Man-made pollution is helping to push the tropics northwards, research finds.
The effect could impact weather and climate, making sub-tropical regions drier and creating wetter and stormier conditions further north.
Scientists already knew the tropics were widening by around 0.7 degrees of latitude per decade.
Ozone depletion in the stratosphere is thought to be the main driver of this expansion in the southern hemisphere.
But the new findings indicate that tropic widening in the northern hemisphere is mainly due to black carbon and ozone lower in the atmosphere.
While stratospheric ozone provides vital protection against harmful solar radiation, the same gas in the lower troposphere is a man-made pollutant and harmful to health.
Professor Robert Allen, from the University of California at Riverside, who led the climate modelling study, said: “Both black carbon and tropospheric ozone warm the tropics by absorbing solar radiation.
“Because they are short-lived pollutants, with lifetimes of one to two weeks, their concentrations remain highest near the sources: the northern hemisphere low-to-mid-latitudes. It’s the heating of the mid-latitudes that pushes the boundaries of the tropics poleward.”
The research is reported in the journal Nature.
“If the tropics are moving poleward, then the sub-tropics will become even drier,” said Dr Allen.
“For example, the southern portions of the United States may get drier if the storm systems move further north than they were 30 years ago. Indeed, some climate models have been showing a steady drying of the subtropics, accompanied by an increase in precipitation in higher mid-latitudes.
“The expansion of the tropical belt that we attribute to black carbon and tropospheric ozone... is consistent with the poleward displacement of precipitation seen in these models.”
Dr Allen added: “We need to implement more stringent policies to curtail their emissions, which would not only help mitigate global warming and improve human health, but could also lessen the regional impacts of changes in large-scale atmospheric circulation in the northern hemisphere.”
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