Scientists investigate historical effects of CO2 on rainfall
Clues left behind in a cave in central America have allowed an international, multidisciplinary team of researchers to produce a new paleoclimate rainfall record that illustrates the contribution of manmade industrial emissions to less rainfall in the northern tropics.
The team, part funded by the European Research Council and led by the UK’s Durham University, reconstructed a rainfall record stretching back 450 years from unprecedented speleothem samples from Yok Balum cave in Belize using uranium thorium dating from the chemical composition in the speleothems containing aragonite, which is high in uranium. The research highlights the effects produced through manmade usage of fossil fuels and signifies a drying trend beginning around 1850 that parallels the steady rise in sulphate aerosols in the atmosphere linked to the industrial boom in Europe and North America.
Harriet Ridley, lead author and academic at the Durham University, said: “The research presents strong evidence that industrial sulphate emissions have shifted this important rainfall belt, particularly over the last 100 years. Although warming due to manmade carbon dioxide emissions has been of global importance, the shift of rain belts due to aerosol emissions is locally critical as many regions of the word depend on this seasonal rainfall for agriculture.”
As part of the research, scientists also identified nine short-lived dry spells lasting from two to ten years in the northern tropics since 1550. The dry spells followed large volcanic eruptions in the Northern Hemisphere that produced similar emissions as those produced by burning fossil fuels.
This data provides strong evidence that any release of sulphate aerosols into the upper atmosphere led to shifts in rainfall patterns due to moderated temperatures in the Northern Hemisphere by reflecting the sun’s radiation. The resulting shift in the Intertropical Convergence Zone (ITCZ), a tropical rainfall belt neat the equator, shifted towards the warmer Southern Hemisphere leading to dryer conditions in the northern tropics.
Overall, the position of the ITCZ responds to the temperature gradient between the poles and the equator. When there is less gradient, the ITCZ moves further north in the north and further south in the south. Yet the relative temperature difference between the two hemispheres also plays a crucial role in modulating the position of the ITCZ. The position of the ITCZ shifts towards the warmer hemisphere.
The research is published in Nature Geoscience.
