A look back at how accurate James Hansen's climate projections were from the 1980s
<a href="http://www.realclimate.org/index.php/archives/2018/06/30-years-after-hansens-testimony/" target=n>RealClimate.org</a> recently took a look back at the first transient climate projections using global climate models by Dr. James Hansen from 30 years ago.
The climate projections were based on three potential emission scenarios with A, being the worst case.
Looking at the chart below, which shows the projected temperature changes for each scenario from 30 years ago versus what has actually happened (observations).
<img src="https://vortex.accuweather.com/adc2004/pub/includes/columns/climatewx/2018/590x495_06291750_screen-shot-2018-06-29-at-1.30.11-pm.png"/>
As you can see, the observed global temperature increase over the past 30 years has been closest to scenario C and at times, between scenario C and B.
The projected changes (1984-2017) for each scenario were the following:
Scenario A: 0.33+/- 0.03 deg. C. per decade
Scenario B: 0.28+/-0.03 deg C. per decade
Scenario C: 0.16+/-0.03 deg C. per decade
The actual observed temperature change for the 1984-2017 period was 0.19+/-0.03 deg C. per decade for NASA GISTEMP and 0.21+/- 0.03 deg C. per decade for Cowtan Way method.
In order to make these projections, the climate models with their three scenarios had to project the changes in the various greenhouse gas emissions over the period.
From the graphs below, we can see that the climate models clearly overestimated the projected increases in nitrous oxide and methane for scenarios A and B. This may be a reason that scenarios A and B had a greater rate of temperature change than what they should have had. According to the report, it is clear that the forcings in scenario A and B will have overshot the real world.
In terms of carbon dioxide, the model projections were pretty much on the mark.
<strong>Key excerpt from the RealClimate article.......</strong>
<em>Can we make an estimate of what the model would have done with the correct forcing? Yes. The trends don’t completely scale with the forcing but a reduction of 20-30 percent in the trends of Scenario B to match the estimated forcings from the real world would give a trend of 0.20-0.22ºC/decade – remarkably close to the observations.
</em>
Overall, the climate models showed positive skill and all scenarios gave better predictions than a forecast based on persistence.
The graph below shows the observed warming versus projected temperatures only influenced by natural drivers.
<img src="https://vortex.accuweather.com/adc2004/pub/includes/columns/climatewx/2018/590x379_06291859_screen-shot-2018-06-29-at-2.59.32-pm.png"/>
As you can see, with just natural forcings alone, the global temperature trend would have been close to neutral if not slightly lower. Greenhouse warming is clear. Hansen was correct in 1988 that we were already seeing the effects of man-made global warming.
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Images courtesy NASA and RealClimate.org. Report credit to Gavin Schmidt.
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