The expansion of the tropical belt towards the north pole may be due to human made factors like black carbon and tropospheric ozone, says a new study.
In the Northern Hemisphere, the main cause appears to be black carbon and tropospheric ozone pollution, where as in the Southern Hemisphere, the cause of tropical expansion was earlier linked to the depletion of stratospheric ozone.
The lead author of the study, Robert J. Allen, notes that continuing expansion will have large-scale impacts on atmospheric circulation worldwide.
“If the tropics are moving poleward, then the subtropics will become even drier,” Allen said. “If a poleward displacement of the mid-latitude storm tracks also occurs, this will shift mid-latitude precipitation poleward, impacting regional agriculture, economy, and society.”
According to recent observations, the tropics have been widening by 0.7 degrees every decade, with global warming reason to some, but not all, of the tropical expansion.
“Both black carbon and tropospheric ozone warm the tropics by absorbing solar radiation,” Allen explained. “Because they are short-lived pollutants, with lifetimes of one-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.”
Black carbon aerosols are tiny particles of carbon created from the burning of biomass, and incomplete fossil fuel combustion, such as in diesel engines. Tropospheric ozone is a pollutant generated from volatile organic compounds (VOCs) reacting with sunlight.
“Greenhouse gases do contribute to the tropical expansion in the Northern Hemisphere,” Allen said. “But our work shows that black carbon and tropospheric ozone are the main drivers here. 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.”
As the tropics spread further, they also carry wind and precipitation patterns with them, potentially drying out the tropics relative to their current state.
“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,” he said. “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 in our work is consistent with the poleward displacement of precipitation seen in these models.”
We should implement policies to reduce the emissions of green house gases, tropospheric ozone and black carbon that are driving the tropical expansion before the tropics expand even more.