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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
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.
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.
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