Runaway Climate Change - Current Risk

Current Risk

The scientific consensus in the IPCC Fourth Assessment Report is that "Anthropogenic warming could lead to some effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change." Note however that this statement is about situations weaker than "runaway change". Text prepared for the IPCC Fifth Assessment Report states that "a 'runaway greenhouse effect'—analogous to Venus—appears to have virtually no chance of being induced by anthropogenic activities."

Estimates of the size of the total carbon reservoir in Arctic permafrost and clathrates vary widely. It is suggested that at least 900 gigatonnes of carbon in permafrost exists worldwide. Furthermore, there are believed to be another 400 gigatonnes of carbon in methane clathrates in permafrost regions with 10,000 to 11,000 gigatonnes worldwide. This is large enough that if 10% of the stored methane were released, it would have an effect equivalent to a factor of 10 increase in atmospheric CO₂ concentrations. Methane is a potent greenhouse gas with a higher global warming potential than CO₂.

Worries about the release of this methane and carbon dioxide is linked to arctic shrinkage. Recent years have seen record low Arctic sea ice. It has been suggested that rapid melting of the sea ice may initiate a feedback loop that rapidly melts arctic permafrost. Methane clathrates on the sea-floor have also been predicted to destabilise, but much more slowly.

A release of methane from clathrates, however, is believed to be slow and chronic rather than catastrophic and that 21st-century effects of such a release are therefore likely to be 'significant but not catastrophic'. It is further noted that 'much methane from dissociated gas hydrate may never reach the atmosphere', as it can be dissolved into the ocean and be broken down biologically. Other research demonstrates that a release to the atmosphere can occur during large releases. These sources suggest that the clathrate gun effect alone will not be sufficient to cause 'catastrophic' climate change within a human lifetime.

James E. Hansen has suggested that the Earth could experience a runaway greenhouse effect and adopt a climate like that of Venus if fossil-fuel use continues until reserves are exhausted.