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143 Chapter 13 Global Warming and the Forests Introduction The fact that the climate is warming because of the buildup of greenhouse gases in the atmosphere is generally accepted. To begin, note that what is happening is called both “climate change” and “global warming.” Take your choice. The merit of the term “climate change” is that it allows for gaps in the warming; it doesn’t sound like nonsense when an unusually cold spell strikes in a limited area. It also emphasizes that rising temperatures in most of the world are not the only change: precipitation and winds are changing too. In any case, there’s no need to use one term consistently; rather, it is reasonable to use whichever is appropriate to the context. Let’s start with a brief outline of the elementary physics of what’s involved , and how rising temperatures affect precipitation and soil, especially permafrost. The Physics of Climate Change The increasing use of fossil fuels by the world’s growing population is adding to the natural concentration of carbon dioxide (CO2) in the atmosphere . Water vapor, CO2, and methane (CH4), in that order, are the three strongest natural greenhouse gases (GHGs), which are opaque to outgoing heat rays (infrared rays) from the surface of the sun-warmed earth. Water 144 the world of northern evergreens vapor (hereafter simply called “vapor”) is the most important ingredient, but it is so commonplace, and present in such ever-varying amounts, that it is often left out of theoretical calculations. Acting together, the gases trap much of the sun’s heat that would otherwise be radiated back to the sky. Over the past 250 years, the concentration of CO2 has been rising rapidly, causing the current greenhouse effect: the average temperature is increasing over the whole earth. Note the word “average.” The occurrence of cold spells here and there, even long ones, does not negate the fact that, averaged over the whole world, the temperature is indisputably rising. Note also that we are not concerned here with temperature trends in the more distant past. Two other GHGs are important; both are in the stratosphere. They are natural ozone, and chlorofluorocarbons (CFCs), industrial chemicals that came into use in the 1930s. Chlorofluorocarbons were found to be destroying ozone in the stratospheric ozone layer that shields the surface life below from the sun’s damaging ultraviolet radiation, so their use was discontinued, worldwide. Even so, the amount still persisting will go on destroying ozone for years. Today, temperatures are rising faster in the Arctic than in the Tropics. Researchers1 predict that the concentration of CO2 is unlikely to become stabilized at less than 650 parts per million by volume (ppmv). The level was about 280 ppmv in the preindustrial era, before 1750. An increase as great as predicted would (or will?) lead to a worldwide average temperature rise of about 4° Celsius (7.2° Fahrenheit), which, it is said, would be “catastrophic.” Possibly, a positive feedback (intensifying the warming) will progress, for a while, like this: WARMTH → MORE VAPOR → INCREASED GREENHOUSE EFFECT → WARMER. But eventually there would surely be a counteracting negative feedback like this: WARMTH → MORE VAPOR → MORE CLOUDS → SUNLIGHT REFLECTED → COOLER. Moreover, as warming peat decomposes, quantities of methane, which is a much stronger GHG than CO2 (on a per volume basis), are liberated. The result is additional positive feedback: WARMTH → PEAT DECAYS → METHANE EMITTED → WARMER. global warming and the forests 145 Global warming theory is bedeviled by positive feedbacks that boost it and negative ones that counteract it. How Will Climate Change Affect the Forests? Three of the possible effects of climatic “warming,” in the narrow sense of the word, were just listed. But “warming” has a wider connotation. It also implies an increase in the total energy received from the sun. Indeed, warmth, another word for heat, is one of energy’s many forms. Some of this energy manifests itself as mechanical energy—in a word, the winds become stronger.2 The strongest, most damaging storm winds to affect North American forests blow in latitudes south of our area or near its southern limits, but as the climate warms, they can be expected to shift northward. Downbursts of wind will probably be the most serious. A downburst consists of a radiating system of horizontal straight winds that spread out from the point where the dense central column of downward-moving air in a thunderstorm hits the ground. A famous recent downburst struck in 1999 in the...


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