T H E    G R E E N H O U S E    E F F E C T

   In 1827, a French scientist named Jean-Baptiste Joseph Fourier made a compelling observation. He concluded that the Earth's atmosphere acts like an immense garden greenhouse, trapping heat to warm the planet. (If he'd lived in the 20th Century he might have called it the "Car Parked in the Open on a Sunny Day with its Windows Rolled Up" but luckily he didn't!) He called the phenomenon "un effet de verre," which means "an effect of glass." But the "greenhouse effect" is an entirely natural phenomenon, and one which, on the whole and in the past, has been very beneficial to humans and all life on our planet. Without it, temperatures on Earth would be too cold. Here's how the greenhouse effect operates: Approximately 30 percent of the Sun's energy that reaches the Earth is reflected and scattered by the surface and the atmosphere back into space.

   The remainder of this solar radiation is absorbed by the so-called "greenhouse gases" in the atmosphere and the Earth's surface where it helps warm the planet.

   The Earth radiates heat back out into the atmosphere. Much of the heat is absorbed by greenhouse gases. As a result, the temperature of the Earth's surface and the lower atmosphere stay warm enough for human life. The heat energy emitted out to space prevents the planet from overheating.

Greenhouse Gases
   Most greenhouse gases such as carbon dioxide, nitrous oxide, and other trace gases like methane occur naturally. Water vapor in clouds is also a contributor to the greenhouse effect but of course water is a molecule of 2 hydrogen atoms and one oxygen, so it's not the same as the other "greenhouse gases." Some are from natural emissions, and some are from human-induced emissions. Without the greenhouse gases, Earth's climate would be about 60° Fahrenheit colder than it is today.

   The greenhouse effect thus serves an essential function, but human activities may intensify this natural phenomenon. An international group of the world's leading climate researchers (the Intergovernmental Panel on Global Climate Change) is now convinced that global climate change is definitely occurring, as seen in rising temperatures and increasing amounts of carbon dioxide in the atmosphere. It is still unclear, however, how much of this is natural and how much human induced.

   But we do know that our activities can increase the concentration of greenhouse gases in the atmosphere. Burning fossil fuels to power our cars, homes, and factories releases carbon dioxide and increases concentrations of other greenhouse gases like methane and nitrous oxide. Humans even manufacture some greenhouse gases, with complicated names such as chlorofluorocarbons (CFCs), hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride. Many of these gases can remain in the atmosphere for decades or even centuries. The concern is that at a certain, as yet unknown level, the greenhouse gases may begin to trigger very unwelcome and perhaps irreversible effects. Planetary scientists look at Venus, with temperatures on its surface hot enough to melt lead, and talk about the "runaway greenhouse effect" that makes that planet completely uninhabitable.

   CFCs have been linked to the reduction of Earth's ozone layer,which is essential in protecting life on the surface of our planet, and in the upper layers of the ocean, from ultraviolet (UV) radiation.

   An unprecedented series of international conventions has reduced the rate of emission of CFCs into the atmosphere considerably. Many climate researchers think that we should control such emissions as a kind of "global insurance policy" in case, some decades from now, we find out that, without a doubt, we humans are indeed making our planet’s climate and weather very different from the past.

(Adapted, with permission, from the ERA/NASA/NOAA Climate Change Presentation Kit CD-ROM.)

Images provided by the Atmospheric Chemistry & Dynamic Branch of NASA