CLIMATE CONTRARIAN: Sen. James Inhofe has called global warming the greatest hoax ever perpetrated on the American people.
Senator James Inhofe

On November 18, with the United Nations Global Warming Conference in Copenhagen fast approaching, U.S. Sen. James R. Inhofe (R–Okla.) took the floor of the Senate and proclaimed 2009 to be "The Year of the Skeptic." Had the senator's speech marked a new commitment to dispassionate, rational inquiry, a respect for scientific thought and a well-grounded doubt in ghosts, astrology, creationism and homeopathy, it might have been cause for cheer. But Inhofe had a more narrow definition of skeptic in mind: he meant "standing up and exposing the science, the costs and the hysteria behind global warming alarmism."

Within the community of scientists and others concerned about anthropogenic climate change, those whom Inhofe calls skeptics are more commonly termed contrarians, naysayers and denialists. Not everyone who questions climate change science fits that description, of course—some people are genuinely unaware of the facts or honestly disagree about their interpretation. What distinguishes the true naysayers is an unwavering dedication to denying the need for action on the problem, often with weak and long-disproved arguments about supposed weaknesses in the science behind global warming.

What follows is only a partial list of the contrarians' bad arguments and some brief rebuttals of them.

Claim 1: Anthropogenic CO2 can't be changing climate, because CO2 is only a trace gas in the atmosphere and the amount produced by humans is dwarfed by the amount from volcanoes and other natural sources. Water vapor is by far the most important greenhouse gas, so changes in CO2 are irrelevant.

Although CO2 makes up only 0.04 percent of the atmosphere, that small number says nothing about its significance in climate dynamics. Even at that low concentration, CO2 absorbs infrared radiation and acts as a greenhouse gas, as physicist John Tyndall demonstrated in 1859. The chemist Svante Arrhenius went further in 1896 by estimating the impact of CO2 on the climate; after painstaking hand calculations he concluded that doubling its concentration might cause almost 6 degrees Celsius of warming—an answer not much out of line with recent, far more rigorous computations.

Contrary to the contrarians, human activity is by far the largest contributor to the observed increase in atmospheric CO2. According to the U.S. Geological Survey, anthropogenic CO2 amounts to about 30 billion tons annually—more than 130 times as much as volcanoes produce. True, 95 percent of the releases of CO2 to the atmosphere are natural, but natural processes such as plant growth and absorption into the oceans pull the gas back out of the atmosphere and almost precisely offset them, leaving the human additions as a net surplus. Moreover, several sets of experimental measurements, including analyses of the shifting ratio of carbon isotopes in the air, further confirm that fossil-fuel burning and deforestation are the primary reasons that CO2 levels have risen 35 percent since 1832, from 284 parts per million (ppm) to 388 ppm—a remarkable jump to the highest levels seen in millions of years.

Contrarians frequently object that water vapor, not CO2, is the most abundant and powerful greenhouse gas; they insist that climate scientists routinely leave it out of their models. The latter is simply untrue: from Arrhenius on, climatologists have incorporated water vapor into their models. In fact, water vapor is why rising CO2 has such a big effect on climate. CO2 absorbs some wavelengths of infrared that water does not so it independently adds heat to the atmosphere. As the temperature rises, more water vapor enters the atmosphere and multiplies CO2's greenhouse effect; the IPCC notes that water vapor (pdf) may “approximately double the increase in the greenhouse effect due to the added CO2 alone.”