br>By Alister Doyle
OSLO, Feb 9 (Reuters) - Air pollution tied to industrialisation in the northern hemisphere almost certainly reduced rainfall over Central America in new evidence that human activity can disrupt the climate, a study suggested on Monday.
"We identify an unprecedented drying trend since 1850," the scientists wrote in the journal Nature Geoscience after studying the rate of growth since 1550 of a stalagmite found in a cave in the tiny nation of Belize.
Stalagmites are pointed rocks formed by mineral-rich water dripping from the cavern roof.
The experts, from Britain, the United States, Switzerland and Germany, said the drying in Belize "coincides with increasing aerosol emissions in the northern hemisphere" as the Industrial Revolution pushed up fossil fuel use.
The findings indicate that growing air pollution in countries such as China and India may cause further disruption, especially in Asia, to a band of tropical rains that encircles the globe around the equator and is vital to farming.
The scientists linked the drying to sun-dimming pollution because the nine biggest volcanic eruptions in the northern hemisphere since 1550, spewing out ash that veiled sunshine, also showed up as dry periods in the stalagmite's growth.
For example, an eruption of the Laki volcano in Iceland in 1783, which has also been associated with lower water flows in the Nile, coincided with drought in Belize, they wrote.
Sun-masking pollution cools the northern hemisphere, where most industry is based. That tends to push the Intertropical Convergence Zone, a band of rain encircling the globe, south because it moves towards the warmer hemisphere, they said.
However, the scientists wrote that some unknown natural variations might also provide an explanation for the drying.
In a report published in 2013, a U.N. scientific panel said it was 95 percent likely that greenhouse gases were the main cause of recent global warming.
Many industrialized nations have introduced clean air acts since the 1970s, meaning the world has seen a shift in pollution towards fast-growing emerging nations led by China and India.
"Geographic changes in aerosol emissions should be considered when assessing potential future rainfall shifts in the tropics," they wrote.
Lead author Harriet Ridley of Durham University told Reuters that colleagues had being examining Belize stalagmites for signs of drought about 1,000 years ago, often suggested as the cause of the collapse of the Mayan civilisation.
That study is still continuing.
Baitoushan volcano, on China's border with North Korea, had one of the biggest eruptions in history around the year 1000, according to NASA. (Reporting by Alister Doyle; Editing by Crispian Balmer)