Most of us know the drill by now about global warming and greenhouse gases. Human activity has caused large amounts of greenhouse gases, including carbon dioxide and methane, to be released into the atmosphere, where they can trap heat. The excess carbon dioxide comes from coal and oil power plants and automobile gas, as well as the destruction and burning of forests. Most scientists agree that this process has led to dangerous and rapid global warming that is adversely affecting the world’s climate.

There are two ways to reduce carbon dioxide buildup: 1) reduce carbon dioxide emissions through burning less fossil fuel, and 2) re-absorb carbon dioxide from the air. Trees can help in both of these endeavors.

Trees absorb carbon dioxide from the air and store it in their woody stem, branches and roots (also known as woody biomass), which is important because that ultimately leads to a cooler atmosphere. Their leaves absorb sunlight, which heats the Earth’s surface. And they also suck in water from the soil that evaporates into the atmosphere and creates a mechanism called evotranspiration—the creation of low-lying clouds reflecting the sun’s rays—causing atmospheric cooling.

In other words, trees act as carbon sinks through their ability to absorb and store carbon (called “carbon sequestration”), and to grow large and live for decades or centuries. Trees that grow quickly and live long are the ideal carbon sinks, but unfortunately, most trees don’t possess both of these characteristics.

Another problem is that, as indicated by a recent study reported in the Proceedings of the National Academy of Sciences, the ability of trees to absorb and store carbon dioxide doesn’t produce the same results in the earth’s three main climate zones. In tropical forests, trees do cause a cooling effect, but in boreal forests of the subarctic (including portions of Canada, Russia and the northern U.S.), they can actually contribute to global warming. And in the mid-latitudes that include the U.S., trees don’t make much of a difference in their role as carbon sinks one way or the other.

Because of the high level of energy consumption in the U.S., it has been estimated that planting 44 million trees per year for fifty years would only absorb one quarter of one percent of the carbon dioxide from fossil fuels that the U.S. will emit over the next fifty years. So planting trees will not make a big dent in carbon dioxide absorption.

The most significant environmental value of trees lies in their ability to help in the burning of less fossil fuel, resulting in a reduction in greenhouse gas emissions.

As a homeowner, by planting shade trees or shrubs around your house you can reduce air conditioning energy use by up to 70%, and trees placed to slow down the wind can reduce energy needed for heating by 30%. If you live in an area with snowy winters, trees used as snow fences can reduce the energy required for snow plowing.

The U.S. Department of Energy recommends that you plant deciduous (leafy) trees on the south and west sides of your house to keep it cool in summer and help the sun warm it in winter to reduce your energy use. The U.S. Forest Service Center for Urban Forest Research suggests that three trees planted around a house can save as much as 30% in energy use.

A good rule of thumb is that, whether you’re planting community trees or trees on your own property, that you choose trees appropriate to your location, climate and soil. You should also plant trees that are low maintenance and don’t require much in the way of fossil fuel-burning power equipment like chainsaws and trucks.

If you are interested in trees as local agents of greenhouse gas reduction, what kinds of trees should you plant? Good choices for urban trees that are efficient carbon sequestration agents, according to a 2002 study by the U.S. Forest Services’ Northern Research Station (Syracuse, NY) are the Common Horse Chestnut, Black Walnut, American Sweetgum, Ponderosa Pine, Red Pine, White Pine, London Plane, Hispaniolan Pine, Douglas Fir, Scarlet Oak, Red Oak, Virginia Live Oak and Bald Cypress. Hardwoods like ash, cottonwood and elm also perform well as carbon sinks.

But no matter what kinds of trees you decide to plant, the important lessons to learn are that the continued existence of healthy trees worldwide does make a difference in reducing global warming and decreasing our reliance on non-renewable fossil fuels, and that you should do all you can as a global citizen to protect these precious natural resources.

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