Feedback loops (see variation 39) Here are some other reasons why predicting the effect of greenhouse gases on Gaia’s temperature is not at all simple.

Water vapour and clouds Water vapour is a powerful greenhouse gas but the amount of it in the air depends on the temperature. If it is cold it is removed from the air as rain or snow. If it is hot much more water vapour can exist as a gas in the air. This complication can be dealt with but water vapour also condenses as clouds, mists, and fogs, and at all levels from the surface to the high stratosphere. Clouds near the surface tend to reflect light back to space, whereas those high up in the stratosphere reflect heat back to the Earth. In general, but not always, high clouds heat and low clouds cool. Ice formation at high levels adds yet another complication: the distribution of ice and water in clouds can sufficiently affect their radioactive properties to halve the predicted warming by greenhouse gases.

The oceans If clouds complicate our predictions, so do the oceans. The ability of the oceans to store heat is 1000 times that of the air. The extent of greenhouse gas heating will greatly depend upon how much of the added heat is distributed in the oceans. If all of the predicted rise of surface temperature were mixed in with the oceans uniformly, there would be no perceptible rise in surface atmospheric temperature during the next century or so.

The polar ice Snow is brilliant in its whiteness and reflects heat in the form of sunlight back to space. Snow tends to sustain itself by keeping cold. Increasing warmth should in a simple world melt the snow, leaving dark ground, that would then absorb sunlight and with positive feedback hasten the melting. In the real world there are mountain regions, like Greenland and the Antarctic plateau, covered with thousands of metres of solid ice. These regions are so cold that a few degrees of global warmth is not enough to melt them but, more important, increasing warmth brings more water vapour and more snow. Ice can give rise to negative as well as positive feedback on global warming.

The organisms The involvement of living organisms in albedo, and especially in regulating cloud cover, whether through evapotranspiration over the forests, or through DMS production in the algal fields of the sea, is a major complication in any prediction of climatic change. Living organisms are involved intimately too in the removal of CO2 from the air, and methane production. All the great ecosystems — the photosynthesizers, the consumers, and the methanogens — have interacted with climate throughout Gaia’s long history.

Obviously, the prognosis for the fevered planet is uncertain. What is certain, however, is that we have already administered enough poisons to make the patient sick, and Gaia’s temperature is bound to rise, in the short term. The system may then respond to offset the fever — or it may be set on an irreversible course of runaway heating, to a new, hot state, an inhospitable one for humans.