In the 1960s, when stations all over the world began to measure the level of solar radiation on the Earth's surface, instruments tended to register declining values. In an earlier, widely respected study, Atsumu Ohmura and his research team at the Institute for Atmospheric and Climate Science at ETH Zurich (IAC) (1) were able to show that the level of solar radiation on the Earth's surface, decreased by a few per cent between 1960 and 1990. A new Swiss-US American collaboration, with the participation of Atsumu Ohmura and Martin Wild, also from IAC, now demonstrates that this trend seems to have gone into reverse. The findings are published in the latest issue of "Science" (2).

Differentiated picture

To carry out their study, the researchers collated and analysed data from hundreds of stations around the world. These show that the solar radiation on the Earth's surface began to rise again in most regions in the mid-1980s. The authors surmise that above all the better quality of air in many places as well as changes in cloud cover could play a role in this reversal.

Air pollution, particularly in industrialised countries, was one of the major factors identified as a cause of "global dimming", as the decrease in solar radiation is called, between the 1960s and 1980s. "Subsequent policies introduced in Western Europe and North America in the 1980s had a positive effect on the quality of the air. On the other hand, political and economic collapse in many eastern European countries led to lower emissions," explains Martin Wild. "This could explain why the stations in North America and Europe began to measure higher levels of solar radiation."

Satellite data confirms it

The fact that solar radiation continues to decline in India fits this assumption; air pollution on the subcontinent has increased strongly in recent years. By contrast, and at first glance, it is surprising that in China, as in Europe, solar radiation has started to increase. Martin Wild suspects that this brightening has to do with the mandatory conversion from brown coal to petroleum and natural gas.

Sunshine is on the increase again. This could have consequences for the global climate. large

The ETH study is supported by a second investigation, which was also published in the same issue of "Science" (3). Rachel Pinker and her team at the University of Maryland analysed satellite records to reconstruct the global solar radiation since the mid-1980s. The US scientists also reaches the conclusion that radiation has increased globally since the 1990s. "Of course, we're gratified that another study using a completely different method confirms our results," says Wild with regard to the US study.

Masked greenhouse effect

It is still very difficult to predict the consequences of the ascertained tendency. Wild starts with the premise that the air pollution from the 1960s until the 1980s masked the greenhouse effect, at least partially. "During this period average global temperatures were not what one would really expect them to have been with the growing concentration of greenhouse gases." In the 1990s global warming was clearly accelerated. This could have something to do with the decline of global dimming," thinks Martin Wild. "The real consequences of the greenhouse effect are now becoming visible." It was not yet possible today to predict how serious the consequences will be of the brightening of the atmosphere.