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If it isn't relevant, it begs the question why so many people, professionals in the field, still busy themselves with the topic.

It is relevant, exactly because professionals keep bringing it up, and more importantly,  because the issue on climate sensitivity has not decisively been settled (and also because scientists have staked part of their scientific career on it). Hansen's work in the late eighties is therefore essential for "proofing" ideas on climate sensitivity - it's an important test case for our ability in predicting the future.

As noted previously, Hansen considers his 1988 estimates of climate sensitivity now too large, and now puts the number for sensitivity lower. Whether he's wrong on that number now being too low, or whether it is still too high, the earth will find out as long as we don't switch to a society that is carbon neutral. In, say, 2028 it is likely we will be able to say something useful whether his 3.3 degrees estimate comes closer to describing reality.

The political problem of course is our political strategies for adaptation should be based on that sensitivity number.

by Nomad on Mon Feb 7th, 2011 at 10:32:18 AM EST
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The political problem of course is our political strategies for adaptation should be based on that sensitivity number.

But in the case of severe negative consequences and some uncertainty about the number, should we not proceed on the basis of a somewhat worse case number? Else we are taking the position: "We can't know for sure until it is too late, so we shouldn't do anything because we might be doing the wrong thing."

"It is not necessary to have hope in order to persevere."
by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Mon Feb 7th, 2011 at 12:17:32 PM EST
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I try to be philosophically about it. Doing nothing is an ultimate position - though it is not a position I personally take. However, also doing something is politically and economically motivated, and doing something comes in gradations of how much we should do.

I was talking about adaptation in my previous post. Doing something also includes mitigation - adaptation is not the same. Even when we skip adaptation altogether, we can always do mitigation - although we're mostly choosing not to. Yet: even for mitigation the question ultimately spirals into a debate on how much mitigation over what period of time is wise. And also that is tied to climate sensitivity.

by Nomad on Mon Feb 7th, 2011 at 02:56:52 PM EST
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to one magical number : CO2 sensitivity.

We know it's not that simple. Allowing the discussion to be framed around it : "Hansen's CO2 sensitivity from 1988 is wrong, so the world is not warming all that fast" is a huge mistake.

CO2 is the biggest forcing, but a number of others, both positive and negative, are of the same order of magnitude. As a first approximation, the other forcings more or less cancel each other out, leaving a net forcing roughly equivalent to that of CO2. This does NOT mean that we can disregard other forcings!

In particular, the photochemical and particulate coolings are tied to levels of dirty industrial activity, and have varied a great deal in the past few decades. In my modest opinion, their decline in the 80s and 90s (cleanup of OECD heavy industry, decline of ex-Soviet bloc industry) steepened the global warming curve over that period; this decline has since been swamped by the dirty industry of China, India, et al, which is currently flattening the curve.

If one can't abstract that sort of influence (and especially if one declines to try), I find the idea of deriving a CO2 sensitivity from observed temperatures to be downright silly.

It is rightly acknowledged that people of faith have no monopoly of virtue - Queen Elizabeth II

by eurogreen on Mon Feb 7th, 2011 at 12:41:54 PM EST
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We know that our way of living introduces increased green house forcings to our planetary system. Uncertain is: "How much will that warming be?" And this is an essential question for policy-makers (who don't like uncertainties), and it is directly tied to this one number, whether we like it or not.

Models and measurements are the only two methodologies climate scientists have got for understanding the earth's sensitivity. And it is so focused on CO2 since other forcings are converted to CO2 equivalents. There is nothing dangerous to this choice, it is one of practicality - for as far I understand the matter. Understanding the sensitivity for CO2, should enable correlation to other forcings and reach a total sensitivity for the earth. It is true that the role of particulates, soot, clouds may play a significant influence, and they too remain relatively large unknowns. Perhaps a correlation to atmospheric temperatures may turn out to be a foolish exercise altogether, and are we better off with oceanic warming. I can't say. This is why this topic is such a battleground.

Even when sensitivity calculations are fraught with uncertainties, this does not reduce the importance of understanding CO2 sensitivity - because it largely determines the severity and rate of anthropogenic climate change, and hence its understanding underpins political solutions in response to a warming planet.

On the focus on CO2, we are in agreement. I've long argued at ET that CO2 as the main greenhouse bogeyman is a too narrowed and too limited approach of the issue at stake. As there are plenty articles in the science literature mapping out other important forcings and influences, this implies that our approach to anthropogenic climate change should also be multifaceted, and not strictly limited to the mitigation of greenhouse gases.

A positive corollary of this is that our world's fate is not decided purely by rising atmospheric CO2 concentration. Neither should it trivialize the role it does play.

by Nomad on Mon Feb 7th, 2011 at 05:33:45 PM EST
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I find the Lyman et al (2010) paper eurogreen cited and the included graph shown below to be particularly concerning. If this is a reasonable sample of the top 700 meters of the ocean it would seem something is likely happening. After all, the oceans are the biggest source of thermal inertia in the entire weather system and more ocean heat leads to greater energy in the atmospheric system. Is there any basis for making an argument that this sort of change is cyclical over a few decades or less?

And what about the rate of loss of Arctic sea ice:

From the discussion at the first link:

In this case, the match is not very good, and possibly getting worse, but unfortunately it appears that the models are not sensitive enough.

"It is not necessary to have hope in order to persevere."
by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Mon Feb 7th, 2011 at 05:51:48 PM EST
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