Summary: Two recent studies on the capability of forests to impact climate change, one published by the American Geophysical Union in December 2005 and one in Nature in January 2006, have garnered attention in the popular press. The first examines an unlikely scenario: the effect on the earth's climate of replacing all of the world's grasslands and croplands with forests. It concludes that global temperatures would rise 1.3° C. According to the study, the warming effect would be much more dramatic in boreal (high latitude) than in temperate forests, and tree planting in the tropics would actually lead to cooling. However, these finding hold only for the long term-in the first few decades after forests are planted, the result would be cooler temperatures, as the effects of sequestration dominate the reduction in albedo. The second study reports the startling finding that plants emit large quantities of methane even in the presence of oxygen. (Methane is also called "swamp" gas, on account of its production in low-oxygen environments.) Nonetheless, the authors conclude that the emission of methane by plants reduces the carbon uptake attributable to sequestration by only four percent.

Two recent studies have addressed the capability of forests to impact climate change, either by virtue of the effects of albedo and evapotranspiration, or by changing the concentrations in the atmosphere of carbon dioxide (CO2) and methane, both of which are greenhouse gases. The studies are "Climate Effects of Global Land Cover Change," by Gibbard (and others) of the Lawrence Livermore National Laboratory and the Carnegie Institution, published by the American Geophysical Union on December 8, 2005; and "Methane Emissions from Terrestrial Plants under Aerobic Conditions," by Keppler (and others) of the Max-Planck Institute for Physics in Germany, Utrecht University in the Netherlands, and the Department of Agriculture and Rural Development for Northern Ireland, published in Nature on January 12, 2006.

The Lawrence Livermore/Carnegie Institution study investigated whether converting all of the world's grasslands and croplands to forests would cause warming or cooling. Forests interact with the climate in a variety of ways, in some respects providing a cooling effect and in other respects, a warming one. This study tried to determine which of the effects would dominate.

Albedo is a measure of reflectivity, quantified as a percentage from zero to 100. The higher the albedo, the more reflective a body is. A body with a lower albedo absorbs more energy and, therefore, will warm more than one with a higher albedo. Fresh snow has a high albedo-close to 90 percent-while the albedo of a dark object like a forest canopy is low.

Through changing land use patterns, human activities have altered the earth's albedo from pre-agricultural times. According to the Lawrence Livermore/Carnegie Institution study, previous studies have concluded that by converting forest to cropland, humans have increased the albedo in the temperate (mid-latitude regions), thereby causing cooling in those areas before the twentieth century as compared to pre-industrial times. Other studies have shown that expanding forest cover in boreal (high latitude) regions can lower the area’s albedo and lead to warming, while studies of tropical (low latitude) regions have been inconclusive on the effect of forestation or deforestation on albedo.

The issue of latitude comes into the picture because a body's albedo depends in part on the angle at which the light strikes it. Thus, the same plant at the same time of year but at a different latitude will have a different albedo. In addition, the ground at different latitudes has different albedo, as the snow typical in boreal regions has much higher albedo than the soil or rock in tropical regions.

The removal of CO2 from the atmosphere by trees and plants is one type of carbon sequestration. (Another type is geological sequestration, involving the storage of CO2 underground.) Through photosynthesis, plants grow by converting sunlight and CO2 into carbohydrates. In the process, CO2 is taken out of the air and transformed into sugars and starches. So long as the plant remains intact (i.e., does not decay or get burned), the CO2 is sequestered-it is trapped in the plant rather than remaining in the atmosphere. The removal of CO2 from the atmosphere reduces the greenhouse effect, thereby cooling the earth.

Evapotranspiration is the sum of evaporation and transpiration (the process of water loss from plants through stomata, the small openings used for gas exchange found on the underside of leaves). The U.S. Geological Service estimates that transpiration accounts for about ten percent of the moisture in the atmosphere, with oceans, seas, and other bodies of water (lakes, rivers, streams) providing nearly 90 percent. Evaporation and transpiration are endothermic or energy-absorbing processes, and therefore cool down their surroundings.

The Lawrence Livermore/Carnegie Institution study first looked at whether, in converting all of the world's grasslands and croplands to forest, the albedo effect (warming) or evapotranspiration (cooling) would dominate. The study found:

• Global temperatures rise 1.3° C, as the albedo effect dominates over evapotranspiration.
• This effect is more pronounced in the northern hemisphere, where the conversion to forests would lead to a 3.8° C temperature increase.
• The warming effect is more dramatic in the boreal (very high latitude) forests as compared to the temperate zone. A modeling exercise that looked only at replacing vegetation in the temperate region with trees showed a warming of only 0.27° C.
• Although the albedo effect dominates over evapotranspiration in the temperate zone, leading to warming, this is not true in the tropics. There, the evapotranspiration effect dominates, leading to net cooling (because of the relationship between temperature and saturation water vapor pressure).
• Reversing the experiment-i.e., replacing of all the world's trees with grasslands-would result in a global cooling of 0.4° C.

The main finding-that converting the world's grasslands and croplands to forests will lead to a 1.3° C temperature increase when only albedo and evapotranspiration are considered-is due to the fact that the albedo effect dominates over evapotranspiration, a result which fuels a self-reinforcing loop. Greater forest cover lowers the albedo, which leads to warming. The increased warming melts more snow (which has a high albedo), uncovering bare soil (which has a lower albedo). The decrease in albedo causes more warming, which melts more snow.

The authors then compared these results with another consequence of growing forests: the fact that trees sequester CO2, thereby reducing the greenhouse or warming effect of the gas. By itself, the lower CO2 levels resulting from tree growth would lead to cooling-by 3.5° C globally, with the replacement of all grassland and croplands by trees. However, as the authors have concluded, the albedo effect of forestation (net of evapotranspiration) leads to warming. So this leaves the question as to whether the change in albedo or the effect of sequestering CO2 would dominate.

The study concludes that the answer depends on the time frame. Over the short term (i.e., decades), planting forests is likely to have a cooling effect on the climate, as the trees sequester atmospheric CO2 and reduce the warming effect of the gas. This cooling overcomes the warming attributable to the decreased albedo (again, net of evapotranspiration) that results from the forest growth-but for only so long. According to the authors, although the albedo effect is permanent, atmospheric CO2 concentrations equilibrate over time (in part because of the interaction of ocean and atmosphere), so that after about 80 years global forestation would produce net warming.

The authors suggest that this study has important policy implications, "since incentives for tree plantations in mid- and high latitudes [i.e., temperate and boreal regions], may, on long time-scales, produce the opposite effect to that desired." Yet although the study raises questions about the efficacy of planting trees as a strategy to address climate change, it is worth keeping at least three items in mind.

First, as the authors themselves observe, the simulations conducted in this study are entirely unrealistic. They write:

Our goal here is not to reproduce the observed pattern of land cover change, nor to realistically simulate possible future scenarios, but rather to bracket the magnitude of temperature change that is possible in the climate system due to changes in land cover.

Second, the study reaches significantly different conclusions as to tree planting in the boreal and temperate zones. The albedo effect appears to result in much more significant warming in the far North than in the temperate zones. The authors recommend further study to evaluate whether forestation in the mid-latitudes can actually mitigate climate change.

Third, the study, like some previous ones, suggests that forestation in tropical zones would lead to cooling because, unlike in temperate zones, evapotranspiration dominates over albedo in these areas, even in the long run. Therefore, even if it turns out that forests in temperate and boreal regions cause warming in the long term, that may not be the case in the tropics.

Although there is much less methane in the atmosphere than CO2, it is the second most common greenhouse gas. Moreover, each methane molecule traps over 20 times more heat than a molecule of CO2. The Max Planck Institute study suggests that plants may be a major source of methane emissions. The study found that for every 10° C increase in temperature, the plants' rate of methane emissions doubled. This phenomenon could create a self-reinforcing cycle, with more methane leading to higher temperatures, leading to more methane, and so on.

The study's finding that plants are a major source of methane came as a big surprise for two reasons. First, scientists had previously thought that plants emitted methane only in the absence of oxygen, in places like swamps. (In fact, methane is sometimes called "swamp or "bog" gas.) However, the new study finds that plants emit substantial amounts of methane in the presence of oxygen under normal growing conditions. Second, the amount of methane produced by plants may be enormous, accounting for ten to 30 percent of global methane production. Given the discovery of such a big, previously unknown source of methane, many scientists were astounded. An editorial in Nature pondered in its title, "How could we have missed this?" Major news outlets greeted the news with headlines such as "Global Warming: Blame the Forests," "Plants Gone Bad," and "Plants Revealed as Methane Source," and suggested that the findings would cause a "wholesale revision of the global methane budget."

The authors of the study felt the need to issue a second statement ten days after the study was initially published. Their concern was that the media misunderstood their findings, with the most frequent misinterpretation that methane from plants is responsible for global warming. On the contrary, the authors write, plants are a natural source of methane emissions, present long before humans extended their influence into the atmosphere. In fact, it is the

anthropogenic emissions which are responsible for the well-documented increasing atmospheric concentrations of methane since pre-industrial times. Emissions from plants thus contribute to the natural greenhouse effect and not to the recent temperature increase known as 'global warming.'

Perhaps more importantly, the authors refute the claim that carbon sequestration through forestation is ineffective on account of the trees' methane emissions. According to the authors, the climatic benefits of reforestation vastly outweigh the effects of the associated methane emissions. They write,

[T]he climatic benefits gained through carbon sequestration by reforestation far exceed the relatively small negative effect, which may reduce the carbon uptake effect by up to 4 percent.

As a result, the authors conclude that "the potential for reduction of global warming by planting trees is most definitely positive."

Partly as a result of the authors' public refutation of press reports, some newspapers issued corrections. The U.K. Guardian, which ran the headline, "Global Warming: Blame the Forests," printed a correction four days later stating:

The headline above overstated the more circumspect case outlined in the article below...Scientists have just discovered [that plants may contribute 30 percent of global methane], but to conclude that it is a new cause of rising temperatures is mistaken.

Other scientists have actually questioned the validity of the Max Planck study on various technical grounds. For instance, some have questioned the researchers' use of a temperature range of 30-70° C (with 70° C higher than even the very highest temperatures in tropical forests). Researchers have also asked why the experiment used leaves from ash and beech trees, which are commonly associated with temperate, but not tropical climates.

At least in the popular press, these studies have raised the specter that forests may not be as effective at carbon mitigation as once thought. However, their actual conclusions are more nuanced. Although tree planting in the boreal regions as a strategy to address climate change does, indeed, seem questionable, this may not be the case in temperate areas and appears unlikely to be true in the tropics. The take-away message is that the science in this regard is still murky.

The press jumped on the Max Planck Institute study with catchy headlines about blaming the forests for global warming. However, the study's bottom line is that methane emissions from forestation projects would reduce their effectiveness as a sequestration strategy by a maximum of only four percent.

Although neither of the articles delves into some of the more mundane practicalities, the fact is that forest sequestration projects, many of them relatively small, are plagued by high transaction costs that make them financially shaky propositions. The relevant legal and administrative mechanisms-for example, for bundling smaller projects-may be at least as much in need of development as the underlying science.