With deep snow lingering, now is a good time to perform the following experiment. Look out across a hillside that is a mix of forest and field. Now squint a bit. Which is brighter, the forest or the field?
The snow-covered field, of course, is much brighter than the forest, even on a cloudy day. The darker forest, meanwhile, absorbs more light and reflects less of it back to your eyes.
This simple observation has profound implications for global warming and, in particular, for the various solutions that have been proposed for combating it here in Vermont and New Hampshire.
Since 1750 and the dawn of the industrial age, the concentration of carbon dioxide in the atmosphere has risen from 278 parts per million to 380 parts per million. Carbon dioxide is the most prevalent greenhouse gas in the atmosphere, and this 37 percent increase in carbon dioxide is the chief reason why global warming is front-page news today.
Roughly three-quarters of this increased carbon dioxide has come from the combustion of fossil fuels, with the other quarter coming from deforestation. Trees are comprised primarily of carbon, and as the world’s forests have been cut down for either wood, fuel, or to make way for agriculture, most of this carbon has been released into the atmosphere, primarily as carbon dioxide.
People searching for ways to combat global warming have, naturally enough, turned to reforestation. If all the carbon that has been released into the atmosphere from the cutting of trees over the past two centuries could be sequestered in new trees, and if forest cover worldwide could be increased, then a significant share of the increased carbon dioxide in the atmosphere would be removed. Companies have begun planting trees as a way to mitigate the carbon dioxide emitted from other aspects of their businesses, and individuals can now purchase “green credits” whose proceeds, among other things, go toward reforestation efforts.
But here’s where our snowy field experiment comes back into play. Scientists at California’s Lawrence Livermore National Laboratory recently confirmed that forests do indeed absorb more energy than fields and grasslands. They also noted that snow and ice absorb very little energy, reflecting most of it back into space. Converting snowy pastures into forest, or forests into snowy pastures, therefore, has a measurable effect on climate, independent of the carbon dioxide issue.
The Lawrence Livermore scientists set out to see how important reflectivity is, compared to carbon dioxide. Does planting trees reduce the greenhouse effect (by decreasing carbon dioxide), yet warm the climate anyway because forests absorb more energy than grasslands or snow? Or is the opposite true - is reducing carbon dioxide by planting trees more important than increasing reflectivity by cutting them down? Extending the greenhouse metaphor further, which is more important, the thickness of the greenhouse walls (the carbon dioxide) or the amount of heat in the greenhouse to start with (reflectivity)?
In temperate forests like those in Vermont and New Hampshire, the scientists found that, according to their model, these two effects roughly cancel each other out, meaning that growing or cutting trees here has little net effect on the global temperature. If you travel north, into the boreal forest, clear-cutting trees leads to net cooling because snow is so much more reflective than forest and because snow is on the ground for more days of the year. South towards the tropics, meanwhile, where the reflectivity difference between field and forest is small, tree cover leads to net cooling, as carbon dioxide absorption becomes greater than decreased reflectivity.
The scientists are quick to point out that the atmosphere and climate are very complex systems, that the model is necessarily a simplification, and that further research is needed to verify the results. One obvious line of inquiry: as global warming pushes the annual snow line farther north, will the line where keeping or planting trees becomes more beneficial than cutting them also move north?
These overall findings have two implications for us here in Vermont and New Hampshire, where nearly 90 percent of our landscape is forested. First is that there are many reasons why we might want to maintain our forests, including wildlife habitat, forest products, flood control and other ecosystem services, recreation, solitude, and just plain beauty. Combating global warming, it seems, should not be added to this job description.
The second implication is that the antidotes to global warming available to us locally should focus on the heart of the matter - the combustion of fossil fuels - and not on the minor improvements that might result from changes in land management. In essence, while fossil fuel combustion has contributed 75 percent of the increased carbon dioxide in the atmosphere, from our local perspective, it has accounted for 100 percent of the problem.