Sometime in the last few billion years, disaster struck one of Earth’s nearest neighbours. Planetary geologists think there is good evidence that Venus was the victim of a runaway greenhouse effect which turned the planet into the boiling hell we see today.

A similar catastrophe is almost certain to strike Earth in about 2 billion years, as the Sun increases in luminosity.

But that raises an important question: is it possible that we could trigger a runaway greenhouse effect ourselves by adding carbon dioxide to the atmosphere?

According to the climate scientist James Hansen, that’s a distinct possibility. A couple of years ago, he wrote: “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 is a dead certainty.”

Today, Colin Goldblatt at the University of Victoria in Canada and Andrew Watson at the University of East Anglia in the UK, publish an interesting analysis of this question and, while they are nowhere near as pessimistic as Hansen, their conclusion is not entirely re-assuring.

Here’s the background. The fear is that adding carbon dioxide to the atmosphere is warming the planet and increasing evaporation from the oceans. The extra water vapour, itself a greenhouse gas, causes more warming and more evaporation in a vicious cycle of temperature increases that eventually result in the ocean boiling away.

This runaway greenhouse only stops when the atmosphere reaches some 1400 degrees C, causing it to emit thermal radiation at a wavelength that water vapour does not absorb and so can radiate into space.

In the above scenario, there is nothing to stop a runaway greenhouse whenever there is a small increase in temperature, like the one climate scientists have seen in the last few years. But the historical records shows us that small increases in temperature do not trigger runaway greenhouses.

Atmospheric physcists have known for some time that the physics is a little more complex than this. Goldblatt and Watson point out that when the temperature rises, the Earth emits more heat into space and this cools the planet providing an important balancing mechanism.

The crucial point is that there is a specific limit to the amount of radiation the atmosphere can emit. So a runaway greenhouse can only occur when the Earth is close to that limit.

So the question now becomes this: can the anthropological emission of greenhouse gases into the atmosphere push us close enough to this limit to trigger a runaway greenhouse?

Goldblatt and Watson have an answer: “The good news is that almost all lines of evidence lead us to believe that it is unlikely to be possible, even in principle, to trigger full a runaway greenhouse by addition of noncondensible greenhouse gases such as carbon dioxide to the atmosphere.”

But there is an important caveat. Atmospheric physics is so complex that climate scientists have only a rudimentary understanding of how it works. For example, Goldblatt and Watson admit that the above conclusion takes no account of the role that clouds might play in this process.

And scientists’ ignorance of the processes at work raises a significant question mark. As Goldblatt and Watson put it: “Is there any missed physics or weak assumptions that have been made, which if corrected could mean that the runaway is a greater risk? We cannot answer this with the confidence which would make us feel comfortable.”

That’s something worth worrying about. What’s needed, of course, is a major effort to better understand the physics of warm moist atmospheres and something like this is indeed happening.

Goldblatt and Watson are sufficiently worried to suggest that we start thinking of mitigation strategies, should their reasoning turn out to be flawed. “In the event that our analysis is wrong, we would be left with the situation in which only geoengineering could save us,” they say.

They devote some of their paper to this problem. “In the distant future, modifying Earth’s orbit might provide a sustainable solution,” they conclude.

All the more reason to redouble our efforts to cut greenhouse gas emissions. As Goldblatt and Watson put it in their conclusion: “The imperative to cut greenhouse gas emissions remains.”

Ref: arxiv.org/abs/1201.1593: The Runaway Greenhouse: Implications For Future Climate Change, Geoengineering And Planetary Atmospheres