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Impact on climate from sinking clouds? Too early to say

  • 27 Feb 2012, 12:48
  • Verity Payne

If the sky seems particularly oppressive and grey today, maybe because it's closing in. Or, to be more precise, clouds actually sank a little over the first decade of this century, according to a new scientific study - a finding that hints at a new mechanism that might cool the climate as it warms.

The research, published in the journal Geophysical Research Letters and led by Professor Roger Davies of the University of Auckland, used satellite data to work out global cloud heights between 2000 and 2010. Davies found that the average global height of clouds dropped by around one per cent, or roughly 30 to 40 metres, over that period. Most of this drop in average cloud height was down to fewer clouds occurring at very high altitudes.

Davies suggests that the finding might identify a 'negative climate feedback' - when warming causes a change to climate which counteracts some or all of the warming. Davies does stress, however, that his data is fairly preliminary - it only covers a decade, too short a period to say for certain that such a feedback exists. He explains:

"This is the first time we have been able to accurately measure changes in global cloud height and, while the record is too short to be definitive, it provides just a hint that something quite important might be going on."

The research has not uncovered exactly what caused the drop in average cloud height over the globe, but Davies speculates that it is down to changes in the atmospheric circulation patterns that cause clouds to form at high altitudes.

The changes were not uniform around the planet, but varied over different regions and from year to year. Davies found that these differences were related to El Niño Southern Oscillation (ENSO) - the spread of warmer or cooler than normal sea waters across the equatorial Pacific, which is known to alter large scale climate patterns.

Clouds, and their influence on Earth's climate, have long been an area of uncertainty in climate models, largely because of the difficulties in measuring clouds. Only the deployment of spacecraft such as NASA's Terra satellite, used in this study, has enabled scientists to make reliable observations of things like average cloud height over the globe. Davies suggests that these new observations will help improve climate models:

"Cloud height is extremely difficult to model and therefore hasn't been considered in models of future climate. For the first time we have been able to accurately measure the height of clouds on a global basis, and the challenge now will be to incorporate that information into climate models. It will provide a check on how well the models are doing, and may ultimately lead to better ones."

The research has garnered a bit of media attention in articles turning it to their own news agendas, with the Mail Online asking Is the Earth cooling itself?, and Andrew Bolt, a columnist for the Herald Sun, referring to the study as "the latest attempt to explain the lack of warming that global warmists predicted". In reality, it's rather more prosaic - an early study on a newly discovered issue, with an interesting result.

The Terra satellite will continue to collect data over the coming decade, so could shed more light on the existence of the negative feedback. Davies explains:

"If cloud heights come back up in the next ten years we would conclude that they are not slowing climate change, but if they keep coming down it will be very significant. We look forward to the extension of this climate record with great interest."

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