[Vision2020] 1-26-20: GISS Director Schmidt: "1981 prediction from Hansen et al (1981) continues to underpredict the temperature trends"

[Ted Moffett]

Vision2020 Post: Ted Moffett
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Over and over in climate science so called "skeptic' commentary on the
Internet and elsewhere it is alleged that climate scientist and former
Goddard Institute for Space Studies director James Hansen's temperature
predictions exaggerated the future warming from anthropogenic global
warming, in a manner that largely invalidates his scientific expertise.
This is a false claim, when looking at the specifics of all of Hansen's
actual predictions, and repeating it over and over, in the style, some
might point out, of the current commander in chief, does not make it true:
https://democracyjournal.org/magazine/43/can-truth-survive-trump/
*"Republicans have, for years now, been preparing the ground for this foul
harvest: “We’re an empire now,” Karl Rove said in 2002, “we create our own
reality.” But even Rove could not quite believe what havoc such empire had
wrought in Trump." *

As NASA scientist Gavin Schmidt points out in the following commentary,
some of Hansen's predictions actually "underpredict" the increase in
temperature:

http://www.realclimate.org/index.php/archives/2020/01/update-day-2020/#comments
Excerpt from commentary below:

"To summarize, the 1981 prediction from Hansen et al (1981)
<http://www.realclimate.org/index.php/archives/2020/01/update-day-2020/#ITEM-22959-0>
continues to underpredict the temperature trends due to an underestimate of
the transient climate response. "
Update day 2020!
Filed under:

   - Climate modelling
   <http://www.realclimate.org/index.php/archives/category/climate-science/climate-modelling/>
   - Climate Science
   <http://www.realclimate.org/index.php/archives/category/climate-science/>
   - Instrumental Record
   <http://www.realclimate.org/index.php/archives/category/climate-science/instrumental-record/>
   - Scientific practice
   <http://www.realclimate.org/index.php/archives/category/scientific-practice/>

— gavin @ 26 January 2020

Following more than a decade of tradition (at least
<http://www.realclimate.org/index.php/archives/2009/12/updates-to-model-data-comparisons>),
I’ve now updated the model-observation comparison page
<http://www.realclimate.org/index.php/climate-model-projections-compared-to-observations/>to
include observed data through to the end of 2019.

As we discussed
<http://www.realclimate.org/index.php/archives/2020/01/one-more-data-point/>
a couple of weeks ago, 2019 was the second warmest year in the surface
datasets (with the exception of HadCRUT4), and 1st, 2nd or 3rd in satellite
datasets (depending on which one). Since this year was slightly above the
linear trends up to 2018, it slightly increases the trends up to 2019.
There is an increasing difference in trend among the surface datasets
because of the polar region treatment. A slightly longer trend period
additionally reduces the uncertainty in the linear trend in the climate
models.

To summarize, the 1981 prediction from Hansen et al (1981)
<http://www.realclimate.org/index.php/archives/2020/01/update-day-2020/#ITEM-22959-0>
continues to underpredict the temperature trends due to an underestimate of
the transient climate response. The projections in Hansen et al. (1988)
<http://www.realclimate.org/index.php/archives/2020/01/update-day-2020/#ITEM-22959-1>
bracket the actual changes, with the slight overestimate in scenario B due
to the excessive anticipated growth rate of CFCs and CH4 which did not
materialize. The CMIP3 simulations continue to be spot on (remarkably),
with the trend in the multi-model ensemble mean effectively
indistinguishable from the trends in the observations. Note that this
doesn’t mean that CMIP3 ensemble means are perfect – far from it. For
Arctic trends (incl. sea ice) they grossly underestimated the changes, and
overestimated them in the tropics.
CMIP3 for the win!

The CMIP5 ensemble mean global surface temperature trends slightly
overestimate the observed trend, mainly because of a short-term
overestimate of solar and volcanic forcings that was built into the design
of the simulations around 2009/2010 (see Schmidt et al (2014)
<http://www.realclimate.org/index.php/archives/2020/01/update-day-2020/#ITEM-22959-2>.
This is also apparent in the MSU TMT trends, where the observed trends
(which themselves have a large spread) are at the edge of the modeled
histogram.

A number of people have remarked over time on the reduction of the spread
in the model projections in CMIP5 compared to CMIP3 (by about 20%). This is
due to a wider spread in forcings used in CMIP3 – models varied enormously
on whether they included aerosol indirect effects, ozone depletion and what
kind of land surface forcing they had. In CMIP5, most of these elements had
been standardized. This reduced the spread, but at the cost of
underestimating the uncertainty in the forcings. In CMIP6, there will be a
more controlled exploration of the forcing uncertainty (but given the greater
spread
<http://www.realclimate.org/index.php/archives/2019/11/sensitive-but-unclassified/>
of the climate sensitivities, it might be a minor issue).

Over the years, the model-observations comparison page is regularly in the
top ten of viewed pages on RealClimate, and so obviously fills a need. And
so we’ll continue to keep it updated, and perhaps expand it over time.
Please leave suggestions for changes in the comments below.
References

   1. J. Hansen, D. Johnson, A. Lacis, S. Lebedeff, P. Lee, D. Rind, and G.
   Russell, "Climate Impact of Increasing Atmospheric Carbon Dioxide",
   *Science*, vol. 213, pp. 957-966, 1981.
   http://dx.doi.org/10.1126/science.213.4511.957
   2. J. Hansen, I. Fung, A. Lacis, D. Rind, S. Lebedeff, R. Ruedy, G.
   Russell, and P. Stone, "Global climate changes as forecast by Goddard
   Institute for Space Studies three-dimensional model", *Journal of
   Geophysical Research*, vol. 93, pp. 9341, 1988.
   http://dx.doi.org/10.1029/JD093iD08p09341
   3. G.A. Schmidt, D.T. Shindell, and K. Tsigaridis, "Reconciling warming
   trends", *Nature Geoscience*, vol. 7, pp. 158-160, 2014.
   http://dx.doi.org/10.1038/ngeo2105
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