Analysis of a carbon forecast gone wrong: the case of the IPCC FAR

Re-posted from judithcurry.com, an article by Alberto Zaragoza Comendador

The IPCC’s First Assessment Report (FAR, released in 1990) made forecasts or projections of future concentrations of carbon dioxide that turned out to be too high.

From 1990 to 2018, the increase in atmospheric CO2 concentrations was about 25% higher in FAR’s Business-as-usual forecast than in reality. More generally, FAR’s Business-as-usual scenario expected much more forcing from greenhouse gases than has actually occurred, because its forecast for the concentration of said gases was too high; this was a problem not only for CO2, but also for methane and for gases regulated by the Montreal Protocol. This was a key reason FAR’s projections of atmospheric warming and sea level rise likewise have been above observations.

Click the link to read the full article. The key points are:

  • FAR’s Business-as-usual scenario over-projected forcings for the 1990-2017 period by 55%. This would mean a difference of 0.59 w/m2 between FAR and reality.
  • Lower-than-expected concentrations of Montreal Protocol gases explain about 0.19 w/m2 of the difference. With the big caveat that Montreal Protocol accounting is a mess of CFCs, HCFCs, HFCs, stratospheric ozone, and perhaps other things I’m not even aware of.
  • FAR didn’t account for tropospheric ozone, and this ‘unexplains’ about 0.07 w/m2. So there’s still 0.45-0.5 w/m2 of forcing overshoot coming from something else, if Hausfather’s numbers are correct.
  • N2O is irrelevant in these numbers
  • CO2 concentration was significantly over-forecasted by the IPCC, and that of methane grossly so. It’s safe to assume that methane and CO2 account for most or all of the remaining difference between FAR’s projections and reality.
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