C, and a period of instability can exist between these states. We suggest that this dynamic has influenced climate variations over Phanerozoic time, and that higher steady state surface temperatures may be easier to reach than is commonly demonstrated in simple ‘GEOCARB style’ carbon cycle models.