Long-term stability of the Earth's climate.

Earth's climate has remained reasonably temperate for at least the last 3.5 billion years, despite a large increase in solar luminosity with time. The increase in solar flux has probably been offset by a decrease in atmospheric CO2 concentration caused by a negative feedback in the carbonate-silicate geochemical cycle. The same feedback mechanism implies that an Earth-like planet could remain habitable (i.e. possess liquid water) out to a least the orbit of Mars. The initial atmospheric CO2 concentration may have been much higher than the amount required to offset the lower solar output, in which case the Earth may have originally been much hotter than it is today. However, once the initial accretion period was over, Earth should have been stable against either a runaway greenhouse, that is, complete evaporation of the oceans, or against rapid loss of water. Long-term climatic evolution has thus far been studied only with one-dimensional, globally-averaged climate models. Although such models can provide a qualitative understanding of climate history, they rely on a number of assumptions that may not have been valid in the past. Some problems that deserve to be investigated with more sophisticated climate models are discussed.