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Tuesday, 2 May 2017

Hansen 1988



I am now going to divert away from the Eschenbach blog again and address one of the comments to this blog instead, this one concerning Hansen's 1988 paper. The comment (or at least the relevant bits) is as follows:

"...why not focus on measuring the accuracy of the forecasts based on the models.

Let's start with the Hansen paper of 1988:"

Which is this one:

https://pubs.giss.nasa.gov/docs/1988/1988_Hansen_ha02700w.pdf

"Figure 1, shows a forecast of a surface temperature rise of c. 1.1 deg C from 1980 to 2016 under scenario A"

Oh really? I wonder if he is talking about a different paper? Although the URL seems to be correct. The Fig 1 on my paper is this:


Actually, the correct graph (having looked at the Skeptical Science entry for Hansen 1988) is this one:

"Plate 4 and section 5.2.3 predicts even faster warming of the troposphere, particularly the tropical troposphere. Indeed, the troposphere warming is cited as an 'useful diagnostic' for the greenhouse effect."

This is Plate 4:

This is 5.2.3:


"Now to what happened.

1) Greenhouse gas emissions have been even higher than predicted under scenario A."

Since I can't find the Figure 1 he is referring to, I've no idea what he is referring to in this sentence. So I am just going to have to take the whole lot and treat it as one big conclusion.

"2) According to wft, surface temperatures have risen by about 0.9 deg C (Giss), from 1980 to 2016, but have since fallen as the El Nino effect dissipates. Many challenge the GISS dataset, as the level of warming from 1880 to 2000 shown by that dataset has risen from around 0.5 in 2002 to about 1.2 deg C in 2014. Surely, they knew how to read thermometers in the early 20th century?

https://realclimatescience.com/alterations-to-climate-data/

3) Troposphere temperatures have risen about 0.4 deg C for the same period.

http://woodfortrees.org/graph/gistemp-dts/from:1979.1/offset:-0.32/mean:12/plot/rss/from:1979.1/mean:12/plot/gistemp-dts/from:1979.1/offset:-0.32/trend/plot/rss/from:1979.1/trend

At the same time, estimates of Equilibrium Climate Sensitivity (ECS) to CO2 doubling have fallen to about 1.6 deg C. This is (just) within the IPCC range of 1.5-4.5 deg C, but interestingly quite a bit below the alleged danger limit of 2 deg C.

https://judithcurry.com/2016/04/25/updated-climate-sensitivity-estimates/

In conclusion, Hansen's forecast was wrong. He predicted far more warming than has actually occurred. Crucially, the troposphere has warmed less than the surface, so the prediction about the troposphere being a useful diagnostic for the greenhouse effect has been proven wrong.

Ergo, the greenhouse effect has been over-stated. Moreover, spending trillions to reduce CO2 emissions is probably the wrong solution to global warming. If far more of the warming is natural than we thought, then it would be better to spend on adaptation, since that will also help with CO2 induced warming."

Now it is my turn to make some points.

This URL (http://woodfortrees.org/graph/gistemp-dts/from:1979.1/offset:-0.32/mean:12/plot/rss/from:1979.1/mean:12/plot/gistemp-dts/from:1979.1/offset:-0.32/trend/plot/rss/from:1979.1/trend) isn't hyperlinked, and when I pasted it into Google it went nowhere, so I can't see the graph.


The URL cited above for 'realclimatescience' leads to something called "The Deplorable Climate Science Blog". According to DeSmogBlog, this is operated by Steven Goddard, alias Tony Heller. DeSmogBlog tells us that he has a Masters in Electrical Engineering (not climate science) and a BS in Geology (not climate science). His real name is Tony Heller, as Steve Goddard is a pseudonym. Heller wrote an article in the 2008 edition of The Register stating that Arctic Ice is not receding while also claiming that data from the National Snow and Ice Centre (NSIDC), which drew the opposite conclusion, was incorrect. However, he later retracted that statement:

“Dr. Walt Meier at NSIDC has convinced me this week that their ice extent numbers are solid…. It is clear that the NSIDC graph is correct, and that 2008 Arctic ice is barely 10% above last year — just as NSIDC had stated.”

Heller/Goddard can't be trusted on climate science. Therefore neither can this comment by 'Anonymous'.

Anonymous overall conclusion though is that the Hansen forecast was wrong. Let's see what others say about this.

Skeptical Science answers this point in detail:

"Hansen's 1988 results are evidence that the actual climate sensitivity is about 3°C for a doubling of atmospheric CO2."

"In 1988, James Hansen projected future warming trends. He used 3 different scenarios, identified as A, B, and C. Each represented different levels of greenhouse gas emissions.  Scenario A assumed greenhouse gas emissions would continue to accelerate.  Scenario B assumed a slowing and eventually constant rate of growth. Scenario C assumed a rapid decline in greenhouse gas emissions around the year 2000.  The actual greenhouse gas emissions since 1988 have been closest to Scenario B. As shown below, the actual warming has been less than Scenario B.

As climate scientist John Christy noted, "this demonstrates that the old NASA [global climate model] was considerably more sensitive to GHGs than is the real atmosphere."  However, Dr. Christy did not investigate why the climate model was too sensitive.  There are two main reasons for Hansen's overestimate:
  1. Scenario B, which was the closest to reality, slightly overestimated how much the atmospheric greenhouse gases would increase. This isn't just carbon dioxide. It also includes methane and chlorofluorocarbons (CFCs).
  2. Hansen's climate model had a rather high climate sensitivity parameterClimate sensitivity describes how sensitive the global climate is to a change in the amount of energy reaching the Earth's surface and lower atmosphere.
If we take into account the lower atmospheric greenhouse gas increases, we can compare the observed versus projected global temperature warming rates, as shown in the Advanced version of this rebuttal. To accurately predict the global warming of the past 22 years, Hansen's climate model would have needed a climate sensitivity of about 3.4°C for a doubling of atmospheric CO2.  This is within the likely range of climate sensitivity values listed as 2-4.5°C by the IPCC for a doubling of CO2. It is even a bit higher than the most likely value currently widely accepted as 3°C.

In short, the main reason Hansen's 1988 warming projections were too high is that he used a climate model with a high climate sensitivity. His results are actually evidence that the true climate sensitivity parameter is within the range accepted by the IPCC."

So there you go.

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