World getting darker with global dimming

A research group from the University of Maryland has prepared the first database for many years of aerosol quantities over land, opening a door to new research into the effect on climate of air pollution.

The research indicates that during the last 30 years visibility over land has decreased worldwide, meaning more pollution or aerosols in the air.

Aerosols are solids or liquids in the air, comprising mainly dust, soot and sulphur dioxide, that is, air pollution.

They are the result of manufacturing methods, burning fossil fuels and tropical rainforests on fire.

Aerosols are dangerous for both the environment and health.

They reflect light back into space, cutting solar radiation at the planet’s surface, or take in radiation from the sun, warming the atmosphere.

They also have an impact on rainfall and cloud formation.

Greenhouse gases such as CO2 are transparent and do not affect visibility.

However, the low levels of greenhouse gases make temperatures rise since the gases lock in surface solar energy and stop its transfer into space as heat.

The researchers found, in an initial review of the database, a constant increase in aerosols from 1973 to 2007.

The prevention of the sun’s radiation from the globe’s surface has resulted in a worldwide dimming particularly in Africa, South America, Australia and Asia, although Europe has in fact brightened through restrictions on sulphur in coal the researchers say.

Kaicun Wang, Robert Dickinson and Shunlin Liang, from Maryland and Texas Universities prepared a database of visibility measurements from 1973 to 2007 taken at 3,250 meteorological stations worldwide and released by the National Climatic Data Centre (NCDC).

The researchers compared statistics about visibility, to satellite data from 2000 to 2007 and found visibility figures to be a justifiable source enabling scientists to study the relationship between changes in the climate and air pollution.

The group’s discovery of a regular increase in aerosols over past decades means an increase in sulphate in the air.

However, work by the Intergovernmental Panel Climate Change demonstrates a decrease from 1980 to 2000 of worldwide emissions of sulphate.

Ellsworth Dutton, a scientist from Colorado’s National Oceanic and Atmospheric Administration’s Earth System Research Laboratory, disagreed, pointing out that the sites of the meteorological stations, often close to centres of population, are not indicative of the world’s total land mass geography.

Furthermore, the researchers’ methods do not recognise differences in aerosols higher than 10 metres approximately of the atmosphere, which wind and rain can change.