Constraints on Climate Sensitivity Tightened

There is some really good news in last week's Nature concerning climate
sensitivity. Climate sensitivity is how much the average temperature goes up
when CO2 doubles. If there is high climate sensitivity we might hit a
"tipping point" popularized by Time Magazine giving breathless headlines
about being very worried. One reason for the worry was high climate
sensitivity could produce such an effect because we (the whole world) are
doing such a poor job of controlling CO2. But, now we have this:

Climate sensitivity constrained by temperature reconstructions over the past
seven centuries

Gabriele C. Hegerl1<http://www.nature.com/nature/journal/v440/n7087/full/nature04679.html#a1>,
Thomas J. Crowley
1<http://www.nature.com/nature/journal/v440/n7087/full/nature04679.html#a1>,
William T. Hyde1<http://www.nature.com/nature/journal/v440/n7087/full/nature04679.html#a1>and
David J. Frame
2 <http://www.nature.com/nature/journal/v440/n7087/full/nature04679.html#a2>
Top of page<http://www.nature.com/nature/journal/v440/n7087/full/nature04679.html#top>
Abstract

The magnitude and impact of future global warming depends on the sensitivity
of the climate system to changes in greenhouse gas concentrations. The
commonly accepted range for the equilibrium global mean temperature change
in response to a doubling of the atmospheric carbon dioxide
concentration1<http://www.nature.com/nature/journal/v440/n7087/full/nature04679.html#B1>,
termed climate sensitivity, is 1.5–4.5 K (ref.
2<http://www.nature.com/nature/journal/v440/n7087/full/nature04679.html#B2>).
A number of observational studies3,
<http://www.nature.com/nature/journal/v440/n7087/full/nature04679.html#B3>4,
<http://www.nature.com/nature/journal/v440/n7087/full/nature04679.html#B4>5,
<http://www.nature.com/nature/journal/v440/n7087/full/nature04679.html#B5>6,
<http://www.nature.com/nature/journal/v440/n7087/full/nature04679.html#B6>7,
<http://www.nature.com/nature/journal/v440/n7087/full/nature04679.html#B7>8,
<http://www.nature.com/nature/journal/v440/n7087/full/nature04679.html#B8>9,
<http://www.nature.com/nature/journal/v440/n7087/full/nature04679.html#B9>10<http://www.nature.com/nature/journal/v440/n7087/full/nature04679.html#B10>,
however, find a substantial probability of significantly higher
sensitivities, yielding upper limits on climate sensitivity of 7.7 K to
above 9 K (refs
3–8<http://www.nature.com/nature/journal/v440/n7087/full/nature04679.html#B3>).
Here we demonstrate that such observational estimates of climate sensitivity
can be tightened if reconstructions of Northern Hemisphere temperature over
the past several centuries are considered. We use large-ensemble energy
balance modelling and simulate the temperature response to past solar,
volcanic and greenhouse gas forcing to determine which climate sensitivities
yield simulations that are in agreement with proxy reconstructions. After
accounting for the uncertainty in reconstructions and estimates of past
external forcing, we find an independent estimate of climate sensitivity
that is very similar to those from instrumental data. If the latter are
combined with the result from all proxy reconstructions, then the 5–95 per
cent range shrinks to 1.5–6.2 K, thus substantially reducing the probability
of very high climate sensitivity.

The last sentence is critical as it looks like the doomsday scenario is
excluded. Richard Kerr for the journal Science put it this way:

 While newly climate-conscious news reporters seek signs of apocalyptic
> change in hungry polar bears and pumped-up hurricanes, evidence-oriented
> researchers are working to nail down some numbers. They are concerned with
> climate sensitivity: how much a given increase in atmospheric carbon dioxide
> will warm the world. If it's extremely high, continued emissions of
> greenhouse gases could ignite a climatic firestorm. If it's very low, they
> might merely raise the global thermostat a notch or two.
>
> Now two new studies that combine independent lines of evidence agree that
> climate sensitivity is at least moderately strong--moderate enough so that a
> really scorching warming appears unlikely. Even with the most conservative
> assumptions, says climate researcher Chris E. Forest of the Massachusetts
> Institute of Technology in Cambridge, the studies cool the maximum warming.
> And the reinforced low end of the range, he says, means continued emissions
> will fuel a substantial warming in this century.
>
...

> "Combining multiple lines of evidence is certainly the way to go," says
> Forest. An extremely high climate sensitivity "is probably less likely than
> we thought a year ago," agrees climate researcher Reto Knutti of the
> National Center for Atmospheric Research in Boulder, Colorado. More
> importantly, "we start to see a much better agreement on the lower bound,"
> says Knutti. "We can be pretty sure the changes will be substantial" by the
> end of the century, he says.
>

Maybe now we can get past the "destroying the planet" red herring. Can we
just be good stewards trying to solve a significant but not impossible
problem? One positive thing that came from the NAE debate on the enivornment
was the "what about the poor?" question coming from both sides of the
debate. Any "solution" needs to take that into consideration. By not needing
to adopt draconian solutions we can make sure the poor are not unduly
burdened. The goals expressed by the President last Saturday of being good
stewards AND preserving jobs is a worthy one for this organization.
Received on Thu Apr 27 08:21:59 2006