Memorandum by Professor Richard S Lindzen,
Massachusetts Institute of Technology
I am honoured to be able to share my impressions
of the global warming issue with the members of this esteemed
body. For the past 45 years I have been conducting research into
various aspects of the physics of climate. I currently hold the
Alfred P Sloan Professorship in Atmospheric Physics at the Massachusetts
Institute of Technology, and have previously held professorships
at Harvard University and the University of Chicago.
It goes without saying that few laymen understand
what global warming is really about. However, most of you have
been assured that it is a very serious problem, and that almost
all scientists agree. For example, your Prime Minister has written
that it was quite wrong "to suggest that scientific opinion
is equally split", and he went on to claim "The overwhelming
view of experts is that climate change, to a greater or lesser
extent, is man-made, and, without action, will get worse".
The Prime Minister is certainly aware that there are many sources
of climate change, and that profound climate change occurred frequently
long before man appeared on earth. Moreover, given the ubiquity
of climate change, it is implausible that all change is for the
worse. Nevertheless, on the whole I do not disagree with the Prime
Minister. Indeed, I know of no split whatever, and suspect that
the Prime Minister is simply setting up a straw man in claiming
that there is opposing opinion. Where the Prime Minister is, in
my view, leading you astray is in suggesting that this agreement
constitutes support for alarm.
Indeed, when we analyse the nature of the scientific
agreement we will see that it provides no support for alarm. However,
given the proclivity of governments to respond to alarm with substantial
support for science, we can understand the reluctance of the scientific
community, such as it is, to object to the alarmist interpretation
of their agreement.
WHAT IS
TRULY AGREED
In order to analyse the meaning of the Prime
Minister's claim, it is helpful to break the claim into its component
parts. I won't suggest that there is no controversy over details,
but there are few that would fundamentally disagree with the following.
1. The global mean surface temperature is
always changing. Over the past 60 years, it has both decreased
and increased. For the past century, it has probably increased
by about 0.6 degrees Centigrade (C). That is to say, we have had
some global mean warming.
2. CO2 is a greenhouse gas and its increase
should contribute to warming. It is, in fact, increasing, and
a doubling would increase the radiative forcing of the earth (mainly
due to water vapour and clouds) by about 2 per cent.
3. There is good evidence that man has been
responsible for the recent increase in CO2, though climate itself
(as well as other natural phenomena) can also cause changes in
CO2.
I will refer to this as the basic agreement.
To this extent, and no further, it is legitimate to speak of a
scientific consensus.
BEYOND THE
BASIC CONSENSUS
Various bodies have been unable to resist making
claims that items 1 and 2 are causally connected. This is referred
to as the attribution question. I will show that attribution is
by no means widely accepted or even plausible. However, as we
will see, the alleged attribution, itself, also provides little
or no support for alarm. The reason why the basic agreement
(even when supplemented by the claim of attribution) does
not support alarm hinges on other points of widespread agreement,
which the Prime Minister failed to mention (and very likely was
unaware of).
4. In terms of climate forcing, greenhouse
gases added to the atmosphere through mans activities since the
late 19th Century have already produced three-quarters of the
radiative forcing that we expect from a doubling of CO2. The main
reasons for this are (1) CO2 is not the only anthropogenic
greenhouse gas—others like methane also contribute; and
(2) the impact of CO2 is nonlinear in the sense that each
added unit contributes less than its predecessor. For example,
if doubling CO2 from its value in the late 19th Century (about
290 parts per million by volume or ppmv) to double this (ie, 580
ppmv) causes a 2 per cent increase in radiative forcing, then
to obtain another 2 per cent increase in radiative forcing we
must increase CO2 by an additional 580 ppmv rather than by another
290 ppmv. At present, the concentration of CO2 is about 370 ppmv.
5. A doubling of CO2 should lead (if the
major greenhouse substances, water vapour and clouds remain fixed),
on the basis of straightforward physics, to a globally averaged
warming of about 1C. The current increase in forcing relative
to the late 19th Century due to mans activities should lead to
a warming of about 0.76C, which is already more than has been
observed, but is nonetheless much less than current climate models
predict.
CLIMATE MODELS
AND BASELESS
ALARMISM
This brings us, finally, to the issue of climate
models. Essential to alarm is the fact that most current climate
models predict a response to a doubling of CO2 of about 4C. The
reason for this is that in these models, the most important greenhouse
substances, water vapour and clouds, act in such a way as to greatly
amplify the response to anthropogenic greenhouse gases alone (ie,
they act as what are called large positive feedbacks). However,
as all assessments of the Intergovernmental Panel on Climate Change
(IPCC) have stated (at least in the text—though not in the
Summaries for Policymakers), the models simply fail to get clouds
and water vapour right. We know this because in official model
intercomparisons, all models fail miserably to replicate observed
distributions of cloud cover. Thus, the model predictions are
critically dependent on features that we know must be wrong.
If we nonetheless assume that these model predictions
are correct (after all stopped watches are right twice a day),
then man's greenhouse emissions have accounted for about six times
the observed warming over the past century with some unknown processes
cancelling the difference. This is distinctly less compelling
than the statement that characterised the IPCC Second Assessment
and served as the smoking gun for the Kyoto agreement: The
balance of evidence suggests a discernible human influence on
global climate. This is simply a short restatement of the
basic agreement with the addition of a small measure of
attribution. While one could question the use of the word "discernible",
there is no question that human influence should exist, albeit
at a level that may be so small as to actually be indiscernible.
As we have already noted, however, even if all the change in global
mean temperature over the past century were due to man, it would
still imply low and relatively unimportant influence compared
to the predictions of the models that are drawn on in IPCC reports.
Another example of the misuse of the basic
agreement to promote alarm consists in the opening lines of
the executive summary of the US National Research Council (NRC)
2001 report: Climate Change Science: An Analysis of Some Key
Questions. This hurried report was prepared at the specific
request of the White House. The brief and carefully drafted report
of 15 pages was preceded by a totally unnecessary 10 page executive
summary. The opening lines were appended at the last moment without
committee approval. Here they are:
Greenhouse gases are accumulating in Earth's
atmosphere as a result of human activities, causing surface air
temperatures and subsurface ocean temperatures to rise. Temperatures
are, in fact, rising.
The changes observed over the last several
decades are likely mostly due to human activities, but we cannot
rule out that some significant part of these changes is also a
reflection of natural variability.
To be sure, this statement is leaning over backwards
to encourage the alarmists. Nevertheless, the two sentences in
the first claim serve to distinguish observed temperature change
from human causality. The presence of the word "likely"
in the second statement is grossly exaggerated, but still indicates
the lack of certainty, while the fact that we have not emerged
from the level of natural variability is, in fact, mentioned albeit
obliquely. What, as usual, goes unmentioned is that the observed
changes are much smaller than expected.
The response from many commentators was typical
and restricted to the opening lines. CNN's Michelle Mitchell characteristically
declared that the report represented "a unanimous decision
that global warming is real, is getting worse, and is due to man.
There is no wiggle room". Mitchell's response has, in fact,
become the standard take on the NRC report. Such claims, though
widely made in your country as well as mine, have no basis: they
are nonsensical.
MISLEADING INFERENCES
How is it that model based alarm has been "justified"
despite the fact that the observed warming over the past century
is much less than was anticipated by the models? As usual, the
argument involves obscuring this latter fact. The argument also
ignores the fact that the climate is capable of unforced internal
variability. That is to say, the climate can vary without any
external forcing at all. El Niño is an example but there
are many others besides. Reference to any temperature history
of the earth shows fluctuations that are not connected to any
known forcing, and these fluctuations amount to as much as half
a degree Centigrade.
The most common defense is based on studies
from the UK's Hadley Centre, and appears in Chapter 12 of the
IPCC's Third Scientific Assessment. I would like to comment on
this line of argument.
In these studies, we are shown three diagrams.
In the first, we are shown an observed temperature record (without
error bars), and the results of four model runs with so-called
natural forcing for the period 1860-2000. There is a small spread
in the model runs (which presumably displays model uncertainty—it
most assuredly does not represent internal variability). In any
event, the models look roughly like the observations until the
last 30 years. We are then shown a second diagram where the observed
curve is reproduced, and the four models are run with anthropogenic
forcing. Here we see rough agreement over the last 30 years, and
poorer agreement in the earlier period. Finally, we are shown
the observations and the model runs with both natural and anthropogenic
forcing, and, voila, there is rough agreement over the whole record.
It should be noted that the models used had a relatively low sensitivity
to a doubling of CO2 of about 2.5C. In order to know what to make
of this exercise, one must know exactly what was done. The natural
forcing consisted in volcanoes and solar variability. Prior to
the Pinatubo eruption in 1991, the radiative impact of volcanoes
was not well measured, and estimates vary by about a factor of
3. Solar forcing is essentially unknown. Thus, natural forcing
is, in essence, adjustable. Anthropogenic forcing includes not
only anthropogenic greenhouse gases, but also aerosols that act
to cancel warming (in the Hadley Centre results, aerosols and
other factors cancelled two thirds of the greenhouse forcing).
Unfortunately, the properties of aerosols are largely unknown.
This was remarked upon in a recent paper in Science, wherein
it was noted that the uncertainty was so great that estimating
aerosol properties by tuning them to optimise agreement between
models and observations (referred to as an inverse method) was
probably as good as any other method, but that the use of such
estimates to then test the models constituted a circular procedure.
In the present instance, therefore, aerosols constitute simply
another adjustable parameter (indeed, both its magnitude and its
time history are adjustable). However, the choice of models with
relatively low sensitivity, allowed adjustments that were not
so extreme.
What we have is essentially an exercise in curve
fitting. I suppose that the implication is that it is possible
that the model is correct, but the likelihood that all the adjustments
are what actually occur is rather small. The authors of Chapter
12 of the IPCC Third Scientific Assessment provided the following
for the draft statement of the Policymakers Summary: From the
body of evidence since IPCC (1996), we conclude that there has
been a discernible human influence on global climate. Studies
are beginning to separate the contributions to observed climate
change attributable to individual external influences, both anthropogenic
and natural. This work suggests that anthropogenic greenhouse
gases are a substantial contributor to the observed warming, especially
over the past 30 years. However, the accuracy of these estimates
continues to be limited by uncertainties in estimates of internal
variability, natural and anthropogenic forcing, and the climate
response to external forcing.
This statement is not too bad—especially
the last sentence. To be sure, the model dependence of the results
is not emphasised, but the statement is vastly more honest than
what the Summary for Policymakers in the IPCC's Third Assessment
Report ultimately presented: In the light of new evidence and
taking into account the remaining uncertainties, most of the observed
warming over the last 50 years is likely to have been due to the
increase in greenhouse gas concentrations. In truth, nothing
of the sort can be concluded. The methodology, by omitting any
true treatment of internal variability, misses a crucial point.
One can represent the presence of internal variability simply
by plotting an horizontal line with the average value of the temperature
for the period 1850-2000, and broadening this line to have a thickness
of about 0.4C to represent the random internal variability of
climate (in nature if not in the models). One can then plot the
observations with a thickness of about 0.3C (corresponding to
an observational uncertainty of about +/-0.15C). The two appropriately
broadened lines will now overlap almost everywhere (a certain
percentage of non-overlap is statistically expected) leaving no
evident need for forcing at all.
Thus, the impact of man remains indiscernible
simply because the signal is too small compared to the natural
noise. Claims that the current temperatures are "record
breaking" or "unprecedented", however questionable
or misleading, simply serve to obscure the fact that the observed
warming is too small compared to what models suggest. Even the
fact that the oceans' heat capacity leads to a delay in the response
of the surface does not alter this conclusion.
FROM ALARMISM
TO FANTASY
We still have not really addressed the interesting
question of how modest warming has come to be associated with
alarm. Here we must leave the realm where fudging and obfuscation
are the major tools to a realm of almost pure fantasy. A simple
example will illustrate the situation.
According to any textbook on dynamic meteorology,
one may reasonably conclude that in a warmer world, extratropical
storminess and weather variability will actually decrease. The
reasoning is as follows. Judging by historical climate change,
changes are greater in high latitudes than in the tropics. Thus,
in a warmer world, we would expect that the temperature difference
between high and low latitudes would diminish. However, it is
precisely this difference that gives rise to extratropical large-scale
weather disturbances. Moreover, when in Boston on a winter day
we experience unusual warmth, it is because the wind is blowing
from the south. Similarly, when we experience unusual cold, it
is generally because the wind is blowing from the north. The possible
extent of these extremes is, not surprisingly, determined by how
warm low latitudes are and how cold high latitudes are. Given
that we expect that high latitudes will warm much more than low
latitudes in a warmer climate, the difference is expected to diminish,
leading to less variance. Nevertheless, we are told by advocates
and the media that exactly the opposite is the case, and that,
moreover, the models predict this (which, to their credit, they
do not) and that the basic agreement discussed earlier signifies
scientific agreement on this matter as well. Clearly more storms
and greater extremes are regarded as more alarming than the opposite.
Thus, the opposite of our current understanding is invoked in
order to promote public concern. The crucial point here is
that once the principle of consensus is accepted, agreement on
anything is taken to infer agreement on everything advocates wish
to claim.
Again, scientists are not entirely blameless
in this matter. Sir John Houghton (the first editor of the IPCC
scientific assessments) made the casual claim that a warmer world
would have more evaporation and the latent heat (the heat released
when evapourated water vapour condenses into rain) would provide
more energy for disturbances. This claim is based on a number
of obvious mistakes (though the claim continues to be repeated
by those who presumably don't know better).
For starters, extratropical storms are not primarily
forced by the latent heat released in convection. However, even
in the tropics, where latent heat plays a major role, the forcing
of disturbances depends not on the evaporation, but on the evaporation
scaled by the specific humidity at the surface. It turns out that
this is almost invariant with temperature unless the relative
humidity decreases in a warmer world. Incidentally, this would
suggest that the feedbacks that cause models to display high climate
sensitivity are incorrect. The particularly important issue of
whether warming will impact hurricanes, is a matter of debate.
As the IPCC has noted, there is no empirical evidence for such
an impact. State of the art modeling suggests a negative impact,
while there are theoretical arguments that suggest a slight positive
impact on hurricane intensity. This is all of significant intellectual
interest, but it is not the material out of which to legitimately
build alarm.
Perhaps the most reprehensible attempt to generate
alarm over global warming has been seen in connection with the
recent tragic tsunamis in South Asia, where statements were made
attempting to link this essentially geological event to global
warming. However specious such links are, they follow what has
become an almost self-parodying habit of those proclaiming alarm
of attaching any severe, unusual or even common but not well known
event to global warming while suggesting rather dishonestly that
the event had indeed been predicted by models.
SUMMING UP
So where does all this leave us?
First, I would emphasise that the basic agreement
frequently described as representing scientific unanimity
concerning global warming is entirely consistent with there being
virtually no problem at all. Indeed, the observations most simply
suggest that the sensitivity of the real climate is much less
than found in models whose sensitivity depends on processes which
are clearly misrepresented (through both ignorance and computational
limitations). Attempts to assess climate sensitivity by direct
observation of cloud processes, and other means, which avoid dependence
on models, support the conclusion that the sensitivity is low.
More precisely, what is known points to the conclusion that a
doubling of CO2 would lead to about 0.5C warming, and a quadrupling
(should it ever occur) to about 1C. Neither would constitute a
particular societal challenge. Nor would such (or even greater)
warming be associated with more storminess, greater extremes,
etc.
Second, a significant part of the scientific
community appears committed to the maintenance of the notion that
alarm may be warranted. Alarm is felt to be essential to the maintenance
of funding. The argument is no longer over whether the models
are correct (they are not), but rather whether their results are
at all possible. Alas, it is impossible to prove something is
impossible.
As you can see, the global warming issue parts
company with normative science at a pretty early stage. A very
good indicator of this disconnect is the fact that there is widespread
and even rigorous scientific agreement that complete adherence
to the Kyoto Agreement would have no discernible impact on climate.
This clearly is of no importance to the thousands of negotiators,
diplomats, regulators, general purpose bureaucrats and advocates
attached to this issue.
At the heart of this issue there is one last
matter: namely, the misuse of language. George Orwell wrote that
language "becomes ugly and inaccurate because our thoughts
are foolish, but the slovenliness of our language makes it easier
for us to have foolish thoughts". There can be little doubt
that the language used to convey alarm has been sloppy at best.
Unfortunately, much of the sloppiness seems to be intentional.
The difficulties of discourse in the absence of a shared vocabulary
are, I fear, rather evident.
25 January 2005
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