Climate models hindered by inaccurate UV satellite measurements

The SORCE satellite Credit: NASA

Current satellite technology has been overestimating how much variation there is in UV radiation from the Sun, according to a new analysis.

Scientists measure the varying amount of ultraviolet light that comes to Earth from the Sun because it plays a role in our climate.

It does this by creating or destroying ozone molecules in the top layers of the atmosphere. This is turn affects the surface temperatures on Earth, so scientists include this factor in their models of climate change.

The SORCE satellite, launched in 2003, measures the patterns of radiation from the Sun hitting the upper atmosphere, including UV. A new analysis by a team of scientists from Imperial College London, Switzerland, and the Czech Republic has revealed that SORCE is not measuring the spectrum of solar UV very accurately.

Current satellite measurements show a large variation in UV radiation, which would have an indirect impact on climate. The effect is particularly important over the tropics, where changing levels of ozone affect the surface temperature of the North Atlantic region and Europe.

However, the new analysis suggests that satellites are overestimating the variability. This means that UV radiation plays a smaller role in influencing our climate. The results are published today in Nature Geoscience.

"If the SORCE data were correct, the variation in UV radiation could have a tangible role in offsetting regional climate change. However, we have shown that SORCE is overestimating the Sun’s variability, meaning we may need to improve our models or identify another way in which the Sun influences regional climate, for example over the North Atlantic," said Dr William Ball, first author of the study and a visiting researcher in Imperial’s Department of Physics.

"Nevertheless, this doesn't change our conclusions that the long-term effects of the Sun are likely small relative to those of human-induced climate change."

Getting better information

“What we thought was our best information turns out to be wrong. It’s a sad conclusion to have to make, but we hope it can lead to a better understanding of the impact of UV. It’s information we want to know, but we’re not quite there yet,” said co-author Professor Jo Haigh of the Grantham Institute at Imperial.

“Accurate information on UV radiation is important for creating the best possible models of our climate,” she added.

Ozone levels in the upper atmosphere are affected by reactions with UV radiation, but also by patterns of wind moving ozone molecules around. The team looked at historical data for ozone levels where wind patterns were also known.

They fed these values into a climate model and changed the input of UV variance until it matched the observed change in ozone. They conclude that SORCE is actually overestimating the variability of the UV radiation spectrum and its effect on the climate system.

“UV radiation could play a role in reducing the effect of human-induced global warming, but this was always expected to be a small influence and humans are still the primary drivers of modern climate change. Climate models do a good job of representing future changes in climate from human activity. It is also clear that the Sun has little impact on global climate when compared to this human influence,” said Dr Ball.

One of the reasons that SORCE may not be as accurate over time is that solar radiation causes it to break down in space. “The instrument is actually degraded by what it’s trying to measure,” said Professor Haigh.

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'High solar cycle spectral variations inconsistent with stratospheric ozone observations' by W. T. Ball, J. D. Haigh, E. V. Rozanov, A. Kuchar, T. Sukhodolov, F. Tummon, A. V. Shapiro andW. Schmutz is published in Nature Geoscience.