Geology Reference
In-Depth Information
Figure 24: Global temperature with and without life over the last 600 million years.
The upper curve reveals the actual temperature—it shows that there was a significant
cooling around 300 million years ago, to the extent that a major glaciation gripped the
planet. This event, which marks a major transition in Gaia's evolution into greater ma-
turity, was caused by a massive increase in the biologically assisted weathering of gran-
ite and basalt as deep-rooted land plants took hold of the planet's land surfaces. It took
Gaia about 100 million years to recover from this self-imposed perturbation. By around
200 million years ago there was much less granite and basalt available for weathering,
and so carbon dioxide once again began to accumulate in the atmosphere. Amazingly,
temperatures before and after the cooling were remarkably similar. Before the event
some 600 million years ago, under a dimmer sun, higher levels of carbon dioxide kept
temperatures well within habitable levels. Since her recovery from the cooling, Gaia has
