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December 8, 2009; 106 (49)

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Cover image: Pictured are four natural elements (clockwise from right: a calving glacier, dust over the Atlantic Ocean, methane bubbles in the Arctic Ocean, and burning in the Amazon rainforest) related to highly nonlinear environmental change that could be triggered by anthropogenic global warming. In an introduction to the Tipping Elements in Earth Systems Special Feature, Hans Joachim Schellnhuber reviews the rugged aspects of interaction of human civilization and its planetary matrix. Papers in the Special Feature analyze individual tipping elements or prompt scientific and policy recommendations for stabilizing Earth's environment. See the article by Schellnhuber on pages 20561–20563. Images: glacier courtesy of Ulrich Heim (Luebeck, Germany), methane bubbles courtesy of Alfred Wegener Institute, dust cloud courtesy of NASA, and Amazon rainforest courtesy of Greenpeace.

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