Brookhaven National Laboratory: Why Hasn’t Earth Warmed as Much as Expected?

It's just cooling masking the effects of al gore warming!!!111!one

 

Here's how I read that piece of research, based only on press release and abstract. (I'd have to pay $35 or so to download it.)

Take the forcings as estimated by NASA GISS.

[IMGLINK]http://data.giss.nasa.gov/modelforce/efficacy_fig28.gif[/IMGLINK]

If you only looked at the red GHG bars, and ignored the aqua aerosol bars, you'd predict a very large forcing, and consequently would predict much more warming than has actually occurred.

So, focusing on GHGs alone yields too high a projected temperature change. That seems like a pretty reasonable conclusion.

They then go on to parse out the difference between the actual, observed temperature change and the GHG-only temperature change. Per the abstract, of the difference, they attribute no more than 15% to natural variation, 25% to thermal lag. The majority of the discrepancy is due to some combination of uncertainties in the impact of aerosols and the estimated climate sensitivity.

Not being an expert on the impact of aerosols, that all sounds fairly reasonable to me.

Their take on the importance of this is that you need to pin down these uncertainties better if you want to get a grip on how much longer we can continue to emit GHGs without exceeding some reasonable temperature threshold limit.

But, boy howdy, if you're looking for a comforting message, I think you're looking in the wrong place. What's the estimated spread of time available to avoid their threshold level of warming (which looks to me to be in line with the consensus of no more than 2 degrees C additional warming from now):

From the press release:

"The paper describes three scenarios: If Earth’s climate sensitivity is at the low end of current estimates as given by the Intergovernmental Panel on Climate Change, then the total maximum future emissions of heat-trapping gases so as not to exceed the 3.6° threshold would correspond to about 35 years of present annual emissions of CO2 from fossil-fuel combustion. A climate sensitivity at the present best estimate would mean that no more heat-trapping gases can be added to the atmosphere without committing the planet to exceeding the threshold. And if the sensitivity is at the high end of current estimates, present atmospheric concentrations of heat-trapping gases are such that the planet is already committed to warming that substantially exceeds the 3.6° threshold."

Message: Depending on the climate sensitivity chosen, either we're already way screwed, we're about to be screwed, or we've got 35 years to get our act together to avoid being screwed (IF emissions do not trend upward, as they have been doing.)

That's all still in the very long run - it would still take quite a while to reach the additional 2 degrees C.

That's substantially more pessimistic than (e.g.) Hansen's estimate that holding GHGs at current levels would yield just a 1 degree C increase in the long run. Yet consistent (for the last two scenarios) with his judgment that we need to reduce emissions immediately.

In fact, that's so pessimistic that I may spring for the article if I don't see a detailed analysis of it on the internet soon.

Yet the point is well taken and dovetails with the article on ship trails posted earlier. Over the very long term, what we've seen to date is GHG increases correlated with aerosol increases. So the historical temperature trends reflect the (by chance) association of the two. If we move to an era of increased GHG concentrations but no higher atmospheric aerosol content, then ... you could plausibly get to a more pessimistic estimate of the time left to avoid the 2 degree C threshold.

Not a cheery picture. I think I'll side with the optimists on this one.

 

Looks like great fodder for misinterpretation.