The Environmental Protection Agency's decision to classify rising carbon-dioxide emissions as a hazard to human health is the latest twist in a debate that has raged for decades among politicians, scientists and industry: whether a natural component of the earth's atmosphere should be considered a pollutant.
The EPA's finding doesn't say carbon dioxide, or CO2, is by itself a pollutant -- it is, after all, a gas that humans exhale and plants inhale. Rather, it is the increasing concentrations of the gas that concern the agency.
Carbon-dioxide levels in the Earth's atmosphere have fluctuated wildly for millennia; at one point billions of years ago, it was the dominant gas in the atmosphere.
However, the EPA ruled that today's higher concentrations are the "unambiguous result of human emissions." Concentrations of carbon dioxide and other gases "are well above the natural range of atmospheric concentrations compared to the last 650,000 years," the agency said.
Over the years, many pro-business groups have discouraged regulation of carbon-dioxide emissions by arguing that CO2 is an essential ingredient of life. In its decision, the EPA stressed that it considers CO2 and other so-called greenhouse gases to be pollutants because of their role in propagating climate change, not because of any direct health effects.
In 1998, the Clinton administration EPA studied the question and determined that the Clean Air Act was "potentially applicable" to CO2 and other greenhouse gases. But despite continued pressure from environmental groups, the administration never moved to regulate the gases.
According to the bulk of scientific research, such as that assembled by the Intergovernmental Panel on Climate Change, the more greenhouse gases there are in the atmosphere, especially carbon dioxide, the more heat is trapped. That leads to rising temperatures. The EPA endorsed the IPCC research and specifically said that "natural variations" in climate, such as solar activity, couldn't explain rising temperatures.
The EPA lumped carbon dioxide with five other gases -- methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride -- into a single class for regulatory purposes. That's because they share similar properties: All are long-lived and well-mixed in the atmosphere; all trap heat that otherwise would leave the earth and go into outer space; and all are "directly emitted as greenhouse gases" rather than forming later in the atmosphere.
Alternatively, tropospheric ozone wasn't included in the class, even though it creates smog and contributes to global warming. But that gas isn't emitted directly; rather, it is created in the atmosphere when sunlight reacts with greenhouse gases emitted by human activity such as engine combustion and industrial processes.
Similarly, the EPA declined to consider regulating water vapor or soot, also known as "black carbon," both of which are big contributors to the greenhouse effect but which don't share common properties with the six greenhouse gases.
The EPA did acknowledge some positive impacts from higher CO2 concentrations.
One is faster-growing trees in tropical forests, which helps offset deforestation. Another is marshes that can more quickly grow above rising sea levels, providing an insurance policy of sorts for some low-lying areas against the potential ravages of rising sea levels resulting from warmer global temperatures.
The EPA also acknowledged some positive aspects of rising temperatures, but concluded that on balance, the negative impacts of climate change outweigh the positive.
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