It's New Year's Day, 2101. Somehow, humanity survived the worst of global warming—the higher temperatures and sea levels and the more intense droughts and storms—and succeeded in stabilizing the Earth's climate. Greenhouse gas concentrations are peaking and are expected to drift downward in the 22nd century. The rise in global temperatures is slowing and the natural world is gradually healing. The social contract largely held. And humanity as a whole is better fed, healthier, and more prosperous today than it was a century ago.

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This scenario of an imagined future raises a key question: What must we do in the 21st century—especially in 2009 and the years just following—to make such a future possible, and to head off the kind of climate catastrophe that many scientists now see as likely? This question inspires the theme of the Worldwatch Institute's State of the World 2009 report: how climate change will play out over the coming century, and what steps we most urgently need to take now.

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The year 2009 will be pivotal for the Earth's climate. Scientists have warned that we have only a few years to reverse the rise in greenhouse gas emissions and help avoid abrupt and catastrophic climate change. The world community has agreed to negotiate a new climate agreement in Copenhagen in December 2009. Early that same year, Barack Obama will be sworn in as the 44th U.S. President. The United States, one of the world's largest producers of greenhouse gases, will have its best chance to provide global leadership by passing national climate legislation and constructively engaging with the international community to forge a new consensus on halting emissions.