Robert Hoff’s letter in which he called to “just have the facts” on global warming moved me to set straight a number of his (HCN, 4/30/07). The Intergovernmental Panel on Climate Change (IPCC), to which he refers, is considered by most scientists to produce one of the most comprehensive surveys of climate science, and the 2007 report was recently released. Within the Physical Science Basis Summary for Policymakers, Hoff will find the following information:

1) In 2005, the radiative forcing (heating) caused by changes in the sun was 0.12 watts per square meter, while human effects were responsible for 1.6 watts per square meter. This means the sun is contributing roughly 7 percent of the current warming, not 40 percent.

2) The likelihood that heavy precipitation events were more frequent in the late 20th century due to human influence is “more likely than not” (greater than 50 percent likelihood). This is also true for increases in drought-affected area, tropical cyclone activity, and extreme high sea level — all “extreme weather events.” The likelihood that human influence will be a factor in future trends is “very likely” (greater than 90 percent) for increased heavy precipitation and “likely” (greater than 66 percent) for the others. Thus, Hoff’s statement concerning the conclusions of the IPCC is false.

3) Anthropogenic activities, including agriculture and fossil fuel use, increased methane concentrations from 715 parts per billion (pre-industrial) to 1774 ppb in 2005. For comparison, methane concentrations over the previous 650,000 years remained between 320-790 ppb. One can make a strong argument that cows, a domesticated animal, exist primarily for human food and leather, and their methane emission is usually considered a human influence (agricultural).

4) The 20th century sea-level rise is estimated to be 0.12-0.22 meters, while estimates of the 21st century’s sea-level rise range from 0.18-0.59 meters. While these numbers do overlap, they are not the same.

Hoff is correct that forests give off carbon dioxide. However, forests also uptake carbon dioxide. 2004 research in the Journal of Ecology shows that there is a net uptake of carbon by our land, equaling about 2.3 gigatons of carbon per year. However, this is mostly offset by deforestation, which releases about 1.6 gigatons of carbon each year. I am glad that Mr. Hoff considers global warming a serious issue, but I hope that he checks his own facts next time.

Twila Moon
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