Science Q & A

How much carbon dioxide can our forests absorb?

New York TimesJanuary 28, 2013 

Q: My family owns a 40-acre woodlot of relatively mature deciduous trees. How many pounds of carbon dioxide does such a forest absorb in a typical year? How many pounds of oxygen are emitted?

“An approximate value for a 50-year-old oak forest would be 30,000 pounds of carbon dioxide sequestered per acre,” said Timothy J. Fahey, professor of ecology in the department of natural resources at Cornell University. “The forest would be emitting about 22,000 pounds of oxygen.”

Every little bit matters, he said. “In the grand scheme of things, forests … are counteracting a considerable amount of fossil fuel burning by cars, slowing down the rate at which the greenhouse gas carbon dioxide is accumulating in the atmosphere.”

The contribution varies with the age of the forest and the species involved. There is no real rule of thumb on the difference between conifers and deciduous trees, Fahey said. Some conifers grow faster, providing more impact sooner.

The Environmental Protection Agency has calculated the amount of carbon dioxide emitted by the average car as of 2007 at about 5 metric tons, more than 11,000 pounds, so a single acre of woodlot would be countering the emissions of about 2.7 cars. For 40 acres, that would be about 109 cars.

Q: Is there a scientific reason that adopting year-round daylight saving time would not be feasible?

There is no reason that any time plan cannot be adopted by human beings, but whether year-round daylight saving time would achieve its chief goal – saving energy – is another question.

A later sunset time leaves a shorter time between darkness and bedtime for use of lights and appliances.

In many places, such energy saving does occur. A study done by California in 2001 found small but significant energy savings in both winter and summer with a later sunset.

Another study, however, published in 2008 after Indiana switched the whole state to daylight time, concluded, “Our main finding is that – contrary to the policy’s intent – DST increases residential electricity demand.” While lighting use decreased, use of energy for heating and cooling increased. After a 2008 report by Energy Department officials, some researchers said that even though extending daylight time had helped save energy nationally, regional variations made it impossible to predict whether year-round daylight time would do so.

Because of air-conditioning use, for example, the South showed smaller savings.

The study also listed potential nonenergy problems with extended daylight time, including children having to travel to school in darkness and higher traffic accident and crime rates.

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