CCNet
Editor: Benny Peiser Faculty
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Forecasting the Earth’s Temperature
David Whitehouse
9
September 2009
The recent spate of
scientific papers that are attempting to predict what the earth’s temperature
might be in the coming decades, and also explain the current global temperature
standstill, are very interesting because of the methods used to analyse
temperature variations, and because they illustrate the limitations of our
knowledge.
Recall that only one
or two annual data points ago many scientists, as well as the most vocal
‘campaigners,’ dismissed the very idea that the world’s average annual
temperature had not changed in the past decade. Today it is an observational
fact that can no longer be ignored. We should also not forget that nobody
anticipated it. Now, post facto, scientists are looking for an explanation, and
in doing so we are seeing AGW in a new light.
The main conclusion,
and perhaps it’s no surprise, to be drawn about what will happen to global
temperatures is that nobody knows.
The other conclusion
to be drawn is that without exception the papers assume a constantly increasing
AGW in line with the increase of CO2. This means that any forecast will
ultimately lead to rising temperatures as AGW is forever upward and natural
variations have their limits. But there is another way of looking at the data.
Instead of assuming an increasing AGW why not look for evidence of it in the
actual data. In other words let the data have primacy over the theory.
Lean and Ride (2009)
try to isolate and analyse the various factors that affect decadal changes in
the temperature record; El Nino, volcanic aerosols, solar irradiance and AGW.
Their formula that links these factors together into a time series is quite
simple (indeed there is nothing complicated about any of the papers looking at
future temperature trends) though in the actual research paper there is not
enough information to follow through their calculations completely.
El Nino typically
produces 0.2 deg C warming, volcanic aerosols 0.3 deg C cooling on short
timescales, solar irradiance 0.1 deg C (I will come back to this figure in a
subsequent post) and the IPCC estimate of AGW is 0.1 deg C per decade.
It should also be
noted that natural forces are able to produce a 0.5 deg C increase, although
over a longer period. The 0.5 deg C warming observed between say 1850 and 1940
is not due to AGW.
The temperature
increase since 1980 is in fact smaller than the rise seen between 1850 - 1940,
approx 0.4 deg C. This took place in less than two decades and was followed by
the current standstill. A fact often overlooked is that this recent temperature
increase was much greater than that due to the postulated AGW effect (0.1 deg C
per decade). It must have included natural increases of a greater magnitude.
This is curious. If
the recent temperature standstill, 2002-2008, is due to natural factors
counteracting AGW, and AGW was only a minor component of the 1980 -1998
temperature rise, then one could logically take the viewpoint that the increase
could be due to a conspiracy of natural factors forcing the temperature up
rather than keeping the temperature down post 2002. One cannot have one
rule for the period 2002 - 2008 and another for 1980 -1998!
Lean and Rind
estimate that 73% of the temperature variability observed in recent decades is
natural. However, looking at the observed range of natural variants, and their
uncertainties, one could make a case that the AGW component, which has only
possibly shown itself between 1980 - 98, is not a required part of the dataset.
Indeed, if one did not have in the back of one’s mind the rising CO2
concentration and the physics of the greenhouse effect, one could make out a
good case for reproducing the post 1980 temperature dataset with no AGW!
Natural variations dominate
any supposed AGW component over timescales of 3 - 4 decades. If that is so then
how should we regard 18 years of warming and decades of standstills or cooling
in an AGW context? At what point do we question the hypothesis of CO2 induced
warming?
Lean and Rind (2009)
look at the various factors known to cause variability in the earths
temperature over decadal timescales. They come to the conclusion that between
2009-14 global temperatures will rise quickly by 0.15 deg C - faster than the
0.1 deg C per decade deduced as AGW by the IPCC. Then, in the period 2014-19,
there will be only a 0.03 deg C increase. They believe this will be chiefly
because of the effect of solar irradiance changes over the solar cycle. Lean
and Rind see the 2014-19 period as being similar to the 2002-8 temperature
standstill which they say has been caused by a decline in solar irradiance
counteracting AGW.
This should cause
some of the more strident commentators to reflect. Many papers have been
published dismissing the sun as a significant factor in AGW. The gist of them
is that solar effects dominated up to 1950, but recently it has been swamped by
AGW. Now however, we see that the previously dismissed tiny solar effect is
able to hold AGW in check for well over a decade - in fact forcing a
temperature standstill of duration comparable to the recent warming spell.
At least the
predictions from the various papers are testable. Lean and Rind (2009) predict
rapid warming. Looking at the other forecasts for near-future temperature
changes we have Smith et al (2007) predicting warming, and Keenlyside et al
(2008) predicting cooling.
At this point I am
reminded that James Hansen ‘raised the alarm’ about global warming in 1988 when
he had less than a decade of noisy global warming data on which to base his
concern. The amount of warming he observed between 1980 and 1988 was far
smaller than known natural variations and far larger than the IPCC would go on
to say was due to AGW during that period. So whatever the eventual outcome of the
AGW debate, logically Hansen had no scientific case.
There are
considerable uncertainties in our understanding of natural factors that affect
the earth’s temperature record. Given the IPCC’s estimate of the strength of
the postulated AGW warming, it is clear that those uncertainties are larger
than the AGW effect that may have been observed.
References:
Lean and Rind 2009,
Geophys Res Lett 36, L15708
Smith et al Science
2007, 317, 796 - 799
Keenlyside et al
2008, Nature 453, 84 - 88