Climate change may eventually change global cloud heights, but scientists need a longer data set to know whether that's happening already. NASA

We are all familiar with clouds, the fluffy-looking objects in the sky that produce rain, thunder and lightning. They act as a sunshade for Earth to keep it cool and also insulate the planet by trapping a lot of its heat. However, we don’t fully understand the impact the changing climate will have on the cloud cover, except that the two are linked.

Speaking about that linkage, a NASA statement Friday said: “Currently their [clouds] cooling effect prevails globally. But as Earth warms, the characteristics of clouds over different global regions — their thickness, brightness and height — are expected to change in ways that scientists don’t fully understand. These changes could either amplify warming or slow it. Pinning down some of the uncertainties around clouds is one of the biggest challenges in determining the future rate of global climate change.”

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Scientists tried to do precisely that, using data from the Multi-angle Imaging Spectroradiometer (MISR) instrument on NASA’s Terra satellite which was launched in December 1999. An open-access study published in February 2012, in the Journal of Geophysical Research – Atmospheres, suggested the height of global cloud cover had decreased in the 10 years between March 2000 and February 2010.

However, a new study published recently in the same journal — the researchers were led by Roger Davies of the University of Auckland, New Zealand, who also led the earlier study group — analyzed data from the same satellite for 15 years, and found “no significant overall trend in global heights during the first 15 years of Terra operation.”

One of the reasons for the reversal of Davies’ earlier conclusion was due to a technical factor, involving when the Terra satellite crossed the equator every day. Over the first two years after its launch, the satellite’s timing of crossing the equator was brought forward by 15 minutes. This changed MISR’s ability to image high clouds by reducing the amount of reflecting off Earth’s surface, from the satellite’s point of view. Called sun-glint, this reflection makes it easier to detect thin, high clouds, which appeared in larger numbers in earlier images, before the time correction was made.

Once the data was corrected for the sun-glint and time difference, and researchers also added five additional years of data, they found “no statistically significant trend in cloud height over the 15-year period,” the NASA statement said.

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There are significant regional variations, however, in the heights of clouds, and these are affected by a number of weather and climate phenomena, such as La Niña and El Niño events, which decrease and increase cloud heights, respectively.

Davies, who thinks a lot more time and data would be needed to understand the possible global effects of climate change on cloud heights, said in the statement: “All we can say at the moment is that the global trends in cloud heights, if they are there, are being swamped by El Niño-La Niña fluctuations. It will take a lot longer till we can tease out these long-term trends.”

The MISR instrument has nine cameras pointing at Earth at different angles, which record images in four visible and near-infrared wavelengths. These images allow researchers to distinguish the different characteristics of clouds, such as amounts, types and heights.