CLIMATE CHANGE IS not always a gradual process. Just as the weather includes both the ordinary passage of seasons and unpredictable, extraordinary events, such as devastating hurricanes and droughts, so, too, does climate change entail both gradual processes and the sudden, sharp changes called climate thresholds. These thresholds are hypothetical—that is, they have not been observed directly, though it is believed that they have happened in the past.

Methane is often implicated in climate threshold theories. A powerful greenhouse gas, methane is contained on the Earth in a number of forms that could be unlocked all at once by sufficiently warm temperatures. For instance, in western Siberia, permafrost peat bogs that have remained frozen since the end of the last ice age are beginning to thaw; ...