Tuesday

A cold morning at Station Rooftop. Rocket leans into the small glass greenhouse the crew built for seedlings and sighs happily.

"It is warm in here," he says. "The glass lets sunlight in, and the heat does not all get back out."

Raven stands outside in her coat. "What do you notice about the sky? It has no glass."

"But the air is still warmer than space," Rocket says. "Way warmer."

Nova drifts up between them. "Something in the air is doing what the glass does," she says. "Not all of the air. Just certain gases."

Rocket presses his nose to the pane. "Then if you add more of those gases..."

Raven finishes it. "The planet keeps a little more heat."

NASA describes the greenhouse effect as what happens when certain gases in the atmosphere trap heat near Earth's surface.

During the day, sunlight passes through the atmosphere and warms the ground. Some sunlight bounces back to space.

At night, the warm surface releases heat back into the air. Some of that heat is trapped by greenhouse gases.

NASA explains the numbers: about half the sunlight reaching the atmosphere gets to the surface and is absorbed.

The surface radiates that energy as infrared heat. About 90 percent of this heat is absorbed by greenhouse gases and re-radiated.

That slows the escape of heat to space. It is why Earth is warm enough for life as we know it.

The greenhouse effect is natural and necessary. NOAA notes that without carbon dioxide, Earth's average temperature would be below freezing.

NASA lists five key greenhouse gases: carbon dioxide, methane, nitrous oxide, chlorofluorocarbons and water vapor.

The effect grows when more greenhouse gases are added to the air. NASA says that is what has happened over the last 100 years.

The extra gases have come mainly from burning coal and oil for energy and burning gasoline to run cars and planes.

Volcanoes add some too, but NASA notes those natural additions are far too small to explain the amount we measure today.

NASA reports that industrial activities have raised atmospheric carbon dioxide by nearly 50 percent since 1750.

NOAA calls carbon dioxide Earth's most important long-lived greenhouse gas.

Scientists can tell the added carbon came from fossil fuels. NASA says it leaves a distinctive isotopic fingerprint in the air.

This is the cause NASA and NOAA name: the human expansion of the greenhouse effect since the mid-1900s.

Good scientists test other explanations. Could a brighter Sun be warming Earth instead?

NASA measured the energy Earth receives from the Sun. It has followed its natural 11-year cycle with no net increase since 1880.

Over the same years, global temperature rose markedly. NASA concludes the Sun cannot explain the warming trend.

There is another clue. A brighter Sun would warm all layers of the atmosphere. Instead, the upper atmosphere has cooled while the surface warmed.

That pattern is what greenhouse gases produce, because they slow heat leaving the lower atmosphere.

The heat-trapping power of these gases is not new science. NASA notes that in 1856, Eunice Foote showed that carbon dioxide and water vapor trap escaping heat.

In 1896, Svante Arrhenius first predicted that changing the amount of carbon dioxide in the air could change Earth's surface temperature.

Today, NASA instruments measure how much heat is trapped compared with how much escapes. They find more heat is staying than leaving.

Clear thinking. Tomorrow's Station Lab turns your own log into anomalies.