University of Auckland: Cloud height changes may lower global
February 24, 2012
from The University of Auckland on changes in cloud height in the decade
to 2010 has provided the first hint of a cooling mechanism that may be
in play in the Earth’s climate.
Published in the journal Geophysical Research Letters, the analysis of
the first ten years of data from the NASA Terra satellite revealed an
overall trend of decreasing cloud height. Global average cloud height
declined by around 1 per cent over the decade, or around 30 to 40 metres.
Most of the reduction was due to fewer clouds occurring at very high
“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.
A consistent reduction in cloud height would allow the Earth to cool to
space more efficiently, reducing the surface temperature of the planet
and potentially slowing the effects of global warming. This may
represent a “negative feedback” mechanism – a change caused by global
warming that works to counteract it. “We don’t know exactly what causes
the cloud heights to lower,” says Professor Davies, “but it must be due
to a change in the circulation patterns that give rise to cloud
formation at high altitude.”
Until recently however, it was impossible to measure the changes in
global cloud heights and understand their contribution to global climate
“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.”
University of Auckland physicists Professor Davies and Matthew Molloy, a
BSc Honours student, analysed measurements of the Multiangle Imaging
SpectroRadiometer (MISR), one of the instruments on the Terra satellite
launched by NASA in December 1999. The instrument uses 9 cameras at
different angles to produce a stereo image of clouds around the globe,
allowing measurement of their altitude and movement.
results to date reveal a complex pattern of decreases in cloud altitude
across some regions of the globe and increases in others, with the El
Niño / La Niña phenomenon in the Pacific producing the strongest effect
and greatest variation from year to year. After taking into account all
these differences, however, the overall trend was of decreasing cloud
height from 2000 to 2010.
The Terra satellite is scheduled to continue gathering data through the
remainder of this decade. “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.”
Professor Davies holds the Buckley Glavish Chair in Climate Physics at
The University of Auckland. The current research was funded by NASA’s
Jet Propulsion Laboratory, which is managed for NASA by the California
Institute of Technology.