Dr Jim Haywood, Aerosol Research Manager
Global dimming is the observed widespread reduction in sunlight at the surface of the Earth. Global dimming shows significant regional variations.
The amount of sunlight received at the Earth's surface has declined at some observing stations, but not at others.
Continuous dimming has been observed at Hong Kong.
No dimming has been seen at some sites (such as Lerwick, UK).
Other sites have shown dimming until around 1980, but subsequently show significant brightening (e.g. Uccle, Belgium and Israel).
The most likely cause of global dimming is the interaction with sunlight of microscopic aerosol particles from human activities. Aerosols reflect and absorb sunlight, reducing the intensity of sunlight at ground level.
Some aerosol particles are produced by human activities such as the burning of fossil-fuels and industrial processes. Global dimming can, therefore, be related to increased pollution, while brightening is related to a reduction in pollution. Sulphur is an example of an aerosol-forming pollutant. Sulphur emissions from European countries have fallen from 18 Tg/y in 1980 to 4 Tg/y in 2000 (see Fig. 1). This partly explains the brightening seen at Uccle in Belgium.
The effects of global dimming (and global brightening) are included in the global climate models that are used to simulate past and future changes in climate. The Met Office Hadley Centre's computer models include many aerosol species such as aerosols from sulphur dioxide, smoke from deforestation and stubble burning, mineral dust, and soot.
Results from our climate models are consistent with observations of global dimming and global brightening. Climate model simulations show regional dimming over many areas from 1950-1980, due to the increased aerosol emissions.
Air quality regulations introduced in many western countries reduced atmospheric aerosol pollution between 1980 and 2000 causing brightening over Europe.
However, atmospheric aerosol pollution has continued to increase in the developing countries since 1980, causing continued global dimming.