Steve Johnston's letter to the editor ("Global Warming Hoax" June 7) was little more than a regurgitation of shopworn climate denier talking points. Take, for example, his egregiously false claim that "Human activity contributes only 3.4 percent to CO2 levels."
Human activity has thus far raised atmospheric CO2 levels from pre-industrial levels of 280 parts per million to roughly 405 parts per million. That's a nearly 50 percent increase, and is responsible for the more than 1C warming the planet has seen since the mid 19th century.
Mr. Johnston also makes demonstrably false about my own scientific work reconstructing temperature changes over the past 1,000 years (not 1,500 years as Mr. Johnston erroneously states). This work resulted in the well-known "Hockey Stick" curve, which demonstrates the unprecedented nature of recent warming over the past millennium. The Hockey Stick has been attacked by climate change deniers owing to the simple, undeniable message it conveys about the dramatic impact human activity is having on Earth's climate.
The highest scientific body in the U.S., the National Academy of Sciences, affirmed my research findings in an exhaustive independent review published in June 2006 (see e.g. "Science Panel Backs Study on Warming Climate," New York Times, June 22, 2006).
The most recent report of the Intergovernmental Panel on Climate Change concluded that recent warmth is likely unprecedented over an even longer timeframe (at least the past 1,300 years).
Readers interested in the truth behind the science should consult scientist-run websites like skepticalscience.com, or books on the topic like my own "Dire Predictions: Understanding Climate Change." Let's get past the fake debate about whether climate change is real, and onto the worthy debate over what to do about it.
Michael E. Mann
Distinguished Professor, Department of Meteorology,
Penn State University
