Runaway climate change

Wikipedia: Runaway climate change describes a scenario in which the climate system passes a threshold or tipping point, after which internal positive feedback effects cause the climate to continue changing, even absent further external forcings. The runaway climate change continues until the feedback effects run out, possibly at a new climate state. The phrase “runaway climate change” is used to describe a theory in which positive feedbacks result in rapid climate change. Some astronomers use the similar expression runaway greenhouse effect to describe a situation where the climate deviates catastrophically and permanently from the original state – as happened on Venus.

Clathrate gun hypothesis

Wikipedia: Clathrate gun hypothesis is the popular name given to the hypothesis that rises in sea temperatures (and/or falls in sea level) can trigger the sudden release of methane from methane clathrate compounds buried in seabeds and permafrost which, because the methane itself is a powerful greenhouse gas, leads to further temperature rise and further methane clathrate destabilization – in effect initiating a runaway process (positive feedback loop) as irreversible, once started, as the firing of a gun.

In its original form, the hypothesis proposed that the “clathrate gun” could cause abrupt runaway warming in a timescale less than a human lifetime, and might be responsible for warming events in and at the end of the last ice age. This is now thought unlikely.

However, there is stronger evidence that runaway methane clathrate breakdown may have caused drastic alteration of the ocean environment and the atmosphere of earth on a number of occasions in the past, over timescales of tens of thousands of years; most notably in connection with the Permian extinction event, when 96% of all marine species became extinct 251 million years ago. 

Yet, it is still argued that a potential positive feedback mechanism from clathrate dissociation would amplify future global warming. However, past hydrate dissociation at Svalbard eight thousand years ago has been attributed to isostatic rebound (continental uplift following deglaciation).

The SWIPA 2017 report notes, “Arctic sources and sinks of greenhouse gases are still hampered by data and knowledge gaps.”

The runaway greenhouse: implications for future climate change, geoengineering and planetary atmospheres

Study: Wide attention has recently been drawn to such a concern by one of our most eminent climate scientists, James Hansen: ‘… if we burn all reserves of oil, gas, and coal, there’s a substantial chance that we will initiate the runaway greenhouse. If we also burn the tar sands and tar shale, I believe the Venus syndrome [the runaway greenhouse] is a dead certainty’. In this study, we begin by reviewing the physical basis of the runaway greenhouse in order to directly address this issue. We then consider how geoengineering could be appropriate to address a threat such as a moist or runaway climate state. Lastly, we discuss the runaway greenhouse for other planets.

Moist greenhouse

The earlier-mentioned considerations all apply to the case of a strict runaway greenhouse. A transition to a moist greenhouse (§2e) or other hot climate state is not excluded by theory and must be seen as a potential threat, until proved otherwise. In our understanding, this is the physically correct interpretation of the severe hazard of which Hansen warns.

The question here is simply how much could human action increase the strength of the greenhouse effect? Kasting & Ackerman found that, with carbon dioxide as the only non-condensible greenhouse gas, over 10 000 ppmv would be needed to induce a moist greenhouse. This is likely higher than could be achieved than by burning all the ‘conventional’ fossil fuel reserves—though the actual amount of fossil fuel available is poorly constrained, especially when one includes ‘exotic’ sources such as tar sands (which are already being exploited) and methane clathrates. Greenhouse gases other than carbon dioxide, cloud or albedo changes could all contribute further warming. Likewise, the exhibition of multiple equilibria in the relevant temperature range complicates matters.