Clouding the issue: skeptics mistake clouds paper for proof of fringe climate theory
- 23 Aug 2012, 11:36
- Verity Payne
New research monitoring cloud cover over the world's land areas
for the last four decades agrees with previous studies indicating
that the planet's tropical climate zones are expanding, and that
consequently the atmospheric jet streams are shifting towards the
poles, altering patterns of cloud cover. But skeptic blogger
Anthony Watts claims the research provides evidence for the
unconventional suggestion that clouds are responsible for global
warming. We asked the researchers if they agree with his
paper, currently in press at the Journal of Climate, also finds
that cloud cover over land has decreased by around 0.4 per cent per
decade, due to a decline in middle and high atmosphere clouds in
the mid-latitudes - roughly between 30 and 60 degrees latitude.
The paper's authors write that their "dataset offers few
surprises": it shows similar trends to the existing record of cloud
observations that it updates. The researchers explain that the
decrease in mid-latitude cloudiness over the last four decades is
consistent with expanding tropical climate zones, adding further
weight to the evidence that the planet is warming.
But climate skeptic blogger Anthony Watts interprets the new
research differently. Watts
suggests the research bolsters a hypothesis proposed by skeptic
scientist Dr Roy Spencer
that runs counter to mainstream scientific thinking. Spencer
claims that clouds, not greenhouse gases, are causing the
planet to warm.
We asked the paper's author Ryan Eastman,
research scientist at the University of Washington, whether he
agrees with Watts's interpretation of his research. Eastman, who
seems surprised by the attention his research has generated, says
that Watts' conclusions are "misleading".
The effect of clouds on Earth's climate is not totally
straightforward. Where clouds form in the sky changes their effect
on the climate, as Eastman explains:
"[H]igh clouds tend to produce a warming
effect on climate while low clouds tend to cool [climate]. But that
is even too simplistic, since low clouds are only associated with
cooling the surface during daylight hours, while at night they
actually warm. During winter clouds also tend to warm rather than
cool the surface."
Currently, clouds have an overall
cooling effect on the climate system. But it is not yet clear
whether cloud cover will increase or decrease as the climate
continues to warm, nor whether these possible changes will
exacerbate or diminish future warming. Many scientists consider
cloud feedbacks to be one of the
main reasons for why there is a broad range of estimates of how
much the world will warm as the amount of carbon dioxide in the
Scientists continue to investigate the issue, conducting climate
modelling experiments and collecting observational data from
satellites and surface measurements of clouds. So far, though,
study periods tend to be too short to give a robust indication of
how clouds might affect climate in the future.
On one thing, however, the vast majority of scientists agree: clouds
provide a feedback. This means clouds amplify or reduce any
temperature change to the climate system from climate drivers such
as increasing greenhouse gases or changes in solar or volcanic
Spencer's theory: not supported by the
Spencer, on the other hand, suggests that clouds are not just
providing a climate feedback, but that changes to cloud cover are
causing the warming of the climate system. He
"The most obvious way for warming to be
caused naturally is for small, natural fluctuations in the
circulation patterns of the atmosphere and ocean to result in a 1%
or 2% decrease in global cloud cover. Clouds are the Earth's
sunshade, and if cloud cover changes for any reason, you have
global warming - or global cooling."
Spencer's hypothesis attracted a lot of media attention when he
published a paper on the topic last year. But the paper was heavily
climate scientists, and the editor of the journal which had
published the paper resigned writing that
the paper is "fundamentally flawed" and "ignore[s] the scientific
arguments of its opponents."
So far Spencer's hypothesis does not seem to have
held up to closer scrutiny. Here's Professor Andrew Dessler,
atmospheric scientist at Texas A&M University, explaining
Eastman tells us he is concerned by Watts' suggestion that this
new research supports Spencer's hypothesis, since:
"The yearly average global total cloud
cover trend cannot be directly used to determine any temperature
trend. Information about the seasonality and locations of the
trends as well as the cloud types changing must be taken into
Eastman also points out that his dataset only covers less than a
third of the Earth's surface, since it only considers how clouds
have changed over land.
Eastman also says:
"Figure two in the [Watts Up With That?]
post is taken completely out of context from the paper."
Figure two shows trends in some types of cloud over Russia that
were included in a previous paper, but have since been found to
based on "spurious data". The new research paper includes figure
two as part of an explanation into why the data showing changes in
some types of cloud are flawed.
But Watts writes:
"Interestingly, some types of clouds
have been on the increase, while others have been on the decrease.
Now, a cause needs to be identified as to why some clouds increase
and others decrease."
Overall, it appears that the conclusions of the paper are much
more conventional than Watts' interpretation suggests.