How aerosols affect tropical rainfall

The inter-tropical convergence zone (ITCZ), a belt of precipitation caused by the trade winds (which blow from east to west in the north and southern hemispheres near the equator) has been shifting southwards in Central America since 1900, when the industrial revolution and associated atmospheric pollution began in real earnest.

The reason for this shift according to a new study is the cooling effect of aerosols which, produced in large quantities due to industrialisation reflect sun’s heat back into space and work in contrast to green house gases which trap atmospheric heat and cause global warming.

Cooling of the atmosphere results in less rainfall and dry conditions while warming leads to evaporation, convection and rainfall. The study found that since 1900 there has been a steady increase in rainfall in the southern tropics, in contrast to a steady decrease in the northern tropics and the ITCZ has shifted southwards in the Central American region.

The study was conducted by lead- author Dr. Harriet E. Ridley, Department of Earth Sciences, University of Durham, Durham, UK and others. The researchers analysed a stalagmite found in a cave in Belize (a Central American nation) to construct a record of rainfall patterns in the region over the past 450 years. This site is near the northernmost extent of the ITCZ, a remarkably sensitive location for reconstructing even minor variations in ITCZ position.The work was published recently in the journal Nature Geoscience.

The team measured Carbon -13 (δ13C) isotope levels over this period in the various layers of the stalagmite. Carbon isotope serves as a good proxy for rainfall as recorded in the stalagmite over the thousands of years of its formation. The authors support this view using instrumental data from the region.

Stalagmites grow incrementally as drops of water seep through the overlying rock. The growth of the stalagmite is therefore linked to the amount of water reaching it, which is in part controlled by rainfall. Furthermore, every drop of water reaching the cave has a unique chemical signature which is controlled by the prevailing climate, most often temperature and rainfall amount. This chemical signature is then incorporated into the stalagmite layers as it grows. By 'chemical signature' in this case, is meant oxygen and carbon isotopes. At the Belize cave site the carbon isotope value of each layer of the stalagmite is controlled by the amount of water dripping onto the stalagmite and therefore the amount of rainfall. As this carbon isotope values change through time one can see how rainfall has changed.

The stalagmite portions were dated by measuring Uranium-Thorium ratios over the past centuries.

U-Th dating is based on the radioactive decay of uranium-234 to thorium-230. This decay is part of a much longer decay series. In order to conduct Uranium-Thorium dating, powder samples (19 in this case) spread between the top and bottom of the stalagmite were taken.

”A key factor in the method is that uranium is soluble in water while the daughter products are non-soluble. This means that uranium is present in water which seeps into limestone caves and is incorporated into stalagmites but it's non-soluble daughter products are not. Daughter isotopes present in the sample increase through time as the uranium decays and the ratio of the uranium to the thorium is measured to provide an age estimate,” clarified Dr. Ridley in an email to this correspondent.

The study revealed that rainfall in the Northern Tropics where Belize is situated indeed declined dramatically since 1900 when the industrial revolution began.

The authors point to increased aerosol concentrations in the Northern Tropics of Central America as the likely cause. There have been drying events even before 1900, but these coincided with Northern Hemisphere volcanic eruptions which sent aerosols into the atmosphere causing cooling and therefore, dry conditions. Similarly, volcanic eruptions in the southern hemisphere resulted in temporary cooling in the southern tropics.