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Part 2 – Reforestation, Albedo and the Lawrence Livermore Study / Does Reforestation Contribute to Global Warming?

( We would like to invite all of those interested in this subject to join our ongoing discussion at Treehugger.com . )

The Lawrence Livermore study has created quite a buzz among bloggers and those that are concerned in Global Warming, Greenhouse Gas Emissions and Carbon Offset Programs. Acceptance of this report is widespread. To make matters worse and the findings have been consistently misinterpreted for the sake of attention getting headlines.

What those that have embraced this report have failed to realize is that the entire study is based on an unrealistic hypothetical predication.

The following is an analysis of the Lawrence Livermore study by M. J. Bradley and Associates

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

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.

Reduction of greenhouse effect through terrestrial sequestration

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

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.

Findings

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.

Part 1 – Does Reforestation Contribute to Global Warming?
Part 2 – Reforestation, Aldedo and Lawrence Livermore Study
Part 3 – Rain Forests: The World’s Air Conditioner
Part 4 – Planting Trees in Cities
Part 5 – Does Reforestation Contribute to Global Warming? – A second look at the Livermore Study
Part 6 – Getting Our Priorities Straight – Common Sense Solutions to Climate Change

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