The primary argument for the attribution of recent warming to anthropogenic increases in CO2 is
due to the Hadley Centre, the UK Met Office’s climate research group. Their argument is quite
simple. It begins with the assumption that their climate model is correct. They then subject their
model to forcing by volcanos and solar variations, and find that they can replicate the observed
global mean temperature until about 19761, but that the increase in global mean temperature of a
few tenths of a degree since then could not be reproduced without additional climate forcing.
This additional forcing, they assert, is due to man. The argument is based fundamentally on the
assertion that the model is correct. The confirmation for this assertion is that the model was
capable of replicating earlier changes in global mean temperature in the instrumental record for
the period 1880 to 1976. Hence, they are confident that the attribution of the recent warming to
man is correct, and that the forecasts for future warming are correct as well. Although this
sounds simple enough, the problems with the argument are huge, and leave one without any
logical grounds to stand on. The following are the major problems (and all of them have already
been noted by the IPCC):
1. Forcing by volcanoes and solar variability are essentially unknown. Hence, the ability to
replicate observations prior to about 1976 depends on arbitrary choices which are tantamount to
‘tuning.’ The claim that models are capable of replicating the past record is really a statement
that the models can be adjusted to replicate the record. Even with such adjustments, the models
fail to replicate regional changes in climate (such as the fact that much of the continental US has
been cooling over the past 60 years)..
2. Although it is claimed that models cannot replicate global mean surface temperature since
about 1976 without additional forcing, it is found that the model response to increasing CO2 is
so sensitive that anthropogenic greenhouse forcing leads to several times as much warming as
needed to replicate the data2. This presents a political problem. Even if the warming since 1976
were due to greenhouse gas additions to the atmosphere, it suggests relatively low sensitivity.
On the other hand, high sensitivity is needed to produce alarming scenarios. Modelers at the
Hadley Centre dealt with this by replacing anthropogenic greenhouse forcing with just plain
anthropogenic forcing which they claim includes aerosols sufficient to cancel about two thirds of
the anthropogenic greenhouse forcing. However, the community of aerosol scientists maintain
that aerosol forcing is thus far unknown. Thus, aerosols too form an arbitrary adjustment
designed to bring models and observed global mean temperature into agreement. In order to
maintain the politically crucial alarm, it is proposed that aerosols will cease cancelling
greenhouse forcing shortly.
3. Finally, in what sense does the fact that a model cannot duplicate a warming of a few tenths of
a degree constitute evidence that anthropogenic forcing is necessary? The alternative hypothesis
is that the warming is simply natural unforced internal climate variability. It is well known that
the climate does indeed fluctuate without any external forcing. There are several reasons for
this. At the most fundamental level, the atmosphere and oceans are turbulent fluids, and it is a
general property of such fluids that they can fluctuate widely without external forcing. There are
moreover specific features of the oceans and atmosphere that lend themselves to such changes.
The most obvious is that the oceans are never in equilibrium with the surface. There are
exchanges of heat on all time scales between the abyssal oceans and the near surface thermocline
region. Such exchanges are involved in phenomena like El Nino and the Pacific Decadal
Oscillations, and produce large variable forcing for the atmosphere. In addition, the turbulent
motions of the atmosphere randomly deposit heat in locations having varying water vapor and
cloudiness (the two main greenhouse substances in the atmosphere) thus potentially leading to
fluctuations in global mean temperature. In general, models simulate such phenomena rather
poorly. Thus, it should be no surprise that they might fail to replicate a natural cause for recent
warming, and this constitutes no meaningful demand for anthropogenic forcing. How do
modelers deal with this logical problem? In general, the response consists in the embarrassing
assertion that they cannot think of any alternative to anthropogenic forcing. This was explicitly
the response of Alan Thorpe, head of NERC, the main UK funding agency for climate research.