Amidst warnings that the impacts of climate change for Britain are likely to include more frequent and severe flooding, scientists have found that air pollution has had a significant impact on the amount of water flowing through rivers in the Northern Hemisphere, but that clean air legislation could be reducing flows.

image expired

The paper on air pollution, which was published last week, was produced by scientists from the Met Office, Centre for Ecology and Hydrology (CEH), Universities of reading and Exeter and the Laboratoire de Météorologie Dynamique in France.

It is already established that increased burning of sulphurous coal up to the late 1970s led to additional aerosols in the atmosphere. These are reflective and therefore reduce the amount of sunlight reaching the Earth's surface, an effect known as 'solar dimming'.

This dimming then started to reverse in Europe and North America with the introduction of clean air legislation and a widespread switch to cleaner fuels. In the new study, researchers found that solar dimming increased river flows (relative to projections based on the situation on the ground – surface meteorology), as the reduced amount of sunlight affected the rate of evaporation from the Earth's surface.

However, when the dimming began to reverse, reductions in river-flows were observed.

The Met Office's Nicola Gedney, lead author of the paper, commented, "We detect the impact of solar dimming on enhanced river flows over regions in the heavily industrialised northern extra-tropics. We estimate that, in the most polluted central Europe river basin, this effect led to an increase in river flow of up to 25% when the aerosol [air pollution] levels were at their peak, around 1980.

Gedney added, "With water shortages likely to be one of the biggest impacts of climate change in the future, these findings are important in making projections for the future."

Chris Huntingford, a co-author based at the Centre for Ecology and Hydrology, added, "This study involved using detection and attribution techniques which were able to show a link between aerosols and changes in river flows.

"These studies normally involve looking at how different factors affect temperature, but here we've been able to attribute this man-made influence to an environmental impact."

The study also tested for the effects of deforestation and carbon dioxide increases on river-flow. Peter Cox, from the University of Exeter, said the researchers found "A further indication that increases in carbon dioxide may have increased river-flows by reducing water loss from plants."

A study which appeared in a special edition of the Bulletin of the American Meteorological Society last week suggested that a heat-wave that hit Western Europe last summer could be linked to climate change.

Though water scarcity is expected to present a major challenge in many European regions, Met Office scientists have said that shifting rainfall patterns in Britain mean we are likely to receive more rainfall, but in fewer, more intense bursts as climate change progresses. Although it remains unclear whether summers are to be hotter and drier, or marked by heavy deluges, heavier, more intense rainfall is likely to further increase the risk of flooding, particularly if it falls in summer, when rain cannot percolate as easily through soils.