Posted by
Aaron McLucas on Friday, May 09, 2008 3:07:13 PM
I was slightly confused by the Nature article, and now I am thoroughly confused after reading this article from
Grist. The graphing used is suspect, confusing and unnecessary. Here is their explanation on how the graph works:
(Nice chart huh, even though you are looking at a total change in the chart of 1 degree over 65 years.)
Let me try to explain.
The first thing to note about the figure -- indeed, one major source of confusion -- is that "each point represents a ten-year centered mean." That is, each point represents the average temperature of the decade starting 5 years before that point and ending 5 years after that point.
Second, the red line is the actual global temperature data from the U.K.'s Hadley Center for Climate Prediction and Research. Why does the red line stop in 1998 and not 2007? Again, it is a running 10-year mean, and the authors use data from a Hadley paper that ends around 2003, I believe, so they can't do a ten-year centered mean after 1998.
Third, the black line is one of the IPCC scenarios, A1B. It is a relatively high-CO2-growth model -- but actual carbon emissions since 2000 have wildly outpaced it.
Fourth, the solid green line is the "hindcast" of the authors -- how well their model compares to actual data (and the A1B scenario). It is then extended (in dashes) through 2010 and finally to 2025, where it meets up with A1B, since their model only imposes decadal variability on the inexorable climb of human-caused global warming.
(Fifth, the short purple line is with radiative forcing [i.e., greenhouse
What? Now I am certainly confused. Why did they use this one particular study from the IPCC for the black line? Did it fit good in the graph? Also, I understand why the red-line stops in 1998, but for purposes of explaining that to the public, why did they do it that way?
I think it was intended to confuse.
Our results suggest that global surface temperature may not increase over the next decade, as natural climate variations in the North Atlantic and tropical Pacific temporarily offset the projected anthropogenic warming.
But what they mean by that statement is not what a simple reading of that sentence would suggest: They do not mean that "the global surface temperature may not increase over the next ten years starting now." What they mean is what the lead author, Dr. Noel Keenlyside, wrote me last night when I asked for a clarification:
Thus, based on our results we don't expect an increase in the mean temperature of the next decade (2005-2015).
They are predicting no increase in average temperature of the "next decade" (2005 to 2015) over the previous decade, which, for them, is 2000 to 2010! And that is, in fact, precisely what the figure shows -- that the 10-year mean global temperature centered around 2010 is the roughly the same as the mean global temperature centered around 2005.
Here is my reading of that statement, in a much simplified form:
Based on our studies using the science we use, since temperatures have been going down steadily over the last five years, we are giving you (the Al Gore's of the world) an excuse to cover your tracks while not abandoning global warming and the push to get funding for our climate projects.
That's just what I read anyway?!
Summing it all up:
He reiterates that his paper is not designed to make such detailed year-by-year predictions. Indeed, the paper was designed to show that any such predictions are complicated by decadal-scale climate factors.
Complicated is an understatement.