Ask Mr. Smarty Plants

As you probably suspected, there isn't an easy answer for this question since there are many variables involved. For instance, what is the "average" American? How much does he/she drive? fly? How big is his/her house, etc.? You can visit the Conservation International web site and calculate your own (or someone else's, if you like) CO2 footprint. (Using this calculator, Mr. Smarty Plants' carbon footprint was ~12 tons of CO2/year!) You will see that there isn't universal agreement on how to calculate a person's CO2 footprint and calculating the amount of CO2 absorbed by trees isn't easy either.

The next variable is the type and size of tree you pick. The type of tree (hardwood or conifer) and its age determines the amount of carbon it can sequester. For instance, according to the U. S. Department of Energy's Method for Calculating Carbon Sequestration by Trees in Urban and Suburban Settings, a 1-year old fast-growing hardwood (e.g., Ulmus americana (American elm) sequesters 4.0 lbs (1.8 kg) carbon/tree/year; whereas, for an equivalent conifer (e.g., Taxodium distichum (bald cypress) the rate is 2.2 lbs (1 kg) carbon/tree/year. A 50-year old fast-growing hardwood sequesters 122.7 lbs (55.8 kg) carbon/tree/year and the equivalent conifer sequesters 106.3 lbs (48.3 kg) carbon/tree/year.

Carbon-info.org has a generic model for the CO2 absorption for a young tree (0-5 years) that gives 5.5 lbs (2.5 kg) CO2/tree/year and for a mature tree (45-50 years) the CO2 absorption is 30.8 lbs (14 kg) CO2/tree/year. The Colorado Tree Coalition claims that a mature tree can absorb CO2 at a rate of 48 lbs. (21.8 kg)/year. They also state that each person in the U.S. produces about 2.3 tons of CO2/year (considerably smaller than Mr. SPs calculation). If we use their figures then, it would require about 96 mature trees to offset each person's CO2 footprint. (If we take their CO2 mature tree absorption rate and Mr. SPs calculated carbon footprint, it will take a whopping 500 mature trees to offset it!)

Trees do hold the potential for offsetting the CO2 you produce since they (as do other plants) use carbon dioxide from the air in their process of photosynthesis and then release oxygen. Trees also offer many other benefits—shade for dwellings that can lower summer energy costs, habitat and food for wildlife, erosion control and aesthetic appeal, to name a few.

There are critics, however, that question the validity of depending on tree planting for reducing the CO2 profile. Some of the problems cited are:

•The tree will release its own CO2 when its leaves fall to the ground and decompose and, when the tree itself dies, its decomposition will release CO2.

•Trees are often planted as monocultures, reducing biodiversity.

•Plantation trees are sometimes non-natives.

•Tree plantations use a great deal of water and reduce soil nutrients that other plants require.

•Tree plantations at higher latitudes may actually absorb enough energy from the sun to increase global warming.

Here are several discussions of the use of tree planting to offset increases in CO2 and global warming:

"Forests: Debate grows over tree planting's carbon-offset impacts."

"Tree planting: A key weapon against global warming."

There are many organizations that offer the opportunity to offset your carbon dioxide production by planting trees. If you decide to use one of them, here are some suggestions to consider from an article ("Tree-planting projects may not be so green") by James Randerson:

· If you want to support forestry make sure the plantation will use native species/promote biodiversity

· Make sure it is protected from future logging or fire

· When donating to a company offering carbon offset projects check the cash is actually needed to get the project off the ground · Check the project has the support of local people · Ensure it represents a cost effective way of reducing carbon - has a responsible company or not-for-profit organisation audited the project?

· Consider supporting other carbon reduction options, such as funding energy efficient equipment or businesses selling low carbon technology.

Planting enough trees on your own to offset your carbon dioxide production isn't going to be easy. You would essentially need to plant a forest. Supporting organizations seeking to stop deforestation of old growth forests might be an alternative, or additional, strategy. However, planting your own native trees in your own area will be a beneficial, positive step.