Forest fires in Australia and California, droughts and water shortages in the Mediterranean – in the last few years, events such as these have become more frequent. Researchers attribute this to the fact that the tropics, the warm region surrounding the Equator, appear to be expanding. And that leads to the affected areas becoming hotter and drier. According to the official definition, the tropics extend across the Equator between the latitudes of 23 degrees North and 23 degrees South. The central area is humid, with a great deal of precipitation, while the marginal regions in the north and south are hot and dry. As a result of climate change, however, for some time now the dry regions have been expanding northwards in the Northern Hemisphere – as far as Southern California – and southwards in the Southern Hemisphere.
But up to now, climate researchers have had a problem. They couldn’t conclusively explain this obvious expansion of the tropics using their climate models. The models simply didn’t show the magnitude and the regional characteristic of the observed expansion. A team working with the physicists Hu Yang and Gerrit Lohmann at the Alfred Wegner Institute, Helmholtz Centre for Polar and Marine Research in Bremerhaven (AWI) has now discovered the likely cause. As the AWI experts report in the Journal of Geophysical Research Atmospheres, the reason for the expansion appears to be an altered warming of the ocean. To date, experts assumed that processes in the atmosphere played a major role – for instance a change in the ozone concentration or the aerosols. It was also thought possible that the natural climate fluctuations that occur every few decades were responsible for the expansion of the tropics. For many years researchers had been looking in the wrong place, so to speak.