Climate Thresholds

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; climate change also entails 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; if they continue to do so, they will release large amounts of dissolved methane in a sharp spike, rather than the gradual increase seen in the past. The clathrate gun hypothesis focuses on an even larger supply of methane: the methane clathrate (ice containing methane) found in enormous amounts on the cold ocean floor. Sufficient warming would melt the ice, releasing enough methane over a short period of time to severely accelerate global warming.

The most severe mass extinction, much greater than the extinction of the dinosaurs, is the Permian-Triassic Extinction. The event caused the extinction of over 90 percent of sea life and twothirds of terrestrial vertebrates, about 250 million years ago (before the birth of the dinosaurs). This was also the only known mass extinction of insects, at a time when they enjoyed their greatest diversity and largest size. The sudden emptying of so many niches in the ecosystem likely accounts for the success of fungi, and of bivalves such as oysters, both of which were rare before the extinction, but which survived and thrived after it.

There is evidence of an extraordinary release of methane at about this time. The only known possible cause of such a release would be the melting of oceanic methane clathrate, which could explain why the Permian sea life was affected much more than land-dwelling life forms. Seen in this light, catastrophic scenarios for global warming no longer seem unprecedented. Any positive feedback to the greenhouse effect can force a climate threshold. Such an occurrence is called a runaway greenhouse effect. Most positive feedbacks, even strong ones, are not runaway effects; it must be a self-perpetuating feedback, such that the effects are stronger every time it “loops” back around. Runaway greenhouse effects not only cause a rapid rise in temperature, they are also fed by rising temperatures until something changes the system sufficiently to reduce the effect or achieve equilibrium. The planet Venus resembles the aftermath of a runaway greenhouse effect, whether or not that is how it developed.