no-trees.jpgEverybody loves trees. They are so popular as offsets they even make Wikipedia’s definition:

When one is unable or unwilling to reduce one’s own emissions, Carbon offset is the act of reducing (“offsetting”) greenhouse gas emissions elsewhere. A well-known example is the planting of trees to compensate for the greenhouse gas emissions from personal air travel.

But does planting trees reduce global warming? Not in most places on the earth. The Carnegie Institution’s Ken Caldeira summarized the result of a major 2005 study (PDF) this way: “To plant forests to mitigate climate change outside of the tropics is a waste of time.”

Why? Because forest canopies are relatively dark, compared to what they replace outside the tropics — grass, croplands, or snowfields — and so they absorb more of the sun’s heating rays that fall on them. That negates the “carbon sink” benefit trees have soaking up carbon dioxide. Worse, the study found that planting a large number of trees in high latitudes would “probably have a net warming effect on the Earth’s climate.” Ouch!

So what about an offset project involving tree planting in the tropics where water evaporating from trees increases cloudiness, which keeps the planet cool, according to models? Tropical-tree-planting offset projects suffer from a different problem:

How can we be sure that the project is resulting in a net increase in tropical trees? Imagine planting 1,000 acres of trees in Brazil, where the full extent of annual deforestation is not known precisely. How do we know that an extra 1000 acres won’t be chopped down somewhere else in the country?

Until countries with tropical forests join an international greenhouse gas treaty and are subject to rigorous verification strategies, tree-related offset projects will not deliver guaranteed, quantifiable benefits.

So if you are thinking about purchasing offsets, be wary of any company that says it plants trees.

As for the study mentioned earlier, “Climate Effects of Global Land Cover Change” (PDF), by the Lawrence Livermore National Laboratory and the Carnegie Institution of Washington, here’s the abstract:

There are two competing effects of global land cover change on climate: an albedo effect which leads to heating when changing from grass/croplands to forest, and an evapotranspiration effect which tends to produce cooling. It is not clear which effect would dominate in a global land cover change scenario. We have performed coupled land/ocean/atmosphere simulations of global land cover change using the NCAR CAM3 atmospheric general circulation model. We find that replacement of current vegetation by trees on a global basis would lead to a global annual mean warming of 1.6 C, nearly 75% of the warming produced under a doubled CO2 concentration, while global replacement by grasslands would result in a cooling of 0.4 C. These results suggest that more research is necessary before forest carbon storage should be deployed as a mitigation strategy for global warming. In particular, high latitude forests probably have a net warming effect on the Earth’s climate.

Offset projects should simply not include tree planting.

This post was created for ClimateProgress.org, a project of the Center for American Progress Action Fund.