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A Continent Split by Climate Change:

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  • Mike Neuman
    2005-9 FOR IMMEDIATE RELEASE MAY 24, 2005 A Continent Split by Climate Change: New Study Projects Stronger Drought in Southern Africa, More Rain in
    Message 1 of 1 , May 24, 2005
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      2005-9 FOR IMMEDIATE RELEASE MAY 24, 2005

      A Continent Split by Climate Change:

      New Study Projects Stronger Drought in Southern Africa, More Rain in
      Sahel

      Contacts:

      Anatta, NCAR Media Relations
      303-497-8604
      anatta@...

      James Hurrell, NCAR Climate and Global Dynamics Division
      303-497-1383
      jhurrell@...

      Cheryl Dybas, NSF Public Affairs
      703-292-7734
      cdybas@...

      BOULDER - A new analysis of Africa's past and future climate shows
      that the Sahel region, which experienced catastrophic drought for
      decades until rains returned in the 1990s, could experience wetter
      monsoons for decades to come. However, drought across southern Africa
      is projected to intensify further. Oceanic warming consistent with an
      increase in greenhouse gases appears to be a factor in these expected
      21st-century changes to Africa's monsoons.

      James Hurrell of the National Center for Atmospheric Research (NCAR)
      presents the findings today in New Orleans at the spring meeting of
      the American Geophysical Union. The study, conducted with Martin
      Hoerling (National Oceanic and Atmospheric Administration), was
      supported by NOAA and the National Science Foundation, NCAR's primary
      sponsor.

      The study, which draws on 80 simulations of global climate from five
      computer models, provides new evidence linking drought in southern
      Africa to the warming of the Indian Ocean. However, it contradicts
      earlier studies that tied Indian Ocean warming to drought in the
      Sahel.
      Instead, the new results relate Sahelian drought to a late 20th-
      century cooling of the North Atlantic Ocean. A subsequent switch to
      North Atlantic warming, partly consistent with the impact of
      greenhouse gas increases, is the main factor behind the Sahel's
      recent swing from drought to moist conditions, the researchers
      believe.

      "Changes in the Indian and Atlantic oceans are causing large regional
      effects in Africa, and these have substantial impacts on people. Now
      we can explain these climatic effects," says Hurrell.

      Recurrent drought since the 1970s has plagued southern Africa,
      including Angola, Zambia, and Zimbabwe. Meanwhile, the nearby Indian
      Ocean has warmed more than 1 degree Celsius (0.6 degree Fahrenheit)
      since 1950. As showers and thunderstorms develop in the rising air
      above the warming ocean, says Hurrell, they help lead to sinking air
      and drought in a surrounding ring that includes southern Africa.

      "In our models, the Indian Ocean shows very clear and dramatic
      warming into the future, which means more and more drought for
      southern Africa," says Hurrell. "It is consistent with what we would
      expect from an increase in greenhouse gases."

      Hurrell and Hoerling compared model results from 1950-99 to several
      control runs that omitted the Indian Ocean warming. None of those
      runs showed the magnitude of drying that actually occurred in
      southern Africa. When the models did include the Indian Ocean
      warming, southern Africa consistently dried out. The models also
      project that by 2049, monsoons across southern Africa could be 10% to
      20% drier than the
      1950-99 average.

      A different process appears to shape rainfall in the Sahel. When sea-
      surface temperatures are warmer in the South Atlantic than in the
      North, it pulls the Sahelian monsoon cycle south as well, depriving
      the region of its usual rains.

      "This was the situation during much of the latter half of the 20th
      century," says Hurrell. "We believe the North Atlantic Ocean cooling
      was natural and masked an expected greenhouse-gas warming effect."

      Since 1990, the sea-surface temperature pattern has reversed, warming
      more rapidly in the North Atlantic than in the South. The models
      examined by Hurrell and Hoerling show this trend intensifying in
      future decades. They project that the Sahel monsoon will be some 20%
      to 30% wetter by 2049 compared to the 1950-99 average.

      The warming of Indian Ocean waters is well beyond the range expected
      from natural processes. This strengthens the case that greenhouse
      gases are involved, says Hurrell. In the Atlantic, natural
      variability affects ocean temperatures more strongly, making it more
      difficult to attribute changes there to greenhouse-gas effects.

      Paleoclimate records show that even greater climate swings have
      occurred in Africa's monsoons, most likely related to past variations
      in solar output and in Earth's orbit. "From a paleoclimate
      perspective, the recent African dryings appear to be neither unusual
      nor extreme," says Hurrell.

      Monsoon rains, critical to life in much of Africa, shift north and
      south with the seasons. They normally reach the Sahel from July to
      September and the southern part of the continent from February into
      April. Low-pressure centers moving west from the Sahel during the
      monsoon often serve as seed for tropical storms and hurricanes in the
      North Atlantic. Hurrell's work does not address the possible impact
      of increased rains in the Sahel on future Atlantic hurricane activity.

      For their study, Hurrell and Hoerling examined output from computer
      models at NCAR, NASA, NOAA, the European Centre for Medium-Range
      Weather Forecasts, and France's National Center for Meteorological
      Research (CNRM).

      NCAR'S primary sponsor is the National Science Foundation. Opinions,
      findings, conclusions, or recommendations expressed in this
      publication do not necessarily reflect the views of the National
      Science Foundation.

      -The End-

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      http://www.ucar.edu/news/releases


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      --

      Anatta
      Media Relations
      National Center for Atmospheric Research
      Phone: (303) 497-8604;
      Fax: (303) 497-8610
      E-mail: anatta@...
      http://www.ucar.edu/news
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