The quality of air in the atmosphere over Vermont may create stresses on our forests, either directly (ozone damage to leaves) or indirectly (acid rain’s effect on the ability of trees to take in nutrients and necessary trace elements). Our local forest ecosystems, on the other hand, constantly interact with the atmosphere to change the composition of the air around us, generally improving air quality.

In the past decade, particularly in the past year or two, global warming has come to the forefront of our consciousness as a reality threatening to alter our worldwide climate in the not-so-distant future. Maintaining healthy forest ecosystems is increasingly seen as one of the necessary components for preserving the near-term stability of our global climate.

Our global climate is the result of a delicate balance: the amount of solar energy arriving at the surface of the Earth and the amount of energy that Earth reradiates (as heat) back to space. Too much reradiation (too little greenhouse warming effect) and the Earth’s mean temperature cools. Too little reradiation (increased greenhouse warming effect) and the Earth’s temperature warms. This balance is dependent, in significant part, on the composition and dynamics of a thin layer of atmospheric gases enveloping our planet. Composition of this thin layer of atmosphere is, in turn, influenced strongly by our human activities and by our forests.

What do we know today that explains our currently recognized global warming trend? A number of greenhouse gases which contribute to warming are emitted to the atmosphere both naturally and through human activities such as burning fuel. These gases include carbon dioxide, methane, nitrous oxide, ozone, and water vapor. Humans also produce a variety of halocarbons, some of which have significant global warming potential. A measure of the importance for potential climate change of each of these gases is its effect on the energy balance of the Earth-atmosphere system. This effect is called “radiative forcing,” measured as watts per square meter (W/m2).

Carbon dioxide is not only the most abundant greenhouse gas present in the atmosphere (0.03 percent by volume), it also has the highest total radiative-forcing effect of any of the greenhouse gases (1.56 W/m2). Methane is next at 0.47 W/m2, followed by nitrous oxides at 0.14 W/m2 and the halocarbons combined at 0.15 W/m2, according to 1992 figures from a report of the Intergovernmental Panel on Climate Change (IPCC). Forest interactions are specific to carbon dioxide. From long-term measurements of carbon dioxide concentrations in the atmosphere, we know levels were around 280 parts per million by volume at the beginning of the Industrial Revolution. Since 1800, atmospheric concentrations of carbon dioxide have increased by 30 percent (Figure 1).

Changes to the atmospheric concentration of carbon dioxide are the result of the net annual carbon dioxide flux from atmospheric interactions with our natural ecosystems (forests, for example) and from direct emissions to the atmosphere. Sources produce and release carbon dioxide and increase its concentration in the atmosphere. Sinks sequester carbon, taking it from the atmosphere and making it unavailable for release, leading to a net reduction of carbon dioxide in the atmosphere. Fossil fuel combustion is the human activity which results in the greatest emissions of carbon dioxide to the atmosphere. Changes in land-use and forestry activities affect the only significant sink for carbon dioxide within human control, our forest ecosystem biomass.

Forest ecosystems act as sources and sinks of carbon dioxide at the same time. Carbon dioxide is removed from the atmosphere by growing vegetation during photosynthesis while using solar energy to produce biomass. But forest ecosystems also release carbon dioxide to the atmosphere through respiration when photosynthesis is shut down at night and through the process of biomass decay.

Biomass accumulates in tree trunks, the understory of the forest, forest floor litter, and in forest soils. When decay or oxidation occurs in these systems, carbon sequestered in the biomass is returned to the atmosphere in the form of carbon dioxide. The difference between how much carbon dioxide is released to the atmosphere and how much is removed from the atmosphere is called the net annual flux. When the annual flux of carbon dioxide involves a net annual removal from the atmosphere, sequestration of carbon occurs.

Whereas the healthy, growing, temperate forests of the northeastern United States act as a sink for carbon dioxide, recent studies suggest that increasing minimum nighttime temperatures have slowed the growth of tropical forests to the point that they may have become overall sources of carbon dioxide. Nighttime respiration may be producing larger releases of carbon dioxide in tropical forests than the day-time photosynthesis has removed. Large-scale harvesting of any forest biomass to be burned for energy or burned as part of land-clearing (widespread in the tropics) further exacerbates the overall release of carbon dioxide to the atmosphere in the short-term.

Harvesting and consumption of forests need not produce long-term increases in atmospheric carbon dioxide. Encouraging active new growth through proper forest management practices will sequester more carbon, allowing a continued net removal of carbon dioxide from the atmosphere if mature and less rapidly growing biomass is replaced by new, vigorously growing younger forests. Harvested biomass used for building structures and products continues to sequester carbon and does not release carbon dioxide to the atmosphere.

Trends in Carbon Dioxide Sources and Sinks

Fossil fuel combustion for energy related activities has accounted for 99 percent of total U.S. carbon dioxide emissions in recent years. Nationally, 20 percent of these were from residential fuel combustion, 16 percent from commercial, 33 percent from industrial, and 31 percent from transportation (Figure 2).

An estimate of the net annual flux of carbon dioxide from forestry in the continental United States is shown in Figure 3. This flux is currently sequestering more than 170 million metric tons of carbon annually. Trees themselves and the understory growth account for roughly 40 percent of the sequestration, while the forest floor and soil layers of forest ecosystems account for the remainder. The net annual sequestration of carbon in the U.S. forest ecosystem in 1996 offset about 12 percent of 1996 U.S. carbon dioxide emissions from fossil fuel combustion.

Although Vermont’s emissions are only 0.1 percent of the U.S. totals, our carbon dioxide emissions have increased significantly since 1982 (Figure 3). At the same time, our forests have been increasing sequestered carbon through increases in total growing biomass.

The result is that Vermont forests are offsetting more than twice the carbon dioxide produced by fossil fuel emissions in the state. Of course, our forests cannot continue to grow and increase biomass indefinitely. If our use of fossil fuels continues to increase, we may soon reach the point where our forests cannot offset our annual emissions.

Global Warming

Is global warming something we should be concerned about in Vermont? After all, surviving Vermont winters can be a struggle. Wouldn’t a little bit of climatic warming be good for Vermont? The answer we would give depends, of course, on how global warming would actually affect Vermont’s everyday climate and environment.

Recent estimates by the IPCC predict the Earth will warm between two and six degrees Fahrenheit in the next 100 years. This warming could produce a shorter ski season, allow incursion of warmer climate tree species which would replace the current mix of hardwoods that produce our spectacular fall foliage, and result in a dramatic change in the quality and quantity of maple sap. There is no scientific study to date that describes in detail the likely effects of global warming on Vermont — but there will almost certainly be effects. Change in global temperatures is sure to create changes in Vermont’s climate, and we should not ignore the potential for significant environmental and economic impacts.

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