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Cloud altitude changing with climate: NZ study

NASA Terra sat data reveals ten-year fall

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A University of Auckland analysis of ten years’ worth of NASA satellite data suggests that changing atmospheric temperatures are impacting cloud formation, with fewer clouds forming at the highest altitudes.

The study, by Professor Roger Davis and honours student Matthew Molloy, is designed to help feed cloud height into climate models. This variable, Professor Davis said in the university’s statement, is very difficult to model. As a result, he says, current climate models do not incorporate cloud height in their predictions.

Using ten years’ worth of data from the NASA Terra satellite, the university’s team identified a global trend, from 2000 to 2010, in which the global average cloud height decreased by between 30 and 40 meters.

Interpreting that result – and in particular, doing so in a way that will help improve climate models – is more tricky. A lower global average cloud height provides a feedback mechanism that lowers temperature – but whether that takes place depends on the trend, and how much effect it might have is an unknown.

Professor Davies told The Register: “We know quite clearly that if cloud heights continue the downward trend, they do provide negative feedback. That part is mature science.

“What is immature is an understanding of why clouds would continue to change in such a way.”

With only a decade’s worth of observations to work with, Professor Davies told The Register, the observations are “leading the way. Until a good explanation for lower cloud heights is developed, we cannot safely extrapolate this effect, and must simply await the observations with interest.”

In particular, Professor Davies warns against interpreting the results as some kind of get-of-jail card for global warming. While some reports focus on the as-yet-unmodelled feedback mechanism in this way, he emphasized to The Register that “I took pains to stress that ten years is too short a time to draw hard conclusions.”

The measurements, which used images taken by the Terra satellites Multiangle Imaging SpectroRadiometer, also revealed that the el Nino / la Nina cycle produces both the strongest effect and the greatest variation from year to year.

“The suggestion made in the paper is that the correlations obtained between cloud height changes and other variables – such as surface pressure, surface temperature, and the southern oscillation index (el Nino/la Nina) might be helpful to future model development.”

The Register also asked whether older satellite data may provide a base on which to extend this data prior to 2000. Professor Davies said it could happen: “There have been lots of measurements of individual cloud heights in the past, but it takes a tremendous amount of sampling (like we have done) to reduce the error in the global average value down to a useful level.”

The research was funded by NASA's Jet Propulsion Laboratory, and is published in the journal Geophysical Research Letters.®

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