SETI [ASTRO] Sahara's Abrupt Desertification Started By Changes In Earth's Orbit


Larry Klaes (lklaes@bbn.com)
Fri, 09 Jul 1999 09:52:08 -0400


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>Date: Thu, 8 Jul 1999 19:54:59 GMT
>From: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
>To: astro@lists.mindspring.com
>Subject: [ASTRO] Sahara's Abrupt Desertification Started By Changes In
Earth's Orbit
>Sender: owner-astro@brickbat12.mindspring.com
>Reply-To: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
>
>
>American Geophysical Union
>Public Information Office
>2000 Florida Avenue, N.W.
>Washington, D.C. 20009
>(202) 462-6900 / FAX 202-328-0566
>
>Contact: Harvey Leifert, (202) 777-7507, hleifert@agu.org
>
>For Immediate Release: July 7, 1999
>
>AGU RELEASE NO. 99-20
>
>Sahara's abrupt desertification started by changes in Earth's orbit,
>accelerated by atmospheric and vegetation feedbacks
>
>WASHINGTON -- One of the most striking climate changes of the past 11,000
>years caused the abrupt desertification of the Saharan and Arabia regions
>midway through that period. The resulting loss of the Sahara to agricultural
>pursuits may be an important reason that civilizations were founded along
>the valleys of the Nile, the Tigris, and the Euphrates. German scientists,
>employing a new climate system model, have concluded that this
>desertification was initiated by subtle changes in the Earth's orbit and
>strongly amplified by resulting atmospheric and vegetation feedbacks in
>the subtropics. The timing of this transition was, they report, mainly
>governed by a global interplay among atmosphere, ocean, sea ice, and
>vegetation. Their research is published in the July 15 issue of Geophysical
>Research Letters.
>
>The researchers, headed by Martin Claussen of the Potsdam-Institut fuer
>Klimafolgenforschung (Potsdam Institute for Climate Impact Research)
>employed a model of intermediate complexity to analyze climate feedbacks
>during the past several thousand years of the current, or Holocene, era.
>Called CLIMBER-2 (for CLIMate and BiosphERe, version 2.1), the model led
>to the conclusion that the desertification of North Africa began abruptly
>5,440 years ago (+/- 30 years). Before that time, the Sahara was covered
>by annual grasses and low shrubs, as evidenced by fossilized pollen.
>
>The transition to today's arid climate was not gradual, but occurred in two
>specific episodes. The first, which was less severe, occurred between 6,700
>and 5,500 years ago. The second, which was brutal, lasted from 4,000 to
>3,600 years ago. Summer temperatures increased sharply, and precipitation
>decreased, according to carbon-14 dating. This event devastated ancient
>civilizations and their socio-economic systems.
>
>The change from the mid-Holocene climate to that of today was initiated
>by changes in the Earth's orbit and the tilt of Earth's axis. Some 9,000
>years ago, Earth's tilt was 24.14 degrees, as compared with the current
>23.45 degrees, and perihelion, the point in the Earth's orbit that is closest
>to the Sun, occurred at the end of July, as compared with early January now.
>At that time, the Northern Hemisphere received more summer sunlight,
>which amplified the African and Indian summer monsoon.
>
>The changes in Earth's orbit occurred gradually, however, whereas the
>evolution of North Africa's climate and vegetation were abrupt. Claussen
>and his colleagues believe that various feedback mechanisms within
>Earth's climate system amplified and modified the effects touched off
>by the orbital changes. By modeling the impact of climate, oceans, and
>vegetation both separately and in various combinations, the researchers
>concluded that oceans played only a minor role in the Sahara's
>desertification.
>
>The CLIMBER-2 models showed that feedbacks within the climate and
>vegetation systems were the major cause of Saharan desertification,
>building rapidly upon the effects of the initial orbital changes. The model
>suggests that land use practices of humans who lived in and cultivated
>the Sahara, were not significant causes of the desertification. Further
>investigation is necessary, the researchers say, to determine more precisely
>the specific effects of latitude and oceanic feedback, as compared with
>biospheric feedback, on the timing of the event.
>
>*****
>
>Notes for science writers and science public information officers:
>
>1. You may receive a copy of this paper, Martin Claussen, Claudia Kubatzki,
>Victor Brovkin, Andrey Ganapolski, Philipp Hoelzmann, Hans-Joachim Pachur,
>"Simulation of an abrupt change in Saharan vegetation in the mid-Holocene,"
>by sending an email to Harvey Leifert <hleifert@agu.org>. If you did not
>receive this press release directly from AGU, please include your name,
>name of your publication or organization, and your phone and fax numbers.
>
>2. For further information on the science in this paper, you may contact
Prof.
>Dr. Martin Claussen, <claussen@pik-potsdam.de>, phone +49 (0) 331 288 2522,
>or one of the co-authors, whose postal and email addresses will be found at
>the end of the paper.
>
>3. This press release and the paper to which it refers are not under embargo.
>
>



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