Ever since a friend sent me a link to this BBC story reporting on the potential adverse consequences of planting trees to mitigate climate change, I've been in a funk. Not only is tree-planting one of the simplest carbon-sequestration strategies available, but it is one in which I was personally involved, when Texaco was first investigating CO2 management. Now it appears that reforestation in the temperate zone could actually make things worse, by trapping additional heat near the earth's surface. If true, this is another example of just how complex the planet's climate is, and how intricately-related its various feedback mechanisms. A month after reading the article, however, the logic of this model-based finding seems less compelling, and its reconciliation with the historical climate record more problematic. Although one of the co-authors of the study published an unequivocal op-ed on this subject in yesterday's New York Times, I still think we need to know a bit more, before we abandon reforestation as a CO2 abatement strategy.
The whole problem starts with how much of the energy that we receive from the sun is retained in the land, oceans and atmosphere, and how much is reflected back into space. De-forestation in the tropics is bad for multiple reasons. It reduces the amount of CO2 being incorporated into plant growth and releases large quantities of stored CO2 into the air, when the logged trees are burned, or when they decay on the ground. It can also lead to reduced cloud cover. Both of these effects increase the earth's retention of solar radiation, apparently by more than the shift in vegetation from forest to food crops or cattle forage enhances the Earth's reflectivity, or albedo. So re-planting the tropical forests, or at least preserving current forests, seems unambiguously beneficial.
But according to research at the Carnegie Institution and the Lawrence Livermore National Laboratory, this is not the case when the forests being re-grown are in the middle and high latitudes, the zones in which the US, Canada and much of Europe fall. There, the addition of warmth-absorbing dark tree cover, especially where highly-reflective snow would otherwise cover the ground for much of the winter, could neutralize or even overwhelm the carbon sequestration benefit. Bottom-line: planting trees in the US to soak up CO2 might do more harm than good.
There are a lot of US businesses, including some very large utilities, that must be very disappointed to hear that, because before this study, tree-planting looked like a cheap and effective means of offsetting industrial emissions that would otherwise be very expensive to reduce directly. At the same time, these findings will be welcomed by those environmentalists who believe that tree-planting and other means of offsetting emissions are simply no substitute for cutting emissions directly, which by extension entails large reductions in humanity's energy consumption and general footprint on the earth. The lead researcher of the study in question appears to share that view. While I'm not suggesting that the findings of the Carnegie/LLNL study were skewed by this, it would be naive to think that climate change skeptics have a monopoly on influence from non-scientific factors.
Even though the general trend of global climate change now appears clear, with supporting evidence accumulating practically every day, it does not mean we must accept every new finding in this area without question, any more than we do in the areas of health or cosmology. Before I buy into the conclusion that reforestation outside the tropics is of no use in combating climate change, I would like to see the model in question validated against the historical and paleo-climate data. If the effect of northern-latitude reforestation is large enough to contribute a couple of degrees of warming by the end of the century, as cited by the BBC, then its influence ought to be discernible in the record of previous eras in which North America was much more heavily forested than today, but the northern hemisphere climate was much cooler, as in the Little Ice Age of the 16th-19th centuries, when the Thames routinely froze over.
Unless the model can account for the forestation effect in earlier periods that lacked today's man-made climate influences, it would be premature to base policy decisions on its future predictions about the efficacy of planting trees now. Until then, I'm at least in complete agreement with Dr. Caldeira that no one should use these findings as an excuse to cut down even more trees.