What are greenhouses gases? The greenhouse effect? How is the greenhouse effect different from global warming?
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Nitrogen and oxygen comprise about 99 percent of Earth’s atmosphere, but these gases are not part of the greenhouse effect because they neither re-emit nor absorb heat radiated from Earth back out into space. “Trace” gases, which make up the remaining one percent of the atmosphere, include greenhouse gases. More than 100 years ago, scientists discovered that these gases trapped heat within the troposphere, somewhat like the window panes of a greenhouse. In fact, without CO2, Earth’s average temperature would be close to zero Fahrenheit. Since the Industrial Revolution in the mid-1800s, we have been enhancing the natural greenhouse effect by burning fossil fuels. It’s this additional heat that’s known as global warming.
The greenhouse gases – known as “radiatively active” because they absorb or re-emit infrared radiation – include water vapor, carbon dioxide, nitrous oxide, methane, chlorofluorocarbons (CFCs), and ozone. Some of these gases last longer in the atmosphere than others, and some absorb more radiation than others, but all can be important in regulating climate. CO2 is especially important because human activities generate so much of it, from so many diverse activities and because these artificial increases in CO2 concentrations persist for so long. In fact, Earth will still be adjusting for decades to come to the CO2 we’ve already added to the atmosphere.
There is a strikingly close correlation between the amount, or concentration, of CO2 in the atmosphere and estimated global mean temperature throughout Earth’s history, with relatively low CO2 concentrations during colder periods and higher concentrations during warmer periods. Because changes in either can reinforce the other, the CO2 concentrations can either lead or lag global temperature.
Naturally driven changes in climate are believed to have raised or lowered CO2 concentrations for millennia, which in turn reinforced the warming or cooling trends. For example, the eccentricity of Earth’s orbit varies on a 100,000-year cycle, which is in sync with the onset and retreat of ice ages. As an ice age begins, oceans cool and absorb more CO2, which supports even more cooling and CO2 absorption. As an ice age ends and the oceans warm, they absorb less CO2, allowing more of the greenhouse gas to build up in the atmosphere.
Similar reinforcements, or positive feedbacks, are occurring now. However, in this case, it appears to be human-driven changes in the carbon cycle, rather than orbital changes, that are driving the feedback loop and leading to rise in global temperature and other changes in climate, according to the Intergovernmental Panel on Climate Change (IPCC).
More information:
“The Carbon Dioxide Greenhouse Effect.” From: The Discovery of Global Warming, by Spencer Weart. The American Institute of Physics
IPCC Fourth Assessment Report – Working Group 1: The Physical Science Basis
UCAR News Release: Climate Change Inevitable in the 21st Century