Dear Umbra,

My wife recently heard a program on NPR where an expert on global warming said that planting trees in the Northern Hemisphere plays a negligible role in fighting global warming, while playing a significant role in the Southern Hemisphere. Do you have any light to shed on this?

Randy Cunningham, Confused Tree-Hugger
Cleveland, Ohio

Dearest Randy,

Tree hugging has no proven deleterious effect on climate change. What you should not do, however, is try to compensate for your carbon production by planting huggable forests, according to the results of a study by the Lawrence Livermore National Laboratory and Carnegie Institution Department of Global Ecology.

Now you seed it, now you don’t.

Photo: iStockphoto

The study was in the news in mid-December, and I too heard it reported on NPR. The study authors put together a large-scale computer model of forests’ role in global warming, and your wife clearly remembered the outcome. Planting gobs of new forests in temperate zones, it seems, will have a negligible or even negative effect on global warming in the long term. Tropical forests, on the other hand, will have a positive effect, the study said.

How is it true that trees could be bad for something?

Doubtless you are familiar with the concept that trees uptake carbon and have helped ameliorate our recent carbon loading of the atmosphere. Trees also evaporate water and contribute to cloudiness, keeping things a bit cooler. What swung these positive effects to the neutral or negative in the large-scale climate model was a third property of forests: albedo.

Who remembers this term from their climate reading? It means reflectivity, basically. It comes from the Latin word for white, and is similar to albino — but not libido. Forests, all dark with leaves and stuff, have a low albedo. Shiny ice sheets have a high albedo. Remember one problem with the (predicted) end of summer ice in the Arctic is the permanent ice sheet’s current role in reflecting sunlight and heat away from Earth. As the ice disappears, warming may increase rapidly when the high-albedo surface provided by miles and miles of whiteness is replaced by darker, lower-albedo ocean water.

Since forests don’t tend to reflect heat, they hold a lot of warmth. When the study authors integrated albedo into their computer model, they found that replacing grasslands and other non-forested areas with forests in temperate zones (e.g., North America, Europe) would eventually lead to net warming of the Earth. In tropical areas (e.g., Brazil), forests exhale more water; scientists say this contribution to cloudiness will help cancel out the warming effect.

Obviously this conclusion could be big trouble for companies and countries intending to rely on tree planting in temperate zones as a carbon offset tactic. It could also be twisted into fodder for those who practice industrial deforestation. For us individuals it is an interesting new piece of information — trees cannot save us from ourselves! — which I suppose can only further inspire our efforts to reduce our fossil-fuel emissions. I know, I know, I make all topics lead to that same conclusion. What can I say?