Abstract
The question of how Earth's climate is stabilized on geologic time scales is important for understanding Earth's history, long-term consequences of anthropogenic climate change, and planetary habitability. Here, we quantify the typical amplitude of past global temperature fluctuations on time scales from hundreds to tens of millions of years and use it to assess the presence or absence of long-term stabilizing feedbacks in the climate system. On time scales between 4 and 400 ka, fluctuations fail to grow with time scale, suggesting that stabilizing mechanisms like the hypothesized "weathering feedback" have exerted dominant control in this regime. Fluctuations grow on longer time scales, potentially due to tectonically or biologically driven changes that make weathering act as a climate forcing and a feedback. These slower fluctuations show no evidence of being damped, implying that chance may still have played a nonnegligible role in maintaining the long-term habitability of Earth.