Wheat fields like this one could yield wheat with less zinc and iron in the future if they are exposed to higher levels of CO2, according to the journal Nature. Zaharov Evgeniy/iStockphoto.com hide caption

itoggle caption Zaharov Evgeniy/iStockphoto.com

Volunteer Tom Strain carries debris from an empty lot as part of an Earth Day cleanup effort in Camden, N.J. The Earth Day events celebrated on April 22 promote a sustainable and clean environment. Mel Evans/AP hide caption

itoggle caption Mel Evans/AP

Police were checking cars throughout Paris on Monday, including near the Arc de Triomphe, as the city tried to cut air pollution by instituting odd-even driving restrictions. Philippe Wojazer /Reuters /Landov hide caption

itoggle caption Philippe Wojazer /Reuters /Landov

A tasseled wobbegong shark (top) lies on the seafloor with the head of a brown-banded bamboo shark in its mouth on the fringing reef of Great Keppel Island on Australia's Great Barrier Reef in August 2011. Reuters/Landov hide caption

itoggle caption Reuters/Landov

An animal's ranking on the food chain depends on where its meals place on the ladder. That puts plants on the bottom (they make all their food), polar bears on top and people somewhere between pigs and anchovies. Lisa Brown for NPR hide caption

itoggle caption Lisa Brown for NPR

Saltwater wetlands that include marshes and shoals on Virginia's Atlantic coast. U.S. coastal wetlands losses were 25 percent greater from 2004-2009, according to a recent federal study. U.S. Fish and Wildlife Service hide caption

itoggle caption U.S. Fish and Wildlife Service

The Chinese border town of Suifenhe is a port of entry for almost all of the hardwood coming from the Russian Far East. Russia is the world's largest exporter of timber, but illegal logging is a growing problem. Courtesty of EIA hide caption

itoggle caption Courtesty of EIA

Former President Bill Clinton hugs House Minority Leader Nancy Pelosi of California as another Democrat, Sen. Barbara Boxer of California, looks on at Wednesday's ceremony naming the Environmental Protection Agency headquarters for him. Pablo Martinez Monsivais/AP hide caption

itoggle caption Pablo Martinez Monsivais/AP

Homeowners can receive up to $4,000 for replacing their lawns with less thirsty plantings, in a rebate program run by the Los Angeles Department of Water and Power. iStock hide caption

itoggle caption iStock

Flames from the Doce Fire rise above a hillside in Arizona's Prescott National Forest at dusk on June 18. Todd Tamcsin/American Red Cross via AP hide caption

itoggle caption Todd Tamcsin/American Red Cross via AP

The new rules' would reduce harmful emissions, the EPA says. David Paul Morris/Getty Images hide caption

itoggle caption David Paul Morris/Getty Images

Heron Island is located on the southern end of the Great Barrier Reef, about 25 miles off the northeast coast of Australia. Ted Mead/Getty Images hide caption

itoggle caption Ted Mead/Getty Images