A Kemp's ridley sea turtle like this one traveled 4,600 miles across the Atlantic ocean in 2008. After being rehabilitated in Portugal, it is being reintroduced into its native Gulf of Mexico waters on Tuesday. US EPA via flickr hide caption

itoggle caption US EPA via flickr

The crippled Fukushima Dai-ichi nuclear power station is seen through a bus window on Nov. 12. The four reactors that failed were stabilized this month. David Guttenfelder/AFP/Getty Images hide caption

itoggle caption David Guttenfelder/AFP/Getty Images

Air travel contributes only 2 to 4 percent of carbon dioxide emissions worldwide. A new ruling says airlines flying into and out of European airports will have to pay a price for the carbon dioxide they emit from burning jet fuel. Above, a plane takes off from the Geneva airport last year. Fabrice Coffrini/AFP/Getty Images hide caption

itoggle caption Fabrice Coffrini/AFP/Getty Images

Nuclear Regulatory Commission Chairman Chairman Gregory Jaczko (center) speaks Wednesday during a meeting of the House Oversight and Government Reform Committee. His fellow commissioners, from bottom left: Kristine Svinicki, William Magwood IV and William Ostendorff. Chip Somodevilla/Getty Images hide caption

itoggle caption Chip Somodevilla/Getty Images

The world's forests act as massive sponges, sucking carbon from the atmosphere. Above, an aerial photo from 2009 shows massive deforestation in the Brazilian state of Para. Antonio Scorza/AFP/Getty Images hide caption

itoggle caption Antonio Scorza/AFP/Getty Images

Christopher Miller samples sediments from an excavation site in South Africa. Archaeologists found layers upon layers of burned bedding material, indicating that the hunter-gatherers who lived here 77,000 years ago stayed for a long time. Courtesy of Lyn Wadley hide caption

itoggle caption Courtesy of Lyn Wadley

Tall grasses in the San Joaquin valley in California suck carbon dioxide out of the air and store it in the soil. It's one option that environmentalists are pursuing for greenhouse gas "offsets" that can be bought and sold in the state. Christopher Joyce/NPR hide caption

itoggle caption Christopher Joyce/NPR

Matt Horton is CEO of Propel Fuels, a company that installs equipment and pumps to handle biofuels. Horton says California is a great market because consumers are interested in renewable fuels. Christopher Joyce/NPR hide caption

itoggle caption Christopher Joyce/NPR

Beneath 8,200 feet of water, the Alvin submarine scopes out the Pacific's seafloor in the 1970s. The geologists aboard weren't searching for life — they were on the hunt for hot spots and undersea thermal vents. Courtesy of Kathy Crane hide caption

itoggle caption Courtesy of Kathy Crane

Nathan Phillips looks at methane data plotted on a map of Boston streets on Nov. 17. Data from a mobile methane "sniffer" and a GPS show a real-time display of the gas levels in Google Earth. The orange spike in the center of the screen, on St. Paul Street, indicates methane levels about two or three times above normal levels, Phillips says. Robin Lubbock/WBUR hide caption

itoggle caption Robin Lubbock/WBUR

The world's population has just hit 7 billion people and continues to grow. Population experts are concerned about the rise in consumption that will accompany the increase in people. One California home builder, ZETA Communities, designs and builds small, highly energy-efficient homes.

Zeta Communities hide caption

itoggle caption Zeta Communities

This abalone shell was found with ocher and a grinding stone. The iron oxide was used as a pigment to paint bodies and walls, as well as to thicken glue.

Science/AAAS hide caption

itoggle caption Science/AAAS