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
