Human activity to blame for global changes in rainfall

2 August, 2007:

Humans – and nobody else – are to blame for the fluctuations in rainfall during the 20th century.

Greenhouse gases created through human activities like the combustion of fossil fuels have had a significant impact on rainfall, a recent study has revealed.

The study has been published in the July 26, 2007, issue of the British science journal Nature.

A group consisting of researchers from Japan, Canada and other countries analysed data using two climate change models based on records of meteorological data taken between 1925 and 1999. One model took into account only the human-linked effects, such as greenhouse gases, while the other included natural phenomena, such as soot emitted by volcanoes and solar insolation.

The results showed that, between the northern latitudes of 40 degrees and 70 degrees, where a large volume of greenhouse gases are emitted, annual precipitation increased by 62 millimetres over a 100-year period. About 50% to 85% of the increase is believed to have been caused by artificial factors.

The annual rainfall in the subtropical and tropical regions between the latitudes of zero degree and 30 degrees north tumbled by 98 millimetres. It is to be noted that 20% to 40% of the decrease is believed to be the result of human activities.

Toru Nozawa, head of the Atmospheric Physics Section of the National Institute for Environmental Studies, Japan, and a member of the research group, said mankind’s emission of greenhouse gases and dust had caused parts of the planet to become either drier or more humid.

According to Francis Zwiers, director of the climate research division of Environment Canada, “the pattern found in the study is one predicted by climate models when they include anthropogenic, human influence on the climate system.”

The researchers found 10% more rain and snow in northern regions, including Canada, Russia and Europe, and the southern tropics region below the equator.

There has been drying away from the equator to 30 degrees North, including Mexico, Central America, sub-Saharan Africa, southern India and Southeast Asia. The main cause behind the shifting patterns is human activity, including a steady rise in greenhouse gas emissions and sulphate aerosols from the burning of fossil fuels.

These shifts may have already had significant effects on ecosystems, agriculture and human health, especially in regions that are sensitive to changes in precipitation, such as the Sahel region in northern Africa, Zwiers said.

As to the future, Zwiers warns that, “we can expect more of the same.”

The overall effect is still one of warming, and warming is changing things.

Human activity is creating a stronger water cycle, moving more water vapour away from the warmest parts of the planet and pushing it towards the poles, essentially making wet areas wetter and dry areas dryer.

The current flooding in Britain is consistent with the predictions, but no specific event can be definitively attributed to climate change.

Human-induced changes have not previously been detected in global studies of precipitation, partly because drying in some regions cancels moistening in others, reducing the global signal, the researchers concluded.