Climate Forecasts in Line
climate forecasting business is always couched in uncertainty. One model shows
this, another one shows that. One federal scientist says one thing, another from
XYZ subagency PDQ (Federal Building J, subbasement G-5c) says something else.
Blah blah blah. The only thing they all seem to agree on is that, if you think
the climate's bad now, just wait 50 or 100 years. Yet they rarely get
fact, not all climatologists feel that way, as readers of WCR well know.
In their wittily titled 2000 book, The
Satanic Gases, WCR Editor Patrick
Michaels and Contributing Editor Robert C. Balling Jr. did something that is
rare for climatologists—they made a few predictions about future climate.
Here's an excerpt:
the next 50 years] scientists will confirm that although the functional form of
the climate models is correct, the amount of warming is already dictated by
nature...In surface temperatures for the last third of this century [remember,
the 2000 is technically still the 20th century], a trend has been established,
and it is near or below the low limit of the model calculations, but it is a
straight line, as the models predicted. There is no reason to suspect this is
going to suddenly stop."
earth's average surface temperature will warm 0.65°C to 0.75°C by
that is controversial stuff. Michaels and Balling are saying that the planet's
warming is linear and therefore predictable. This is no doubt upsetting to the
climate modeling community (and their funding agencies) since—despite the
alleged sophistication of the model's various cloud and surface
parameterization schemes, the integration of the biosphere, and the development
of dynamic, multilayered circulating oceans—those models' prediction of
future climate is the same one you or I could make using a straight edge to
connect the dots (cost: zero). In Prediction 2, given that we now know the
slope, the temperature forecast for the future is easy. We've experienced
about 1.0°C of global warming since pre-industrial times. So by the year
2050, the authors us, the total warming will be a nonastounding 1.6°C to
modelers claim there are far too many uncertainties to make so bold a forecast.
It's difficult to know the real climate sensitivity (how much warming you get,
say, per slug of CO2), how much heat the oceans are storing, the actual cooling
impact of sulphate aerosols, and so forth. Reality is certainly much more
a new paper in Nature by England's
Myles Allen, cowritten by modelers from Hadley Center (the UK Meteorological
Office), the Max-Planck Institut in Germany, and the Geophysical Fluid Dynamics
Lab in Princeton. "Quantifying the Uncertainty in Forecasts of Anthropogenic
Climate Change" is what they call it, and well, they've done just that, and
darn if the numbers don't match Michael's forecast!
to Allen, "We expect global mean temperatures in the decade 2036–46 to be
1–2.5[°C] warmer than in pre-industrial times...This range is relatively
robust to errors in the models' climate sensitivity, rate of oceanic heat
uptake or global response to sulphate aerosols as long as these errors are
persistent over time." Furthermore, they are 90 percent confident of this
prediction (in other words, the real warming would fall within their projected
range nine out of 10 times if the "experiment" of adding greenhouse gases at
the projected rate were repeated). Michaels and Balling's more refined numbers
are dead smack in the middle of this range.
kind of confident estimate is not possible using current climate model
forecasts. Rather, Allen and colleagues arrived at it using simple statistics,
comparing observed temperature patterns for each decade from 1946 to 1996 with
the decadal projections of various climate models.
we all know the model projections of current climate are not correct. So, for
each model and for each decade, Allen and colleagues calculated a "scaling
factor" that best brings the projections in line with the data, using basic
multiple regression. And when that scaling factor is used to correct the model
and then project the future climate change, it turns out that the relationship
Nature's "News and Views"
section (the layperson's introduction to each article), Andrew Weaver and
Francis Zwiers note that:
beauty of this result is that it is independent of the ‘climate sensitivity'
of the model and the rate at which that warming happens. In particular, it is
independent of the rate of oceanic heat uptake, which varies between models with
different representations of ocean physics. More complicated models have indeed
exhibited a linear response to changes in radiative forcing caused by increases
in atmospheric greenhouse gases and sulphate aerosols. For example, when
separate model runs with twentieth-century levels of greenhouse gases and
aerosols are combined, the result is similar to runs which included them
course, a lot of other factors could modify the model's forecasts: Volcanic
eruptions, variability in the sun's output, and ozone depletion are all
legitimate components of climate. But Allen's results suggest that it
doesn't really matter if some or all of these components are missing from the
models, because any feedbacks they might have on temperature have already been
factored into the projection using their approach.
also clear that the various model projections of future temperatures are too
high. The solid line in Figure 1 shows the mean temperature increase of four
climate models; the dashed line, the newly scaled, best-fit projection of future
temperatures. The most likely result, 1.4°C of warming by 2040, fits Michaels
and Balling's prediction precisely.
Figure 1. The mean temperature increase of four climate
models (solid line); the newly scaled, best-fit projection of future
temperatures (dashed line); Allen's most likely result, 1.4°C of
warming by 2040, exactly matches Satanic Gases'.
if it's true we are faced with a future in which we can actually predict the
magnitude of warming, and it turns out to be a small number, can we stop fussing
about our climatic future? Abandon trying to implement a Kyoto Protocol that
would have the net effect of reducing global temperatures by 2050 by only
0.07°C while simultaneously wreaking havoc on the global economy?
demise of the Kyoto Protocol is one forecast that would serve us all quite well.
P.J., and Robert C. Balling Jr., 2000. The
Satanic Gases, Cato Institute Press.
et al., 2000. Quantifying the uncertainty in forecasts of anthropogenic climate
change, Nature, 407,617–620.
A.J. and Zwiers, F.W., 2000. Uncertainty in climate change, Nature,