Why are US trees growing faster?

CHURCHVILLE, VA—Trees in the U.S. are growing 2–4 times as fast as their long-term norm. The Smithsonian Environmental Research Center at Edgewood MD says it is because of global warming, according to a recent press release. Don’t bet on that.

Smithsonian researchers measured a series of forest plots at different stages of growth from 5 to 225 years of age. They found that more than 90 percent of the trees grew 2-4 times faster over the past 20 years than predicted from baseline growth rates.

Helpfully, lead author Geoffrey Parker tells us that there are three main possibilities why the trees might be growing faster:

A) the official temperatures in the region have increased about 0.3 degree C over the past 20 years;
B) There’s been a small expansion of the growing season; and
C) CO2 levels have risen about 12 percent over two decades.

Dr. Parker seems to have chosen A), higher temperatures, as the correct answer. But do facts support the choice?

First, we have to question how much the Maryland forest temperatures have actually increased over the last 20 years. Meteorologist Joe D’Aleo charges that the official U.S. temperature record has been systematically shorn of its low-temperature stations—those at high altitudes, high-latitudes and/or in rural areas. It’s as though NOAA has decided to record only the temperatures at heat-retaining airports and fermentation-warmed sewage treatment plants. D’Aleo warns that a major part of the reported increase in recent U.S. surface temperatures is due to this deliberate bias.

So, where did Dr. Parker get his regional temperature measurement? From Baltimore-Washington International Airport!

How important is this?  A recent comparison of rural and urban sites by physicist Charles R. Anderson used NASA’s published data to pair 28 rural-urban thermometer sites across the U.S., with the rural sites 20–50 miles from their urban pairing. Anderson found that since 1900 the urban thermometers have trended upward about 1.5 degrees C. The rural thermometers show no upward trend! Forests are by definition “rural areas.”  Moreover, Edgewater, Maryland is on the breezy shores of the Chesapeake Bay.

Meteorologist Eugenia Kalnay at the University of Maryland used satellite and high-altitude balloon data to backcast U.S. temperatures from 2003 “without cities or land use changes.” Forty percent of the recent U.S. warming disappeared. Subtract 40 percent of Dr. Parker’s assumed forest temperature increase and the climate warming argument pales severely. Dr. Kalnay has impeccable credentials and this paper was published in the prestigious journal Nature!

But that leaves C) the CO2 increase! Atmospheric CO2 acts like fertilizer for trees. Doubling CO2 raises the growth rates of trees by 50–80 percent, according to Dr. Sherwood Idso, who assessed hundreds of CO2-fertilization studies worldwide. 

Tree-ring experts everywhere have found their historic linkages between temperature and tree growth have changed importantly as the global atmospheric CO2 levels have risen. Dr. Parker has no way to distinguish between the impacts of modestly higher temperatures and the impact of sharply elevated CO2 levels. Once again, it looks as though the man-made warming bias has infected our rank-and-file scientists.

By the way, this research was supported by a grant from a big international bank that is putting up $100 million dollars to encourage tough limits on fossil fuel burning. “Follow the money” has taken on a new meaning.

McMahon et al., “Evidence for a Recent Increase in Forest Growth,” Proceedings of the National Academy of Sciences, www.pnas.org/cgi/doi/10.1073/pnas.0912376107
E. Kalnay and M. Cai, 2003, “Estimating the Impact of Urbanization and Land Use on U.S. Surface Temperature Trends:  Preliminary Report,” Nature 423: 528-531.
K.E. Idso and S. B. Idso, 1994, “Plant Responses to Atmospheric CO2 Enrichment in the Face of Environmental Constraints:  A Review of the Past 10 Years’ Research,” Agriculture and Forest Meteorology 69, 153-203.


About the Author: Dennis Avery

Dennis Avery

Dennis T. Avery, a senior fellow for the Hudson Institute in Washington, D.C., is an environmental economist. He was formerly a senior analyst for the Department of State. He is co-author, with S. Fred Singer, of "Unstoppable Global Warming Every 1500 Years." Readers may write to him at PO Box 202 Churchville, VA 24421; email to [email protected]