Summer 2012    #16
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Water on Earth
“The Rich get richer, as the Poor get poorer”

      As the Earth warmed up, the global water cycle speeded up. This means that, compared to a few years back, more water passes through each stage of the cycle—evaporation from warmer oceans, condensation in clouds, rainfall, snowfall, and river flows. Where it is already wet, the climate becomes wetter, and floods more frequent; and where it is already dry, the climate becomes drier, and drought more common. The laws of physics that govern meteorology anticipate a speedup, but observing whether it is happening is difficult. (See How the water cycle kicked into high gear, below.)

      Paul Durack and his Australian team reported1 in Science that the geographic patterns of salinity in the world’s oceans are now more pronounced: salty regions became saltier, and less salty regions became fresher. From their study, they concluded that the water cycle on Earth accelerated by 8% per degree of global warming over the 50 years from 1950 to 2000. The distribution of salt in the ocean was linked to where evaporation or precipitation had increased; wherever salinity of seawater increased, the net evaporation had increased, but where salinity decreased, more rain had fallen recently than before.

      Durack also wrote, “the ‘rich get richer’ mechanism is already operating, with fresh regions becoming fresher and salty regions saltier in response to observed warming.” The observed speedup of the water cycle is “double the response projected by current-generation climate models, and suggests that a substantial (16 to 24%) intensification of the global water cycle will occur” in a world that will be 2° to 3°C warmer.

      Uncertain availability of fresh water is a greater risk to human society than is warming by itself, the authors add. The United States is grappling with a widespread drought this summer as severe as the historic drought of the 1950s (story, upper right). The message from climate scientists may soon be sinking into the nation’s consciousness.

1. “Ocean salinities reveal strong global water cycle intensification during 1950 to 2000” by Paul J. Durack, Susan E. Wijffels, and R. J. Matear, Science, v. 336, 455 (27 April 2012).

Over 55% of USA Now in Drought

      The central Midwest of the United States is experiencing a widespread but short-term drought, that threatens the nation's supply of food and animal feed in this growing season. By the third week of July, some 55% of the land area of the US was in moderate drought or worse (tan color, above), and the area continues to increase. The affected area is the greatest since 1956, fifty-six years ago. On the map, 38% of the nation is in severe drought (category D2, orange) or worse, and 17% in extreme drought (category D3—red) or worse. At this writing, all forecasts anticipate that the drought will continue through this October, according to the US Seasonal Drought Outlook, of the US Climate Prediction Center.

      The US Department of Agriculture (USDA) reported that 55 percent of the pasture and rangeland was in poor to very poor condition in the US. In the Plains and Midwest states, crop losses mounted, ranchers liquidated herds, and trees continued to drop leaves and branches. On July 25, the USDA designated even more drought disaster areas, bringing the total for the 2012 crop year to 1369 counties in 31 states.

      And with more than 40% of US agricultural land now in extreme or exceptional drought, “Prices of both corn and soybeans soared to all-time highs, . . with corn climbing more than 50 percent in the past four weeks alone due to the worsening drought, squeezing ethanol and livestock producer margins.” (Reuters, July 20, 2012)

How water cycle kicked into high gear

      Evaporation of water increases 7% for every degree C that water is warmer, according to the laws of physical science. As the ocean heats up, its molecules speed up, and more of them leave the ocean surface every second. Driven by evaporation from the oceans, the entire water cycle speeds up. Rates of evaporation, condensation of water vapor into cloud droplets, and precipitation from clouds have all speeded up since 1950.

      As ocean water evaporates, it leaves behind salt. In the hot subtropics where the sky is clear, the evaporation is intense, and the salinity of the ocean, already high, has increased. But wherever rainfall is abundant, the salinity has been decreasing – likely because greater rainfall has diluted the ocean with more fresh water than before.

      It is important to subtract the precipitation from the evaporation over oceans, when estimating the global water budget. The difference—which is called “net evaporation”—does appear to be increasing globally as the temperature of the Earth climbs. Precipitation itself seems to increase more slowly, at about 3% per degree C of warming.

       The first map above assesses the short term drought impacts for about six months or one growing season. The Drought Monitor product of the University of Nebraska at Lincoln, widely used since 2000, attempts to depict both short- and long-term severity and impacts of drought. Its depiction for the same time (next image, below), shows areas of extreme (D3) and exceptional (D4) drought (in red and maroon colors) in Arizona, New Mexico, and Georgia. The short-term depiction above does not depict such extreme categories of drought in the three states. A glance at the long-term drought assessment, next.


Copyright 2012

 Chief Editor:  Michael A Fortune, Ph.D.


 Email:  editor(at) climate-science dot org

Published at Portland, Oregon, USA

Tel:  (503) 922-0003

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