Dear EarthTalk: I keep meeting people who say that human-induced global warming is only theory, that just as many scientists doubt it as believe it. Can you settle the score? -- J. Proctor, London, UK
So-called "global warming skeptics" are indeed getting more vocal than ever, and banding together to show their solidarity against the scientific consensus that has concluded that global warming is caused by emissions from human activities.
Upwards of 800 skeptics (most of whom are not scientists) took part in the second annual International Conference on Climate Change -- sponsored by the Heartland Institute, a conservative think tank -- in March 2009. Keynote speaker and Massachusetts Institute of Technology meteorologist Richard Lindzen told the gathering that "there is no substantive basis for predictions of sizeable global warming due to observed increases in minor greenhouse gases such as carbon dioxide, methane and chlorofluorocarbons."
Most skeptics attribute global warming -- few if any doubt any longer that the warming itself is occurring, given the worldwide rise in surface temperature -- to natural cycles, not emissions from power plants, automobiles and other human activity. "The observational evidence...suggests that any warming from the growth of greenhouse gases is likely to be minor, difficult to detect above the natural fluctuations of the climate, and therefore inconsequential," says atmospheric physicist Fred Singer, an outspoken global warming skeptic and founder of the advocacy-oriented Science and Environmental Policy Project.
But green leaders maintain that even if some warming is consistent with millennial cycles, something is triggering the current change. According to the nonprofit Environmental Defense, some possible (natural) explanations include increased output from the sun, increased absorption of the sun's heat due to a change in the Earth's reflectivity, or a change in the internal climate system that transfers heat to the atmosphere.
But scientists have not been able to validate any such reasons for the current warming trend, despite exhaustive efforts. And a raft of recent peer-reviewed studies -- many which take advantage of new satellite data -- back up the claim that it is emissions from tailpipes, smokestacks (and now factory farmed food animals, which release methane) that are causing potentially irreparable damage to the environment.
To wit, the U.S. National Academy of Sciences declared in 2005 that "greenhouse gases are accumulating in Earth's atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise," adding that "the scientific understanding of climate change is now sufficiently clear to justify nations taking prompt action." Other leading U.S. scientific bodies, including the American Meteorological Society, the American Association for the Advancement of Science and the American Geophysical Union have issued concurring statements -- placing the blame squarely on humans' shoulders.
Also, the Intergovernmental Panel on Climate Change (IPCC), a group of 600 leading climate scientists from 40 nations, says it is "very likely" (more than a 90 percent chance) that humans are causing a global temperature change that will reach between 3.2 and 7.2 degrees Fahrenheit by the end of this century.
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