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
Earth & Space Sciences
University of
Washington
Seattle, Washington
This article appeared in the print edition of the magazine with the headline Global warming fact-check.
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