Of all the current concerns about the environment, global warming is one of the most controversial. Most scientists agree that the surface temperature of the earth has risen between .5 and 1 degree Fahrenheit in the last 100 years. Most of this increase has occurred in the past 20 years. Scientists have measured it using satellite data and thermometers on buoys, ships and weather stations and by studying tree rings, coral and arctic ice core samples.


Image courtesy NOAA
Mean temperature for December through February in the United States from 1895-2005, indicating climate

A degree may not sound like much, but during the last ice age, the surface temperature of the earth was only 9 degrees Fahrenheit cooler than it is now. Many scientists blame an increased greenhouse effect, leading to global warming, for the current trend.

The greenhouse effect is completely necessary to life on Earth. Without it, most of the heat from the sun would be reflected back into space. The Earth would be much colder than it is now, and we would not be able to live on it. Our survival depends on greenhouse gases, like water vapor, carbon dioxide, methane and nitrous oxide.


Image courtesy U.S. Global Change Research Program

But when greenhouse gases are present in too high a concentration, the effect works too well. Scientists estimate that the amount of carbon dioxide in the atmosphere has increased by about 30 percent since the industrial revolution. Most fear that if the global temperature continues to rise, it could cause floods, droughts and disrupted food and water supplies. Increased temperatures may also speed the melting of the polar ice caps, raising sea levels and disrupting the ocean currents that are responsible for our current climate. Scientists already theorize that global warming has caused stronger hurricanes [ref].

Although some theories differ, most scientists believe that the burning of fossil fuels is partially responsible for the increase in greenhouse gases. Deforestation has also increased the concentration of these gases by reducing the number of trees and releasing carbon through burning trees and brush. Livestock also release methane into the air. Check out the US Global Change Research Program and the EPA to learn more.

Some scientists believe that global warming is irreversible. Others believe it requires the same sort of solution as ozone depletion does -- stopping something rather than starting it. If we end rapid deforestation and the burning of fossil fuels, the temperature may return to normal. If that happens, the polar ice caps could replenish themselves through accumulated snowfall. There's a lot of debate over global warming, though, including whether the effect is reversible.

Who's to Blame?
Not everyone agrees with theories on the cause of and solutions for global warming. One recent study suggests that the phenomenon didn't come from human activity. Instead, it blames the 1908 Tunguska Event, when an object entered our atmosphere and exploded. The event destroyed about 60 million trees and disrupted the atmosphere. Some scientists also blame water vapor levels rather than carbon dioxide, which humans may not be able to control.

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