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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 interpretation.

The 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 atmosphere increases.

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 activity.

Spencer's theory: not supported by the evidence

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 writes:

"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 criticised by 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 why:

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 account."

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.

Data misinterpretation

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.

 

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