Chris,
I'm sure others will cover this, but there is apparently still some
confusion in your mind.
The point, as I mentioned in my original email, is that the climate
sensitivity in its fundamental definition does not refer to CO2. It is
the equilibrium surface temperature change per unit of radiative forcing.
As it happens, the forcing for CO2 (as noted by others) is logarithmic
in concentration. In other words, there is the same forcing for any
doubling of CO2 concentration -- from 300 to 600 ppm, or 500 to 1000
ppm, or 700 to 1400 ppm, etc.
Because of this we often "define" the sensitivity as the equilibrium
surface temperature change for a CO2 doubling. Note that the sensitivity
is NOT linear in terms of CO2 concentration. It is only linear as a
function of radiative forcing.
Climate models, of course, take this fully into account. Furthermore,
sophisticated climate models do not specify how the climate sensitivity
varies as conditions change. These variations are internally generated
based on the fundamental physics that is in the models.
Tom
++++++++++++++++++++++++++++++++++
On 2/21/2012 8:35 AM, Robert Chris wrote:
Ken
Thanks for replying to my email. You were right. The Google Group
worked much quicker than I'd anticipated.
There's quite a lot of technical material here for a humble social
scientist to digest. On a quick scan the message I get is that the
forcing is logarithmic but climate sensitivity is linear because of
compensating feedbacks in the climate system. Although it does seem
to me that if the forcing is logarithmic there must come a point where
the absorptive capacity of the GHGs is so small that incremental
emissions will not produce any more global warming. Moreover, there
must be a point where the oceans are also largely saturated and take
up little more CO2. However, the point where climate sensitivity does
reduce to close to zero, might not be reached before we're all fried
and the oceans are nigh on dead. If that brief extrapolation makes
sense, has anyone attempted to estimate where those limits might be?
What has jumped out at me from these replies is the Wasdell paper. He
is highlighting the distinction between the Charney and ESS approaches
to the quantification of climate sensitivity. This is not an academic
peer-reviewed paper although he does refer to several but if he is
correct that climate sensitivity now looks like it might be closer to
10degC rather than 3degC, that should send alarm bells ringing that I
certainly don't hear. Is that because ESS is not yet sufficiently
robust for it to replace Charney; simply because the material has yet
to be published, or because even though CS might be three times higher
than previously thought, the time lag before the equilibrium is
reached is such that things won't warm up that much faster in the
short term and therefore no one's too concerned about it yet?
But if ESS does provide a more accurate valuation of CS and it is
about 10degC, what policy implications might that have for the timing
of geoengineering deployments?
Robert Chris
On Feb 20, 9:48 pm, Ken Caldeira<[email protected]>
wrote:
The attached papers are relevant.
On Mon, Feb 20, 2012 at 11:32 AM, David Mitchell<[email protected]>wrote:
From paleoclimate data with a geologic time-scale, climate sensitivity may
be ~ 2.7 times larger than the Charney value of ~ 3 deg. C ( climate
feedback factor 0.75 deg. C/W m2; characteristic of the fast feedbacks in
GCMs) based on a recent paper from Jeff Kiehl:
Kiehl, J., 2011: Lessons from Earth's Past. Science, Vol. 331, 14 Jan.,
158-159, DOI: 10.1126/science.1199380.
David Wasdell has written about climate sensitivity featuring the work of
Kiehl and others (see attached) but I don't know whether that document ever
got published. Wadham's paper discusses some of the arguments Mike has
made.
These papers do not indicate or suggest that climate sensitivity may save
us from global warming.
David Mitchell
On 2/20/2012 8:57 AM, Mike MacCracken wrote:
Just to be clear--
The radiative forcing due to CO2 increases is logarithmic--that is, the
radiative forcing going from 300 to 600 ppm is the same as going from 600
to
1200 ppm. Thus, the forcing due to the rising CO2 concentration does
decrease on a per ppm basis.
However, forcing is not sensitivity, and like Tom Wigley, I recall papers
that have done a good bit of testing of plausible changes in concentration
and the sensitivity (that is, the temperature change for a doubling of the
CO2 concentration) is, near as it can be estimated, pretty linear. At
lower
temperatures one may have more snow/ice albedo feedback, but at higher
temperatures one has more water vapor and, very likely, carbon cycle
feedback (carbon cycle feedback including thawing permafrost and releasing
CO2/CH4, out-gassing of CO2 from warmer ocean, higher airborne fraction as
ocean overturning slows, etc.). Given the warmth of the Cretaceous, it is
hard to be sanguine about adding more and more CO2 to the atmosphere. And
given the heat of Venus, which absorbs less solar per square meter than
the
Earth even though closer to the Sun, it seems really difficult to argue
that
adding greenhouse gases to an atmosphere leads to a plateau in the
response.
Mike MacCracken
On 2/20/12 7:38 AM, "Tom Wigley"<[email protected]> wrote:
Sensitivity is the equilibrium change in global-mean temperature per
unit of radiative forcing. Linearity has been demonstrated up to much
higher forcings than will ever be reached by even the most pessimistic
scenarios.
Early IPCC reports might cover this. I recall work by Kiehl on this back
in the mid 1980s -- too far back to recall the reference.
Tom.
+++++++++++++++++
On 2/20/2012 5:28 AM, Robert Chris wrote:
I am engaged in discussion with a modestly prominent climate skeptic
who argues that global warming isn't a problem because as CO2
concentrations rise climate sensitivity reduces. I recall coming
across this notion before but I don't know how much peer-reviewed work
has been done on it. I'd appreciate some help with references to peer-
reviewed papers that address the idea that climate sensitivity may be
logarithmic rather than linear so that as atmospheric CO2
concentrations rise the effective climate sensitivity reduces and
discuss the likely levels at which this reduction becomes significant
in terms of reducing the GWP of CO2.
-----------------------------
Robert Chris
The Open University
[email protected]
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Reisinger_et_al_ERL2011.pdf
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Caldeira_Kasting_Nature1993.pdf
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Pagani_et_al_Science2006.pdf
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Matthews_et_al_Nature2009.pdf
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