So, I was at Ikea this weekend, doing some furniture shopping for the new apartment, and noticed that they now charge 5 cents for every plastic bag you take, with the proceeds used to help plant trees around the world. From a global warming standpoint, is planting more trees a good idea? Maybe not, according to Joseph Romm.

Back in 2005, Lawrence Livermore National Laboratory put out a study (PDF) on this, and found that trees planted in higher latitudes may actually have a "net warming effect on the Earth's climate." That's because forest covers are darker than, say, grasslands, and so tend to absorb more heat, and that "albedo effect" may actually outweigh the fact that they suck up carbon dioxide. (That doesn't mean that we should just start mowing down forests to cool the planet, since slashing and burning existing trees can release even more carbon into the atmosphere.)

But that's just for trees in higher latitudes. One of the study's authors has argued that tropical rainforests are a different matter, since they tend to
"evaporate water to the atmosphere and increase cloudiness," which has a cooling effect. (Indeed, the IPCC blamed a quarter of the manmade increase in CO2 over the years on "land change use"--which presumably includes a great deal of tropical deforestation.) Anyway, the LLNL paper isn't the last word on the subject--even its authors say the issue needs to be studied more--but it's not something I've ever really thought about.