What Is the Greenhouse Effect?
Do you discern a pattern in weather records for the earth? Dr. James E. Hansen, director of the Goddard Institute for Space Studies, a research center operated by the U.S. space agency, NASA (National Aeronautics and Space Administration) does. In June 1988, Dr. Hansen said that all this heat is no mere statistical fluke. After dramatic testimony before the U.S. Senate, he said: “It is time to stop waffling so much and say that the evidence is pretty strong that the greenhouse effect is here.”
THE greenhouse effect. Likely you have heard that phrase used a lot. No, it doesn’t refer to a garden greenhouse. It describes atmospheric heating that many scientists fear is already affecting the entire planet. But until Dr. Hansen’s testimony, experts were not willing to say so publicly. “It took a Government forum during a drought and a heat wave and one scientist with the guts to say, ‘Yes, it looks like it [the greenhouse effect] has begun and we’ve detected it,’” says atmospheric scientist Michael Oppenheimer of Dr. Hansen’s testimony. “He felt comfortable saying clearly and loudly what others were saying privately.”
The Global Greenhouse
Have you ever parked your car in the hot summer sun with all the windows closed? When you came back, you got a taste of the greenhouse effect. The windows of your car are transparent to the sun’s rays, which quickly warm the interior. But the hot air inside the car cannot escape, and neither can the heat itself. Why not? Because heat is given off in the form of infrared rays, which are invisible to the eye but can be felt on the skin, for example, when you stand near a fire. The same glass that lets visible light in prevents much of the invisible infrared radiation from getting back out. So the temperature inside your car goes up and up.
The earth’s atmosphere is similar to the glass in your car’s windows. It readily admits visible light but blocks a great deal of invisible radiation, including infrared and ultraviolet light, as well as X rays. In general, this blocking is a good thing. Ultraviolet light and X rays are quite dangerous and are believed to cause cancer. But why block infrared?
When the atmosphere absorbs infrared radiation, it acts like a blanket around our planet. Sometimes we forget that the earth is surrounded by cold, empty space. Even though the sun warms the earth, without our greenhouse “blanket,” that heat would rapidly escape, and the temperature at the surface would be 70 degrees Fahrenheit [40° C.] colder than it presently is. The oceans would freeze!
The problem with the greenhouse effect is that it may become too much of a good thing. A runaway greenhouse effect could mean mass starvation as grain belts turn to dust bowls. It could also mean superkiller hurricanes powered by extra warm oceans, rising oceans flooding coastal areas, rampant skin cancer brought on by an eroding ozone layer, and untold human misery.
Turning Up the Global Thermostat
You probably learned in school that the atmosphere is about 99 percent oxygen and nitrogen. Yet, these gases do not block infrared rays. A few of the gases contained in the remaining 1 percent, along with water vapor, paradoxically both save our globe from deep freeze and threaten to overheat it.
Most scientists agree that increasing the concentration of greenhouse gases in the air will raise global temperatures, although no one can be sure just how this will take place. You might compare these gases to a global thermostat. For over a hundred years, it appears that man has been steadily turning up the global thermostat. “Fossil fuel combustion (along with other industrial and agricultural activities) has caused the atmospheric concentration of carbon dioxide to increase approximately 25 percent since about 1860,” notes Irving M. Mintzer of the World Resources Institute. “The combined atmospheric build-up of carbon dioxide and the other greenhouse gases since 1860 are believed to have already committed Earth’s surface to warm approximately 0.5° to 1.5° C above the average global temperature of the pre-industrial period.”
It is true that one or two degrees doesn’t sound like much, but, in fact, it represents a great deal of heat. “For perspective,” adds Mintzer, “a change in average global temperature of only 1° C separates the current climate regime of North America and Europe from that of the Little Ice Age of the 13th to 17th Centuries.” Additionally, there is no reason to think that the extra heat will be evenly distributed. One extra degree over a year could come in the form of many extra degrees during the hottest summer months, with devastating effects.
The Toronto Conference
As the brutal summer of 1988 continued to bake North America, over 300 delegates from 48 countries attended the International Conference on the Changing Atmosphere, held in Toronto, Canada. In a report on the conference, the Manchester Guardian Weekly noted the following grim prediction of the consequences of global warming:
“The rise in global temperatures will not be even. The high latitudes will warm more quickly than the equator. This will mean a loss of soil moisture in the mid-latitudes of the northern hemisphere, where most of the world’s grain is grown.” In other words, a recipe for global famine.
Global Flooding Forecast
Another major concern is the effect of higher temperatures on the level of the oceans. Most people associate rising sea levels with melting glaciers and ice caps, but in fact the ocean can rise a great deal without any polar melting at all. How? Thermal expansion—the same phenomenon that makes the mercury rise in your thermometer on a hot day. “If we went all out to slow the warming trend, we might stall sea level rise at three to six feet [1-2 m], but that’s the very best you could hope for,” according to Lawrence Livermore National Laboratory scientist Robert Buddemeier.
Sea-level rises of this magnitude have stirred concern worldwide. “A rise of less than two feet [0.6 m] in sea level might inundate 27 per cent of Bangladesh, displacing 25 million people,” reports the U.N. Chronicle. “Egypt could lose 20 per cent of its productive land, the United States, between 50 and 80 per cent of its coastal wetlands. A 6-foot [2 m] rise could wipe out the 1,190-island Maldivian archipelago.”
The above forecasts are on the conservative side. Consider some of the more extreme predictions now being made: “The year is 2035,” goes one of them. “Holland is under water. Bangladesh has ceased to exist. Torrential rains and rising seas there have killed several million people and forced the remaining population into makeshift refugee camps on higher ground in Pakistan and India. In central Europe and the American Midwest, decades of drought have turned once fertile agricultural lands into parched deserts.”—Jeremy Rifkin, in the Manchester Guardian Weekly.
Is this really what the future holds for our planet?
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Not all scientists are sure that the rise in greenhouse gases has caused the global warming. Stephen H. Schneider, a climate modeler at the U.S. National Center for Atmospheric Research cautions: “You can’t say just because one decade warmed up that the cause is the greenhouse effect. But for two decades to have a warming trend, that would be pretty unusual. And if it continues to crack records on an annual basis, then I think most of the skeptics will drop out and agree that there it is.”—Science News, Volume 135, April 8, 1989
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Why the Difficulty in Predicting the Greenhouse Effect
The earth’s overall climate is an enormously complicated system, and scientists freely admit that there are limits to what they can predict. Here are a few of the factors that could greatly influence current computer models of future climate.
MELTING SNOW AND ICE: Ice and snow reflect 40 to 60 percent of the sun’s incoming rays. This has the effect of cooling the planet. But as rising temperatures cause ice and snow to melt, the darker land or water underneath will absorb more heat. This could amplify the greenhouse effect, perhaps by 10 to 20 percent.
CLOUDS: A warmer earth should mean higher global humidity—more clouds. “Cloud feedback is one of the largest sources of uncertainty in the theory of climate change,” admits climate modeling expert V. Ramanathan of the University of Chicago. It is thought, however, that more clouds would tend to cool things off by increasing the reflection of solar energy.
On the other hand, while clouds reflect away some solar energy, they also act as blankets to trap radiation coming up from the surface. So it is hard to predict which effect would predominate in a warmer, cloudier world.
THE OCEANS: Water is an excellent heat absorber, and it appears that the oceans can store enough heat to delay the full onset of the greenhouse effect by decades. Just how much delay is difficult for scientists to predict.
VOLCANOES: Volcano-induced clouds warm the stratosphere and cool the surface of the earth in a complex way. Overall, volcanoes would probably tend to reduce the greenhouse effect, but no one can predict when a big one will erupt.
SOLAR CYCLES: Contrary to what many people think, the sun’s output is not absolutely constant. Its brightness diminished about 0.1 percent between 1979 and 1984. This makes the increasing global temperature during that period seem all the more ominous.
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The Greenhouse Gases
WATER VAPOR: The amount of water vapor in the air depends mostly on the temperature. Warm air can store more moisture than cool air can. Water vapor absorbs heat very effectively, but it cannot give rise to the greenhouse effect by itself. Water vapor serves mostly to amplify the effects of the other gases.
CARBON DIOXIDE (CO2): It is the most prevalent of the heat-trapping gases and is vital to all life on earth because plants need it to live. The amount of carbon dioxide in the atmosphere is currently increasing by a half percent per year. That may not sound like much, but it means that about one ton of carbon for every man, woman, and child on the planet is put into the atmosphere each year from burning fossil fuels, such as coal and oil—5,000,000,000 tons of carbon per year! About half of that carbon is eventually used by plants or is absorbed by the ocean, but the rest stays in the air.
METHANE (CH4): This is the chief component of natural gas. Like carbon dioxide, it contains carbon. It is increasing twice as fast as carbon dioxide in the atmosphere, or about 1 percent per year. There is already twice as much methane in the air as there was in preindustrial times. Scientists are concerned that the increasing concentration of methane may make it more difficult for the atmosphere to decompose other greenhouse gases, such as the infamous CFCs (chlorofluorocarbons).
CFCs: These long-lasting chemicals help destroy ozone after they have risen to the stratosphere. But they are powerful greenhouse gases while in the lower atmosphere. In fact, molecule for molecule, they are about ten thousand times as effective as carbon dioxide when it comes to absorbing infrared rays!
NITROUS OXIDE (N2O): When your dentist used this, he may have called it laughing gas, but its effect on the atmosphere is no laughing matter. It is a by-product of burning fossil fuels and is extremely stable. Once it gets into the atmosphere, it stays there on the average for 150 years. During that time, it absorbs heat while in the lower part of the atmosphere, called the troposphere, but it can also rise to the stratosphere, where it helps to destroy ozone. The concentration is currently growing at 0.25 percent per year.
OZONE (O3): Last but not least is ozone. In the stratosphere, ozone is beneficial because it absorbs dangerous ultraviolet radiation that can cause skin cancer if it penetrates to earth’s surface. But in the lower atmosphere, ozone is a hazard. Ozone is a by-product of combustion, especially in automobiles and jet aircraft.
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The Greenhouse Effect: The earth’s atmosphere, like the glass of a greenhouse, traps the sun’s heat. The sunlight warms the earth, but the heat that is created—carried by infrared radiation—cannot easily escape the atmosphere because the greenhouse gases block the radiation and send some of it back toward the earth, thus adding to the warmth of the earth’s surface
Trapped infrared radiation