Saturday, February 25, 2012

Is The Sky Falling?

The media got wind of a story released by NASA and the headline was irresistible.  It was a NASA-funded university study based on NASA satellite data.  Earth's clouds got a little lower, about one percent on average, during the first decade of this century.

Scientists at the University of Auckland in New Zealand analyzed the first 10 years of global cloud-top height measurements (from March 2000 to February 2010) from the Multi-angle Imaging SpectroRadiometer (MISR) instrument on NASA's Terra spacecraft. The study, published recently in the journal Geophysical Research Letters, revealed an overall trend of decreasing cloud height. Global average cloud height declined by around one percent over the decade, or by around 100 to 130 feet (30 to 40 meters). Most of the reduction was due to fewer clouds occurring at very high altitudes.

Data from NASA's MISR instrument on NASA's Terra spacecraft show that global average cloud height declined by about 1 percent over the decade from 2000 to 2010, or around 100 to 130 feet (30 to 40 meters). The graph here shows anomalies in global effective cloud-top height from the 10-year average, corrected to account for seasonal differences. The solid line represents the 12-month running average of 10-day anomalies, while the dotted line represents the trendline calculated by linear regression. Gray error bars indicate the sampling error (plus or minus 26 feet, or 8 meters) in the annual average. Image credit: University of Auckland/NASA JPL-Caltech.

“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,” explains lead researcher Professor Roger Davies.   Longer-term monitoring will be required to determine the significance of the observation for global temperatures.

Why is this important?   “Clouds are one of the biggest uncertainties in our ability to predict future climate,” says Professor Davies.  “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.”

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

The importance of cloud heights is illustrated below.  High clouds tend to trap more infrared heat from earth while low clouds tend to reflect more of the sun's rays.

Image Credit: Skeptical Science.
It is important to note that clouds are only a feedback mechanism.  Andrew Dessler, a Texas A&M atmospheric sciences professor considered one of the nation’s experts on climate variations, says decades of data support the mainstream and long-held view that clouds are primarily acting as a so-called “feedback” that amplifies warming from human activity.

Dessler adds, “Over a century, however, clouds can indeed play an important role amplifying climate change.”

“I hope my analysis puts an end to this claim that clouds are causing climate change,” he adds.

Other recent studies indicate that as the earth warms low clouds may be decreasing which would in turn warm the planet.  Researchers Amy Clement and Robert Burgman from the University of Miami’s Rosenstiel School of Marine and Atmospheric Science and Joel Norris from Scripps Institution of Oceanography at UC San Diego have been looking at this mystery.  Using observational data collected over the last 50 years and complex climate models, the team established that low-level stratiform clouds appear to dissipate as the ocean warms, indicating that changes in these clouds may enhance the warming of the planet.  Thus for clouds location may be important particularly in tropical regions.

The two biggest uncertainties in climate science are clouds and aerosols.  At least the uncertainties on clouds are beginning to come into focus, yet there is still much work to be done.  This may improve climate models in the near future.


Davies, R. and M. Molloy (2012), Global cloud height fluctuations measured by MISR on Terra from 2000 to 2010, Geophys. Res. Lett., 39, L03701, doi:10.1029/2011GL050506.

Dessler, A. E. (2011), Cloud variations and the Earth's energy budget, Geophys. Res. Lett., 38, L19701, doi:10.1029/2011GL049236.

Clement, A., R. Burgman, and J. R. Norris (2009), Observational and Model Evidence for Positive Low-Level Cloud Feedback, Science, 325 (5939), 460-464, doi:10.1126/science.1171255