By Larry O'Hanlon
In the simplest terms, global warming is just what it sounds like: the worldwide rise in surface temperatures. The National Academy of Science has put the rise at 1 degree F over the course of the 20th century, but measurements from satellites of both land and sea surfaces are showing that the rate of warming is increasing sharply.
It's more than just surface temperatures that are going up, however. A lot of research into temperature changes in the upper layers of the atmosphere, as well as the deep oceans, is showing warming. Then, there are the more obvious signs: the rapid retreat of glaciers in Greenland, Alaska, the Himalaya, the Antarctic Peninsula and on high tropical mountains; the thinning and disappearance of sea ice in the Arctic Ocean during summer; the melting of permafrost in Canada, Alaska and Siberia; and the rise of sea level and an increase in extreme weather.
The cause of global warming is what's called the "greenhouse effect." That's shorthand for the ability of gases in the atmosphere to slow down the release of heat into space at night. Some gases are better at this than others. Carbon dioxide, methane and nitrous oxide are the top three "greenhouse gases." They are very good at absorbing sunlight and converting that energy into heat – rather like a rock does just sitting in the sun.
Surprisingly, the greenhouse effect isn't a bad thing. It's essential for life on Earth – when it's not too vigorous. If not for the greenhouse effect, the temperature on the surface of Earth would be like that of the airless moon – swinging wildly from 225 degrees F (107 C) during the day to -243 degrees F (-153 C) at night. Not a good place for life.
The greenhouse effect is only troublesome when it gets too strong and warms things too much. And that's just what scientists say has happened over the last 150 years or so as the people of industrialized nations have extracted Earth's vast buried stores of fossil fuels and burned them. Since the start of the Industrial Revolution the atmospheric concentration of carbon dioxide has increased nearly 30 percent, methane has more than doubled, and the nitrous oxide concentration is up about 15 percent. All those extra greenhouse gases mean more and more solar energy is being trapped in the atmosphere, exacerbating the greenhouse effect and making things warmer.
The result: 2005 was Earth's warmest year in a century, according to NASA climatologists. The years 1998, 2002, 2003 and 2004 were the next four runners-up. The year 2005 was also a record-breaking year for Atlantic hurricanes in which the coastal city of New Orleans – made all the more vulnerable because of sea level rise – was almost wiped off the map by Hurricane Katrina.
Of course, because the effects of global warming on local climates are very complicated, it remains to be seen exactly how different regions will feel the heat.
"Global warming is a term that's extremely useful when you're running a planet," says John Cox, author of the book Climate Crash. "But it's regional change that affects people. It's the wet and cold and hot and dry."
That's why climate modelers are constantly refining their simulations, and climate scientists continue to refine our view of past climate changes to create a better idea of what to expect.
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