Snowy forests increase warming, while tropical forests cool the planet

Planting trees which trap and absorb carbon dioxide as they grow can help to remove carbon dioxide from the atmosphere. But a new study suggests that, as a way to fight global warming, the effectiveness of this strategy depends heavily on where these trees are planted. In particular, forests in the tropics are very efficient at keeping the Earth at a cool temperature, because not only do they store carbon, they also produce clouds that act like a mirror, reflecting sunlight back into the atmosphere. Planting trees in snowy areas on the other hand may worsen global warming as their canopies absorb sunlight which would otherwise be reflected by the snow, the study suggests. However, while the forests of Europe, Siberia and Canada may contribute to warming, the authors stress they are not advocating chopping down trees.

The researchers, including Ken Caldeira of Carnegie's Department of Global Ecology and Govindasamy Bala at Lawrence Livermore National Laboratory, report their findings in the Proceedings of the National Academy of Sciences. The researchers' work simulates the effects of large-scale deforestation, and accounts for the positive and negative climate effects of tree cover at different latitudes. Protecting, reforesting and afforesting the tropics is strongly advocated:

Latitude-specific deforestation experiments indicate that afforestation projects in the tropics would be clearly beneficial in mitigating global-scale warming, but would be counterproductive if implemented at high latitudes and would offer only marginal benefits in temperate regions. Although these results question the efficacy of mid- and high-latitude afforestation projects for climate mitigation, forests remain environmentally valuable resources for many reasons unrelated to climate.

This is interesting news, because it may obliquely strengthen the case for bioenergy production based on energy trees planted in the tropics. Here's our take: a recent EU-report showed that both tropical Africa and Brazil have more than 92 million hectares of land that can be afforested and reforested with eucalyptus. Couple this potential to the concept of carbon-negative bioenergy ('Bio-Energy with Carbon Storage'), and we may have an extremely effective option to mitigate climate change. The advantages of such a system look as follows:

• afforestation/reforestation with fast-growing trees in the tropics captures carbon from the atmosphere
  
• as they grow, the trees produce dense tropical clouds that reflect the sunlight back into the atmosphere - this is called the 'albedo effect', the importance of which is stressed in the new study; we use the trees as temporary mirrors
  
• once they are harvested, the trees are used as a solid biofuel for the production of energy, while the carbon dioxide captured from the atmosphere that would be released during the combustion is stored underground, in so-called carbon capture and storage (CCS) systems
• the result is a highly efficient carbon-negative energy system that can power our societies while at the same time taking our 'historic' CO2 emissions - all the carbon dioxide from fossil fuels we pumped into the atmosphere since the industrial revolution - out of the carbon cycle; such a system cleans up the past
  
• scientists have found that this kind of carbon-negative bioenergy concepts can take us back to pre-industrial CO2 levels by mid-century

Of course, much more research is needed into the actual albedo effect produced by fast-growing tropical energy trees that are harvested in (long) cycles; plantation trees may not produce the same effect as new forests that are left to stand permanently, as higher soil respiration fluxes resulting from decomposing organic material after crop rotation may offset the benefits:
bioenergy :: biofuels :: energy :: sustainability :: climate change :: tropics :: temperate :: trees :: forests :: deforestation :: reforestation :: afforestation :: biomass ::


So far, we found very few studies focusing on the albedo effect of tropical energy plantations (there are several studies on this effect in extra-tropical plantations). One reference does suggests the following, though:

[on the micro-climate of plantations] There are also situations in which the forests are located in hilly regions along the coast and are subjected to a constant fog, which condenses on the canopy and falls to the forest soil adding to the rainfall level (Lima 1993). This effect also has been observed in some native eucalypt forests of Australia (Costin and Winbush 1961). This could indicate that Brazil's eucalypt plantations may have the same effects on the climate as a native forest located in the same region. Thus, the effect of planting a large area with eucalypts is likely to be the same as if other vegetation of similar structure and albedo were planted. In summary, certain research studies have shown that differences in the microclimate within eucalypt plantations may exist compared with those of other species and native forests, but the data are not conclusive (Poore and Fries 1985). - From a report by the Oak Ridge National Laboratory's Bioenergy Dept. [only accesible via *cache]: "Short-rotation eucalypt plantations in Brazil: environmental issues", s.d., s.l.

In any case, the author of the present study says this on the albedo effect of tropical trees in general: "When it comes to rehabilitating forests to fight global warming, carbon dioxide might be only half of the story; we also have to account for whether they help to reflect sunlight by producing clouds, or help to absorb it by shading snowy tundra."

Forests in colder, sub-polar latitudes evaporate less water and are less effective at producing clouds. As a result, the main climate effect of these forests is to increase the absorption of sunlight, which can overwhelm the cooling effect of carbon storage.

However, Caldeira believes it would be counterproductive to cut down forests in snowy areas, even if it could help to combat global warming. "A primary reason we are trying to slow global warming is to protect nature," he explains. "It just makes no sense to destroy natural ecosystems in the name of saving natural ecosystems."

More information:
G. Bala, K. Caldeira, et al, "Combined climate and carbon-cycle effects of large-scale deforestation" [*abstract], Published online before print April 9, 2007, Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0608998104