Why I Bet against Global Warming
Dr David Evans
I devoted six years to carbon accounting, building models
for the Australian government to estimate carbon emissions from
land use change and forestry (Google on "FullCAM").
When I started that job, in 1999, the evidence that carbon emissions
caused global warming seemed pretty conclusive, but since then
new evidence has weakened the case that carbon emissions are
the main cause. I am now skeptical. As Lord Keynes famously said,
"When the facts change, I change my mind. What do you do,
In the late 1990s the evidence suggesting that carbon emissions
caused global warming was basically:
1. Carbon dioxide is a greenhouse gas. Proved in a laboratory
a century ago.
2. Global warming has been occurring for a century, especially
since 1975, and concentrations of atmospheric carbon have been
rising for a century, especially since 1975. Correlation is not
causation, but in a rough sense it looked like a fit.
3. Ice core data, starting with the first cores from Vostok
in 1985, allowed us to measure temperature and atmospheric carbon
going back hundreds of thousands of years, through several dramatic
global warming and cooling events. To the temporal resolution
then available (data points were generally more than a thousand
years apart), atmospheric carbon and temperature moved in lock-step:
there was an extremely high correlation, they rose and fell together.
Talk about a smoking gun!
4. There weren't any other credible suspects for causing global
warming. So presumably it had to be carbon emissions.
This evidence was good enough: not conclusive, but why wait
until we are absolutely certain when we apparently need to act
now? So the idea that carbon emissions were causing global warming
passed from the scientific community into the political realm,
and actions started to happen. Research increased, bureaucracies
were formed, international committees met, and eventually the
Kyoto protocol was signed in 1997---with the aim of curbing carbon
And the political realm, in turn, fed money back into the
scientific community. By the late 1990s, lots of jobs depended
on the idea that carbon emissions caused global warming. Many
of them were bureaucratic, but there were a lot of science jobs
created too. I was on that gravy train, making a high wage in
a science job that would not have existed if we didn't believe
carbon emissions caused global warming. And so were lots of people
around me; and there were international conferences full of such
people. And we had political support, the ear of government,
big budgets, and we felt fairly important and useful (well, I
did anyway). It was great. We were working to save the planet!
But starting in about 2000, the last three of the four pieces
of evidence outlined above fell away or reversed. Using the same
point numbers as above:
2. Closer examination of the last century using better data
shows that from 1940 to 1975 the earth cooled at about 0.1ºC/decade
while atmospheric carbon increased. But any warming effect of
atmospheric carbon is immediate. By 2003 or so, we had discovered
global dimming, which might be adequate to explain this 35-year
non-correlation. But what had seemed like a good fit between
recent atmospheric carbon and global warming now looks shaky,
in need of the recently-discovered unquantified global dimming
factor to explain 35 years of substantial cooling. I reckon the
last century of correlation evidence now neither supports carbon
emissions as the cause nor eliminates it. Further quantitative
research on global dimming might rescue this bit of evidence,
or it might weaken it further.
3. As more ice core data was collected, the temporal resolution
was improved. By 2004 or so, we knew from the ice core data that
in the warming events of the last million years, the temperature
increases generally started about 800 years before
the rises in atmospheric carbon started. Causality does not run
in the direction I had assumed in 1999---it runs the opposite
way. Presumably temperature rises cause a delayed rise in atmospheric
carbon because it takes several hundred years to warm the oceans
enough for the oceans to give off more of their carbon.
It is possible that rising atmospheric carbon in these past
warmings then went on to cause more warming ("amplification"
of the initial warming), but the ice core data does not prove
that. It could just be that the temperature rose for some other
reason, that this caused the oceans to raise the atmospheric
carbon levels, and that the increased atmospheric carbon had
an insignificant effect on the temperature.
The pre-2000 ice core data was the central evidence for believing
that atmospheric carbon caused temperature increases. The new
ice core data show that past warmings were not
initially caused by rises in atmospheric carbon, and say nothing
about the strength of any amplification. This piece of evidence
casts reasonable doubt that atmospheric carbon had any role in
past warmings, while still allowing the possibility that it had
a supporting role.
4. A credible alternative suspect now exists. Clouds both
reflect incoming radiation (albedo) and prevent heat from escaping
(greenhouse), but with low clouds the albedo effect is stronger
than the greenhouse effect. Thus low clouds cause net cooling
(high clouds are less common and do the opposite). In October
2006, a team led by Henrik Svensmark showed experimentally that
cosmic rays affect cloud formation, and thus that
Stronger sun's magnetic field
=> Fewer cosmic rays hit Earth
=> Fewer low clouds are formed
=> Earth heats up.
And, indeed, the sun's magnetic field has been stronger than
usual for the last three decades. So maybe cosmic rays cause
global warming. But investigation of this cause is still in its
infancy, and it's far too early to judge how much of the global
warming is caused by cosmic rays.
So three of the four arguments that convinced me in 1999 that
carbon emissions caused global warming are now questionable.
The case for carbon emissions as the cause of global warming
now just boils down to the fact that we know that it works in
the laboratory, and that there is no strong evidence that global
warming is definitely not caused by carbon emissions.
Much the same can be said of cosmic rays---we have laboratory
evidence that it works, and no definitely contradictory evidence.
So why did I bet against global warming continuing at the
current rate? Let's return to the interaction between science
By 2000, the political system had responded to the strong
scientific case that carbon emissions caused global warming by
creating thousands of bureaucratic and science jobs aimed at
more research and at curbing carbon emissions. This was a good
and sensible response by big government to what science was telling
But after 2000 the evidence for carbon emissions gradually
got weaker---better temperature data for the last century, more
detailed ice core data, then laboratory evidence that cosmic
rays precipitate low clouds. Future evidence might strengthen
or further weaken the carbon emissions hypothesis. At what stage
of the weakening should the science community alert the political
system that carbon emissions might not be the main cause of global
warming? None of the new evidence actually says that carbon emissions
are definitely not the cause of global warming, there are lots
of good science jobs potentially at stake, and if the scientific
message wavers, then it might be difficult to recapture the attention
of the political system later on. What has happened is that most
research effort since 2000 has assumed that carbon emissions
were the cause, and the alternatives get much less research or
(By the way, I quit my job in carbon accounting in 2005 for
personal reasons. It had nothing to do with my weakening belief
that carbon emissions caused global warming. I felt that the
main value of our plant models was in land management and plant
simulation, and that carbon accounting was just a by-product.)
Unfortunately, politics and science have become even more
entangled. The science of global warming has become a partisan
political issue, so positions become more entrenched. Politicians
and the public prefer simple and less-nuanced messages. At the
moment, the political climate strongly supports carbon emissions
as the cause of global warming, to the point of sometimes rubbishing
or silencing critics.
The integrity of the scientific community will win out in
the end, following the evidence wherever it leads. But in the
meantime, the effects of the political climate is that most people
are overestimating the evidence in favour of carbon emissions
as the cause of global warming. Which makes it a good time to
bet the other way.
I would like to bet against carbon emissions being the main
cause of the current global warming. But I can't bet on that
directly, because all betting requires an unambiguous and measurable
criterion. About the only related measure we can bet on is global
temperature. So I accepted Brian's
bets about trends in global temperatures over the next 10
to 20 years. Basically, if the current warming trend continues
or accelerates, then Brian will win; if the rate of warming slows,
then I will win. Even if carbon emissions are not the main cause
of this global warming, I can still lose:
- Global warming might be due to a side-effect of industrialization
other than carbon emissions. Possible causes include atmospheric
reactions of industrial chemicals that hinder the rate of low
- Global warming might be primarily due to a non-human cause,
such as something related to the sun or to underground nuclear
reactions. If this cause persists over the next 20 years, as
it has for the last 30 years, then I will lose, but if it fades
in the next decade, then I win.
I emphasize that we are making a bet involving odds and judgment.
The evidence is not currently conclusive either for or against
any particular cause of global warming. I think that it is
possible that carbon emissions are the dominant cause of global
warming, but in light of the weakening evidence I judge that
probability to be about 20% rather than the almost 90% as estimated
by the IPCC.
I worry that politics could seriously distort the science.
Suppose that carbon taxes are widely enacted, but that the rate
of global warming increase starts to decline by 2015. The political
system might be under pressure to repay the taxes, so it might
in turn put a lot of pressure on scientists to provide justifications
for the taxes. Or the political system might reject the taxes
and blame science for misinforming it, which could be a terrible
outcome for science because the political system is powerful
and not constrained by truth.
Some people take strong rhetorical positions on global warming.
But the cause of global warming is not just another political
issue that is subject to endless debate and distortions. The
cause of global warming is an issue that falls into the realm
of science, because it is falsifiable. No amount of human posturing
will affect what the cause is. The cause just physically is there,
and after sufficient research and time we will know what it is.
Looking back in another 40 years, we will almost certainly know
the answer, and Brian and I will be in agreement on the issue.
Given that betting is thus possible on this issue, it seems
strange that some people who take strong positions and profit
by those positions are not prepared to bet even a small amount
of their own money. Betting something of one's own money adds,
shall we say, credibility. And people whose own money is at stake
try a little harder---a well known advantage of private business
over public. A good side-effect of widespread betting would be
a market in betting that would represent a community-wide best
guess. Such markets exists in sports betting, and are the best
predictors of game outcomes.
Let's hope for the planet's sake that I win the bets. Meanwhile
let's do more research, and take cheap measures to curb carbon
 The reference is to Brian Schmidt, with whom I made the bet.
Details of the bet itself can be found on Brian's blog, Backseat
A brief biographical statement about
David Evans may be found here.