It is estimated that as much as 120,000 cubic miles (500,000 cu km) of low-salinity water lies beneath the floor of the world’s oceans. “The sea level was [once] much lower than it is today,” says Vincent Post of Flinders University in Adelaide, so the coastline was farther out. At that time, rain would “fill up the water table in areas that are nowadays under the sea.” Scientists hope that these undersea reserves may in time help some of the over 700 million people who have limited access to clean water.

Half the large animal species once found in the Sahara have either disappeared or are now confined to only 1 percent or less of their original range. Regional instability and widespread hunting are only partly to blame. Although the biodiversity of deserts rivals that of forests, researchers say that “the lack of scientific attention given to desert biodiversity is mirrored by a lack of financial support.” As a result, it is difficult for conservationists to monitor endangered desert ecosystems.

It has been estimated that 1 in 8 deaths in 2012 occurred as a result of air pollution. According to the World Health Organization, “air pollution is now the world’s largest single environmental health risk.”