U.S. President Barack Obama, right, smiles after a group of children waved flags and flowers to cheer him during a welcome ceremony Wednesday with Chinese President Xi Jinping at the Great Hall of the People in Beijing. In a surprise agreement late Tuesday, China agreed to a first-ever target for capping its carbon emissions, while the United States pledge to cut faster than previously planned. Andy Wong/AP hide caption

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Data sources: David Ropeik/Harvard University, National Weather Service, World Health Organization, Northeastern University Laboratory for the Modeling of Biological and Socio-Technical Systems, National Geographic, United States Census Adam Cole and Ryan Kellman/NPR hide caption

itoggle caption Adam Cole and Ryan Kellman/NPR

Scenes from an outbreak: Ebola survivor Dr. Kent Brantly; Texas Health Presbyterian Hospital in Dallas; A worker cleans the apartment where Ebola victim Thomas Eric Duncan stayed in Dallas; experimental vaccine; the Carnival Magic cruise ship off Cozumel, Mexico. Jessica McGowan/Getty; Mike Stone/Getty Images; Jim Young/Reuters/Landov; Steve Parsons/AP; Angel Castellanos/AP hide caption

itoggle caption Jessica McGowan/Getty; Mike Stone/Getty Images; Jim Young/Reuters/Landov; Steve Parsons/AP; Angel Castellanos/AP

In the northwestern United States, this crayfish would be just a friendly bit of local fauna. But in Scotland, it's an invasive species wreaking havoc on trout streams. Ari Shapiro/NPR hide caption

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States are moving further apart on hot-button issues such as abortion and health care — and many may resist laws set in Washington. Frankljunior/iStockphoto.com hide caption

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