Sunlight hitting ground not behind recent warming

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The researchers looked how factors like cloud cover and pollution might have affected the Sun's radiation from reaching the Earth (Source: July Velchev/iStockphoto)

Over the past century, rising greenhouse gas levels have caused global average temperatures to increase, climate scientists Kaicun Wang and Robert Dickinson write in theProceedings of the National Academy of Sciences this week.

But the rate of increase has fluctuated over shorter timeframes. While average temperatures increased between 1900 and 1940, they remained flat over the following three decades before increasing steeply from the 1970s to around 2000, the authors say in their article.

And in the first years of this century, they note, there has been "very little additional increase."

While scientists agree that carbon dioxide and other greenhouse gasses are the primary cause of global warming, they have long debated whether factors such as pollution, clouds, or natural ocean variability have caused the shorter-term variations.

In their new paper, Wang, from Beijing Normal University in China, and Dickinson, from the University of Texas, say the warming trend since the 1970s and the lack of warming in the past ten years were not caused by changes in how much solar radiation reached the Earth's surface.

Aerosols and cloud changes

Steven Sherwood, a professor of meteorology and atmospheric climate dynamics at the University of New South Wales, says the new paper is not about whether the Sun drives climate change.

"We already know from direct observations of the power coming from the Sun that it has contributed nothing to global warming since 1979, though it probably made a small contribution to warming early in the 20th century," says Sherwood, who was not involved in the study.

Instead, the aim of the paper was to look at whether atmospheric factors like the extent of cloud cover and sulphur pollution might have affected the rate of warming by stopping the Sun's radiation from reaching the Earth.

Studying this correlation directly is difficult, Wang and Dickinson say in their article. That's because scientists have only measured the amount of solar radiation hitting the Earth in a few places.

So for their analysis they looked at trends in daily maximum and minimum temperatures, using the difference between the highs and lows as a stand-in measure for surface solar radiation.

This is a useful approach because the daytime temperature is mostly determined by radiation from the Sun, while night-time temperatures are mainly determined by the greenhouse effect, says Swiss researcher Martin Wild.

In recent years, Wild and his colleagues used this technique to show that the true scale of greenhouse warming was masked between the 1940s and 1980s by a reduction in surface solar radiation referred to as "global dimming."

"The study of Wang and Dickinson adds to these studies and confirms our earlier findings, and refines some analysis," says Wild.

Highs and lows

In their new analysis, Kaicun and Dickinson show that the surface solar radiation over the Earth's landmasses peaked in the 1930s, but then substantially decreased between the 1940s and the 1970s.

"The cooling effect of this reduction of [surface solar radiation] accounts in part for the near-constant temperature from the 1930s into the 1970s," they write.

Since then, however, neither the rapid increase in temperature from the 1970s through the 1990s nor the slowdown of warming in the early twenty-first century appear to be significantly related to changes of solar radiation reaching the Earth, they say.

They could find no significant trend in surface solar radiation, leading them to conclude that its impact on temperature variations from one decade to another is negligible.

"Our answer is no," says Wang. "Surface solar radiation can't be blamed for the strong warming rate during the recent decades, or the recent [lower] warming rate."