Tropical storms account for two out of every three raindrops that fall on planet earth. Like melting ice caps at the North Pole, the amount of precipitation that falls near the equator is a telling early sign of global warming.
Scientists with the Tropical Rainfall Measuring Mission (TRMM), a joint venture between NASA’s Goddard Space Flight Center and the Japan Aerospace Exploration Agency, pulled together satellite data going back 27 years. Global rain is hard to measure, because most of it falls over vast expanses of open ocean. In 1997, TRMM launched a low earth orbit satellite to provide data on rainfall amounts in the tropics. What they discovered was that overall rainfall for planet earth had changed very little over the past three decades. “But in the tropics, where nearly two-thirds of all rain falls, there has been an increase of 5 per cent,” according to Guojun Gu, lead author of the study, which was published in the American Meteorological Society’s Journal of Climate.
Why is it getting wetter in the tropics? “A warming climate,” said Robert F. Adler, senior scientist at Goddard’s Laboratory for Atmospheres, “is the most plausible cause of this observed trend in tropical rainfall.” Hotter temperatures lead to greater evaporation of water from the ocean, and create conditions, which allow the atmosphere to hold more moisture. The result: more clouds, and more rain in the earth’s tropical zones.
But does an increase of 5 per cent at the equator really matter? Indeed it does. Rain is how the atmosphere of our planet “sweats.” Condensed water vapor releases heat, and this heat energy is the principle engine driving global air patterns in the lower atmosphere, where weather happens. Seasonal rainfall levels directly impact crop harvests, the breeding of livestock, the deployment of fishing boats, and the timing of construction projects, not to mention the sale of swimsuits and umbrellas.
And as we now know, such effects are not strictly local. As far back as the 1920s, British scientist Sir Gilbert Walker was already puzzling over the fact that monsoons in India had a direct effect on everything from droughts in Africa to the severity of winters in western Canada.
While the rest of the world has not seen the same increase in precipitation as the tropics, earlier computer models generated by the National Center for Atmospheric Research (NCAR) show that individual rainstorms worldwide will be heavier and more severe as global warming continues. This increase in “precipitation intensity” will be felt in the northern United States, northern Europe, and northern Asia, according to a study published in Geophysical Research Letters, a publication of the American Geophysical Union. All of these locales will experience “more intense precipitation for a given storm during this century,” researchers concluded.