Divvying Up the Warming
In a MasterResource article a few months back, I walked everyone through a series of recent scientific findings and described how they cast new light on how the total amount of observed global warming to date could be divvied upon among various causes. I ultimately concluded that the high confidence that the IPCC (and later echoed by the EPA) placed on the statement that “Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations” was misplaced.
This line of reasoning was recently incorporated into statements made by Dr. Patrick Michaels when testifying before the U.S. House of Representatives, Committee on Science and Technology, Subcommittee on Energy and Environment.
During the questions and answers portion of the hearing, one of the other panelists, Dr. Benjamin Santer, quickly objected and claimed that Pat was “wrong” because he didn’t take into account the cooling influence of aerosols when determining how much observed warming should be assigned to greenhouse gases.
A day or so following the testimony, Judith Curry hosted a discussion on her blog site Climate Etc. to further examine Michaels’ logic. In her remarks introducing the thread, she too suggested that Pat was “obliged” to include sulfates in the calculation. When I stepped in to offer additional explanation, RealClimate’s Gavin Schmidt commented that he hoped I was “kidding,” and John Nielsen-Gammon of Texas A&M commented that my explanation was “nonsense.”
So with all these erudite folks claiming that Pat Michaels and I are wrong, I figured I ought to take another look into the logic behind our conclusions.
First let’s get a couple of things out of the way up front. The argument about whether or not the inclusion of sulfates is required to arrive at a logically correct conclusion has nothing whatsoever to do with the veracity and/or applicability of the scientific papers from which I’ve drawn some numbers (see my earlier post for details about these findings). I am not suggesting that there isn’t plenty of room to argue that aspect of things, just that such a discussion does not impinge on the discussion of our logic. So I’ll set aside discussion of those issues in order to focus on the topic at hand.
I’ll state the following things simply as given (if it helps I’ll add this disclaimer “The following are for illustrative purposes only”):
1) The observed warming from 1950-2009 is 0.7°C
2) 0.2°C of that is due to a warm bias in the measurements
3) This leaves 0.5°C of actual warming
4) The anthropogenic increase in well-mixed greenhouse gases is responsible for +3.0 W/m2 of extra climate forcing (positive climate forcing imparts a warming pressure on global temperatures)
5) Sulfate aerosols are responsible for -1.5W/m2 of climate forcing (note this is a negative forcing which imparts a cooling pressure)
6) Black carbon (a.k.a. “soot”) aerosols are responsible for 1 W/m2 of added forcing (warming)
7) Stratospheric water vapor changes are responsible for 0.5W/m2 of added forcing (warming)
I’ll stress again, I use these numbers to make the math cleaner. They are in the ball park, but not intended to represent the exact or even best-guess values. So please set aside any heartburn about them (also note that natural variability is not considered here, but most definitely should be in a more formal evaluation).
Also, the IPCC was unaware of numbers 2, 6, and 7 at the time it made its “most of the observed warming” statement quoted above.
In my MasterResource article and in Pat’s testimony, the logic as to how to assign various amounts of observed warming to various factors is as follows:
The 0.5°C of actual warming [in (3)] was caused by three factors, GHGs, black carbon, and stratospheric H2O. In percentage terms, GHG contributed 67%, black carbon contributed 22% and stratospheric H2O contributed 11 percent. This is calculated by taking the positive forcing from each factor and dividing it by the sum of the positive forcings. Thus for GHG, you get 3.0/(3.0+1.0+0.5)=0.67, or 67%. To determine how much of the actual temperature change that GHGs were responsible for, we multiply 0.5°C by 67% and get 0.34°C—which, we pointed out was about 50% of the “observed” warming of 0.7°C (listed in (1)). Thus, the IPCC statement rests on thin ice.
To this, Santer/Schmidt/Nielsen-Gammon/Curry cried “foul!” claiming that we have committed a sin of omission by not factoring in the negative climate forcing contributed by sulfate aerosols.
When calculating the percentage contribution from each climate forcing agent, they maintain that we should have divided each element’s contribution not by the sum of the positive forcing, but by the sum of all forcings (including negative ones). So instead of dividing by (3.0+1.0+0.5)=4.5 like we did, we should have used (3.0+1.0+0.5-1.5)=3.0.
And, they argue, that had we done that, we would have found out that GHGs contribute 100% of the warming, black carbon 33%, stratospheric H2O 17% and sulfates -50%. Therefore, even taking into account measurement errors (listed in (2) above) GHGs still contribute more than half of the observed warming. And the IPCC is right and Michaels and I are wrong (see John Nielsen-Gammon’s explanation at Climate Etc. for further evidence of this line of reasoning).
It is now my turn to reply, “Nonsense!”
Pieces Greater than the Whole?
You can’t divide a physical quantity into pieces that together are greater than the whole. Which is precisely where the latter logic leads you.
The flaw in the Santer/Schmidt/Nielsen-Gammon/Curry logic is in confusing “potential” warming with “observed” or “actual” warming.
I completely agree that using the numbers above, GHGs contribute 0.5°C*100%=0.5°C of potential warming, black carbon contributes 0.5*33%=0.17°C of potential warming, and stratospheric H2O contributes 0.5*17%=0.08°C of potential warming and that of this 0.75°C of potential warming, sulfates offset 0.25°C of it, leaving 0.5°C of observed warming.
But, in employing potential warming to divvy up observed warming is mixing apples and oranges, and leads to results that don’t make practical sense.
Take for instance this hypothetical situation:
GHG = 2 W/m2
Black Carbon = 2 W/m2
Stratospheric H2O = 2 W/m2
Sulfates = -4 W/m2
Observed Warming = 0.5°C
If this were the situation, you would arrive at the answer that GHGs are responsible for 100% of the warming AND black carbon is responsible for 100% of the warming AND stratospheric H2O is responsible for 100% of the warming. This result allows you to assign any and all warming to whatever your favorite postive forcing element is. Certainly this is creative, but it is not practical.
Doing things my way, you get the logical result that each positive forcing element contributes 33.3% to the warming and is equally responsible for what has been observed.
So there you have it, two different ways of describing the observed warming.
I’ll leave it up to you all to decide which one is the more reasonable.
The IPCC’s Take
But before I go, I’ll leave with one additional thing to consider.
Here are two successive statements in the IPCC’s Summary for Policymakers of its Fourth Assessment Report (p. 10).
The first you will recognize:
“Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”
And here is the separate bulleted statement that comes next:
“It is likely that increases in greenhouse gas concentrations alone would have caused more warming than observed because volcanic and anthropogenic aerosols have offset some warming that would have otherwise taken place.”
Clearly the IPCC recognizes the difference between observed warming (as reflected in the first statement) and potential warming (as reflected in the second statement).
The latter IPCC statement will remain true so long as the value of the net climate forcing remains less than that of the GHG forcing (which would take a significant scientific finding to overturn). I think this is the gist of what Santer/Schmidt/Nielsen-Gammon/Curry are saying, and that such a major development has not occurred.
The former statement, however, and the one whose veracity is being assessed by Pat Michaels and I (and the one highlighted by the IPCC and the EPA), is more readily falsifiable through even relatively minor tweaks to our understanding of the various influences on the climate system. And Pat and I are claiming that such minor tweaks have occurred and have led to a falsification of the IPCC statement—or at the very least, demonstration that it is undeserving of the level of confidence placed upon it by the IPCC.
I welcome on-topic comments.