Global Dimming is a recently discovered phenomenon which says that the amount of sunlight reaching the Earth’s surface has been gradually falling.
Although the effect varies greatly from place to place, overall the decline in the amount of sunshine reaching the Earth’s surface amounts to one to two per cent globally, every decade between the 1950s and the 1990s.
The most alarming aspect of global dimming is that it may have led scientists to underestimate the true power of the greenhouse effect. According to a report on Global Dimming on the BBC’s website:
They know how much extra energy is being trapped in the Earth’s atmosphere by the extra carbon dioxide we have placed there.
What has been surprising is that this extra energy has so far resulted in a temperature rise of just 0.6 degree Celsius.
This has led many scientists to conclude that the present-day climate is less sensitive to the effects of carbon dioxide than it was, say, during the ice age, when a similar rise in CO2 led to a temperature rise of six degrees Celsius.
But it now appears the warming from greenhouse gases has been offset by a strong cooling effect from dimming – in effect two of our pollutants have been cancelling each other out.
This is bad because it means that the climate is far more sensitive to greenhouse gases than previously thought and:
CO2 levels are projected to rise strongly over coming decades, whereas there are encouraging signs that particle pollution is at last being brought under control.
“We’re going to be in a situation unless we act where the cooling pollutant is dropping off while the warming pollutant is going up.
“That means we’ll get reducing cooling and increased heating at the same time and that’s a problem for us,” says Dr Cox.
Even the most pessimistic forecasts of global warming may now have to be drastically revised upwards.
If you are interested to learn more about Global Dimming –
- Horizon had a compelling program about it – the transcript of which makes fascinating reading
- The guardian have a detailed feature on Global Dimming
- There’s a Global Dimming FAQ
- The Journal Science published a paper on it: The Cause of Decreased Pan Evaporation Over the Past 50 Years, M Roderick and G Farquhar; Science Volume 298 (2002), pages 1410-1411
- Other papers include: Observed Reductions of Surface Solar Radiation at Sites in the US and Worldwide, B Liepert, Geophysical Research Letters Volume 29 (2002), pages 1421-1433 and
- Global Dimming: A Review of the Evidence, G Stanhill and S Cohen, Agricultural and Forest Meteorology Volume 107 (2001), pages 255-278