New paper looks to the past to estimate the future
- 25 Nov 2011, 10:20
- Verity Payne
A new scientific
paper released yesterday in the journal Science offers a
revised estimate of
climate sensitivity - the amount of increase in the global mean
temperature expected from a doubling of atmospheric carbon dioxide
- suggesting that it may be smaller than some previous
estimates.
The paper hit the blogosphere earlier this month, with
climate skeptic bloggers making much of it at the time. Now it
is gaining a fair bit of media coverage, with some provocative
headlines including "
Climate change fears 'have been exaggerated' say scientists who
claim apocalyptic predictions are unlikely" (the Mail), and the
Australian's "
Climate forecasts 'exaggerated': Science journal". The
Economist and the BBC have more sober headlines, with "Good news at
last?" and "Climate
sensitivity to CO2 probed" respectively.
So, what does the paper show? Scientists used a climate model
to reconstruct land and sea surface temperatures from the Last
Glacial Maximum (LGM), around twenty thousand years ago, when
certain aspects of Earth's climate were quite different. There was
about a third less atmospheric carbon dioxide than before the
Industrial Revolution, sea levels were lower because northern
latitudes were covered in ice and snow, there was less rainfall,
and there was more dust in the air.
The team used a model to examine this environment and work out
what implications it had on how the climate changes as atmospheric
CO2 levels change.
Based on the modelling, the group came up with a new best guess
for climate sensitivity of 2.3 °C (with a two in three chance that
it lies between 1.7 and 2.6 °C). This is a little lower than some
previous estimates. Notably, the Intergovernmental Panel on Climate
Change (IPCC)'s 2007 report (AR4) assessed all of the previous
estimates and suggested that climate sensitivity is likely (two in
three chance) to lie between 2 and 4.5 °C. They concluded that it
was unlikely to be below 1.5 °C, but that values above 4.5 °C could
not be ruled out.
So why is this range of estimates slightly lower than previous
estimated ranges? Andreas Schmittner, an Oregon State University
researcher and lead author on the Science article
explains:
"Many previous climate sensitivity studies have looked at the past
only from 1850 through today, and not fully integrated paleoclimate
date, especially on a global scale... When you reconstruct sea and
land surface temperatures from the peak of the last Ice Age 21,000
years ago - which is referred to as the Last Glacial Maximum - and
compare it with climate model simulations of that period, you get a
much different picture."
Other estimates of climate sensitivity are shown in the image
below:
The new results are not actually so very different from what
previous studies have found as you can see from the image, but
perhaps the most interesting finding of the new study is that the
higher end estimates of previous studies (over 4.5 °C) are
effectively ruled out. This is not in agreement with previous
studies, which couldn't rule out such higher climate
sensitivities - as the image shows.
This should be great news, given the rate at which greenhouse
gases are being released into the atmosphere by human activity.
Schmittner explains:
"If these paleoclimatic constraints
apply to the future, as predicted by our model, the results imply
less probability of extreme climatic change than previously
thought"
There are the usual scientific uncertainties in the paper, for
example, in this case the group's climate model simulations did not
take into account uncertainties arising from how cloud changes
reflect sunlight. And this study is one of many investigating
climate sensitivity, so although it is an important result, it is
important not to overemphasise this finding at the expense of all
the previous work in this area.
This is precisely what some areas of the blogosphere did in their
early online discussions of the article. And, as the Economist puts it,
overhyping the study in this way is pretty hypocritical:
"...Some sceptics complain about the way
ancient data of this type were used to construct a different but
related piece of climate science: the so-called hockey-stick model,
which suggests that temperatures have risen suddenly since the
beginning of the industrial revolution. It will be interesting to
see if such sceptics are willing to be equally sceptical about
ancient data when they support their point of view."
Usefully, the team involved have taken time to explain the work
to
bloggers at Planet 3, where they also address some of the ways
that their work has already been misrepresented or overhyped
online, in a pretty fair and even handed manner.
The scientists involved in the study take pains to point out that
our continued increasing emissions of atmospheric carbon dioxide
will have serious impacts. But, if the study's findings are
correct, Schmittner says, we may get a small bit of breathing
space:
"our study implies that we still have
time to prevent that from happening, if we make a concerted effort
to change course soon."