Picture This: Christmas from space

SAN FRANCISCO — Evening trips to the mall. Christmas parties. Rooftop lights. The December holidays are bright — dazzling enough to be seen by satellites orbiting high above Earth. Researchers recently used satellite data to track when, where and how often we turn on lights. The findings, they say, point to how human activities drive electricity use.

Scientists sent radiometers into space and pointed them toward Earth. These instruments measure the intensity of light. In 2012, the research team released a set of “Earth at Night” maps. They had used data collected on nights with ideal conditions — evenings that were both moonless and cloud-free.

Miguel Román is a physical scientist at NASA Goddard Space Flight Center in Greenbelt, Md. He and his team wanted to analyze how light patterns change from day to day. To do that, his team improved its scans so that the scientists could collect data even on nights with clouds and a bright moon. (Unfortunately, the system can’t cope with snow. Light reflecting off the white stuff “contaminates the signal,” Román says.)

From 2012 to 2014, the satellite snapped daily pictures of 70 U.S. cities. The scientists used those images to measure how much the cities brightened between Thanksgiving and New Year’s Day. They compared the cities’ holiday glow to their light output the rest of the year.

It was “a huge effort,” says Román. “It took three years’ worth of data.” But with the team’s revamped system, “we can do comparisons across cities, even across neighborhoods within cities,” Román says. He described his group’s new findings December 16 at the fall meeting of the American Geophysical Union.

Many cities radiated 20 to 50 percent more light during holiday nights, the researchers found. The light intensity climbed a bit more in the suburbs than in busy city centers. But overall, it seems that everyone in the United States — regardless of income or ethnic background — celebrates the holidays, Román says.

A different picture emerged when his team analyzed another part of the world: the Middle East. There, the major holiday is Ramadan. It’s the ninth month of the Islamic calendar. On those 30 days, Muslims fast from dawn to dusk.

Besides postponing meals, people also shop and go to work later in the day. These behavioral changes showed up on the satellite maps as “a shift in the timing of activity, not just an increase in light sources,” Eleanor Stokes reported at the meeting. Stokes is working on her Ph.D. in forestry and environmental studies at Yale University in New Haven, Conn. She co-led the new light-at-night project with Román.

In their Middle East analysis, nighttime lighting seemed to track social and cultural factors. Richer cities, for instance, glowed more intensely than did poorer ones during Ramadan. Warfare left its mark as well. The satellite maps essentially went dark when conflicts destroyed a region’s electric-power-delivery system.

“What we found here is…human activity is driven not just by energy and electricity prices but also by social and cultural context,” Stokes says. Such factors include both wars and holiday celebrations.

Information like that is important because the planet is getting hotter. Greenhouse gases, such as carbon dioxide, are trapping heat in Earth’s atmosphere. Human activities must take much of the blame. And cities produce more than two-thirds of these greenhouse gases. If humanity is to cope with this extra heat, Stokes says, people need to understand energy-use patterns and the human behaviors that shape them.

Power Words

atmosphere   The envelope of gases surrounding Earth or another planet.

carbon dioxide  A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter (including fossil fuels like oil or gas) is burned. Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

global warming  The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

geophysics    The study of matter and energy on Earth and how they interact.

greenhouse gas   A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.

radiation  Energy, emitted by a source, that travels through space in waves or as moving subatomic particles. Examples include visible light, infrared energy and microwaves.  

radiometer   A scientific instrument that measures the intensity of light.