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Key mistake in his 1988 estimates was using a higher climate sensitivity. With the current uptick in global temperatures last year, we're back into Hansen's scenario C, but just barely.
What always frustrates me in this particular topic that's beaten to death, particularly on sceptic websites, is that I can't seem to find a graph like this with standard deviations added. And of course Hansen has made other projections since.
Yet, at first glance, the temperature records are indicating so far that Hansen's 1988 projections weren't too good.
And no, the argument that scenario C matches global temperatures, doesn't hold water. Because scenario C was based on drastic reductions of atmospheric greenhouse gasses between 1990 and 2000. That didn't happen. In terms of greenhouse gasses exhaust, we follow scenario B. (Hansen first calculates how much extra forcings the earth will get, and bases temperature projections on these calculations.)
So in forcings we track Hansen's scenario B, but even without reductions in GHG, the global temperature barely reaches the line of scenario C at this point. Still, as previously said, we actually can't be really sure about this as the picture doesn't include standard deviations.
Even the recent Real Climate post you link to admits that Hansen's preferred scenario, B, is running out of sync with measurements. (Actually, Hansen stated in 1987 that he was thinking it would turn out to be in between A and B.)
Therefore, the issue is climate sensitivity. Again. It's the single most important figure in climate science, and it is not settled. Despite Gavin's proclamation that the earth's sensitivity lies around 3.3 degrees C, discussion about that figure is far from over. That's the figure the Hansen camp prefers at the moment. (And Hansen's estimates have been climbing down since 1988.)
But is it excessive, or is it being masked by other forcings? Cooling factors (particularly photochemical smog, and particles suspended in the atmosphere) are much better understood nowadays; and they are short-term phenomena. When China and India clean up their industry (and they will, and sooner than we expect), the cooling effect will greatly diminish; the particles will precipitate out of the atmosphere, but the CO2 will still be there. And the net CO2 forcing will make the temperature trend much steeper. It is rightly acknowledged that people of faith have no monopoly of virtue - Queen Elizabeth II
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.
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.
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.
Gavin's proclamation that the earth's sensitivity lies around 3.3 degrees C, discussion about that figure is far from over.
Err, no ... what he actually wrote was (reply to comment #1 at link to RealClimate):
We can even go one step further - what climate sensitivity would have given a perfect forecast given the actual (as opposed to projected) forcings? The answer is 3.3 deg C for a doubling of CO2.
So the 3.3 deg C is a retrospective determination of the value that would have (does) give the best agreement between the 1988 model and the current observations. As such, discussion about that precise figure is over. It is a cold, hard mathematical fact (unless he made an error in running the calculation). Even future measurements do not change this value, because it is specifically an evaluation of an old model against the data available as of early 2011.
A more appropriate comment regarding current opinion on the value for climate sensitivity is in the body of the post:
... and the old GISS model had a climate sensitivity that was a little higher (4.2ºC for a doubling of CO2) than the best estimate (~3ºC).
"best estimate" and "~" acknowledge uncertainty and do not indicate a belief that discussion is over.
The big hangup about climate sensitivity has never been much about the sensitivity of purely CO2 - which is something that can be calculated and measured in lab conditions (it's about 1 degree for doubled CO2). The issue always has been about the effects of feedbacks, positive and negative, to increasing atmospheric CO2, in addition to the higher forcing of increased CO2.
Hansen, and Schmidt, assign certain weights to certain forcings based on certain estimates and certain model runs to explain the observed trendline, thereby finding a certain sensitivity.
There's mathematics involved, but it is far from being all hard.
It's impossible to accurately pin-point a starting date of a transition in a Complex System until after the transition is completed. The abnormal, to expectation, weather and weather patterns and patternings since 2007/2008 suggests, to me, we're already 2 or 3 years into it. She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre
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