Sustainability Indicator: 70% Urban

Today's sustainability indicator, 70 percent, is the proportion of the world's population that will be living in urban areas by 2050. About half of the population is now rural.

This month's indicators:

A NASA visualization of the world's surface ocean currents and their change over time. Courtesy NASA (Source: Bloomberg)

Running Time: 03:02

A soot-covered scientist at the Grand Academy of Lagado successfully extracted sunlight from cucumbers and stored it in aluminum cans. The discovery will allow energy producers to generate solar-powered electricity at night and will provide unlimited on-demand wintertime heat, researchers said.

A lofty idea, but purely fictional. The Grand Academy is an invention of Jonathan Swift, in his 1726 novel, Gulliver’s Travels. The nearly 300-year-old tale comes to life again via Vaclav Smil, the prolific (nonfiction) author and energy-and-environmental systems professor at the University of Manitoba. He discusses Swift’s cucumbers in a 2011 essay, titled "Global Energy: The Latest Infatuations." The story resonates today because it fulfills what many people are looking for from the energy industry -- a simple solution to complex problems.

A contractor hangs upside down during construction of San Diego Gas & Electric Co.'s Sunrise Powerlink, a 117-mile, 500-kilovolt electric transmission line that runs from Imperial county to San Diego, California.

SDG&E, a subsidiary of Sempra Energy, provides energy to 3.4 million consumers in San Diego and southern Orange counties.

The Fukushima nuclear plant meltdown in March 2011 was the largest accidental release of radiation into the ocean in history, according to Woods Hole Oceanographic Institution scientists who studied the area in June. Some water samples showed radioactive particles, or radionuclides, at levels 1,000 times higher than before the earthquake, tsunami and nuclear accident -- but still safely below levels dangerous to humans and much sea life.

This image shows the scientists' route and the locations of their sampling stations. The red and yellow areas mark the Kuroshio Current, a regular feature in waters east of Japan. It carried radionuclides away from the accident, and also blocked them from dispersing to the south. The researchers' study was published this week in the Proceedings of the National Academy of Sciences.

The illuminated Grangemouth Oil Refinery exhales steam and carbon dioxide on March 29 in Grangemouth, Scotland.

U.K. government ministers triggered panic-buying last week by suggesting motorists should store gas in cans at home petrol , after fuel-truck drivers threatened to strike. It contributed to one of the worst weeks that U.K. Prime Minister David Cameron has had since coming to power in May 2010.

Sandhill Cranes gather in a cornfield near the Rowe Sanctuary, south of Gibbon, on Monday, Jan. 16, 2012. The cranes usually spend the winter further south in Texas and Oklahoma but due to drought conditions many cranes have been spending the winter in Kansas and Nebraska.

In a typical winter, the Texas Gulf Coast is packed with tens of thousands of birds-- songbirds, waterfowl, catbirds, gnatcatchers, warblers and other migrants. But this year, an annual count done just before Christmas found the population had dropped steeply.

Sam Courtney, installation supervisor for Wind Turbines of Ohio LLC, unbolts the tail section of a wind turbine tower taken down for repairs yesterday at the KMC Sportsman Lodge in Gurnsey County, Ohio.

Wind farm developers installed 6,810 megawatts of turbines in the U.S. last year, 31 percent more than in 2010, as they rushed to qualify for a expiring U.S. tax grants. That's enough to power more than 5 million homes.

Sir Isaac Newton's law of gravity states that the pull of an object depends on its mass. This means that when the Earth's mass shifts, so does its field of gravity.

The first accurate measurement of melting glaciers in Greenland came from gravitational readings by the twin satellites known as GRACE (Gravity Recovery and Climate Experiment), which produced this rendering of Earth's gravity field. The satellites found that global warming reduced Greenland's ice shield by 240 gigatons of mass each year from 2002 to 2011, corresponding to a global sea level rise of 0.7 millimeters per year.

Romania is looking to exploit shale gas reserves in a drive for energy self-sufficiency. Chevron has a concession covering more than 2,300 square miles in the nation's eastern Barlad region. The company plans to drill the first exploration well in the second half of 2012, depending on the licensing process. As in the U.S., critics of exploration warn of environmental risks.

The village of Banca, Romania, about 170 miles from Bucharest, is pictured here, on March 22.