A new study has determined that the true extent of global warming can only be made when the smog of pollution has cleared from the skies.
According to a report in New Scientist, the cleaner, clearer skies mean measurements of warming temperatures are not confused by smog.
This is the reason why the current measurements of a 0.04 °C warming per year can be taken as the true signal of man-made global warming.
As part of the study, a team led by Martin Wild, of the Institute for Atmospheric and Climate Science in Switzerland, has been monitoring changes in temperatures, the amount of aerosols in the air and the amount of sunlight that is getting through and warming the different continents.
Their data, taken from over 2500 locations, shows that in Europe and the US the amount of solar radiation reaching the ground is increasing.
Satellite data shows that the optical density of the air above Europe, a measure of how dense the suspension of aerosols is, has been stable since 2000.
"This suggests that aerosols are no longer influencing brightness and something else is making it more sunny - possibly a reduction in cloud cover," said Wild.
The stability of smog levels also suggests that ever-cleaner air cannot be the cause of increasing temperatures. Satellite data shows that overall the continents have warmed by 0.04 °C per year between 1999 and 2007.
"This could be the true, unadulterated signal of man-made global warming," said Wild.
Based on optical density data from 6 sites in the German and Swiss Alps, Rolph Philipona of MeteoSwiss, the national weather service of Switzerland, said that as much as half of the warming observed in the 1980s and 1990s could have been due to "global brightening" as the smog cleared.
But, it's a different story in other parts of the world.
Previous research led by Padma Kumari of the Indian Institute of Tropical Meteorology had shown that India became less sunny, starting in the 1980s as its industrial activity increased. The latest data shows the Indian "solar dimming" is continuing.
In China, the situation is slightly different.
Wild's data shows that the country was dimming already in the 1980s, but then the smog levelled off in the 1990s. This may have been because the country cleaned up its act for a bit, or could simply be the consequence of the nation's then economic downturn.
Since 2000, overall the optical density of the air above China has risen dramatically - an observation which correlates well with anecdotes of very low visibility in major Chinese cities.