TREES Feel the Heat.

WE WILL ALL FEEL THE EFFECTS OF CLIMATE CHANGE. HERE'S WHY YOU SHOULD CARE AND HOW TREES WILL FARE.

Last summer, raging fires blazed through more than 6.5 million acres of drought-stricken forests in the western United States. All of that charred land would cover Massachusetts plus Rhode Island, and more. Those fires might signify a changing world--where some forests will grow into new collections of trees, others will fade away to grasslands, animals will invade new territories and bigger, more devastating fires will flare up--all driven by a changing climate.

According to Thomas F. Malone, a meteorologist and member of the National Academy of Sciences, "We're in a period of probably the greatest warming that we've seen in several hundred years." Moreover, we triggered this warming by filling the air with polluting gases, especially carbon dioxide, from fossil fuels. Although our forests will suffer severe casualties from the changing climate, trees can fight back.

The weather always changes: Brisk fall days give way to cold winter weeks, followed by the comforting warmth of spring. Then, the heat sets in for summer. Climate scientists, however, look for longer trends: changes in weather over decades. For instance, the National Oceanic and Atmospheric Administration's records cover 106 years of temperatures, and the first half of 2000 topped the charts, making it the hottest first half of a year ever recorded.

The concerns over what's called global warming, or climate change, began decades ago. In the late 1950s David Keeling of the Scripps Institute of Oceanography wondered if human activities were raising the levels of carbon dioxide in the earth's atmosphere. in 1959, the first full year of Keeling's records, he found carbon dioxide levels of 316 parts per million, or about three-thousandths of a percent. That might not sound like much, but the carbon dioxide levels lingered around 260-280 from 4000 B.C. until 1700 A.D, The levels climbed soon after humans started burning significant levels of fossil fuels. By the late 1990s, carbon dioxide levels surpassed 370. Some climatologists forecast carbon dioxide at 700 by 2100. The trouble arises from carbon dioxide's connection with temperature.

The heating cycle that warms our environment starts with sunlight filtering through the atmosphere. Some of that energy reaches the ground or water and warms it. The earth releases some of the absorbed sunshine as heat that rises into the sky. Some of the rising heat is reflected by greenhouse gases--carbon dioxide, methane, nitrous oxide and so on--instead of flowing into space. Consequently, these gases work like the panels of glass in a greenhouse by holding in some of the heat. In fact, without these gases our world would be about 60 degrees Fahrenheit cooler, which would make it uninhabitable for many organisms.

To keep the earth's temperature steady, the earth must release as much energy as it absorbs. If a greenhouse gas, such as carbon dioxide, increases in the atmosphere, less of the sun's absorbed energy escapes. As a result, the earth's temperature increases.

Scientists pay particular attention to carbon dioxide because it has been skyrocketing--up more than 30 percent since the beginning of the Industrial Revolution. Before that, natural processes balanced the creation and consumption of this gas. Respiration-essentially breathing-by animals and plants and the natural decay of materials, such as leaves, creates carbon dioxide. It is consumed by photosynthetic organisms that use sunlight plus carbon dioxide and water and produce sugar and oxygen. Human actions, though, appear to have tipped that balance. According to the Environmental Protection Agency's website on global warming, "Energy burned to run cars and trucks, heat homes and businesses, and power factories is responsible for about 80 [percent] of society's carbon dioxide emissions ..."

This increasing carbon dioxide is already changing our world. The earth's average temperature climbed by about 1 degree over the last century with a long streak of increasingly hot years at the end. As a result of that heating and consequent melting of glaciers, the oceans expanded and sea level rose, by as much as 10 inches, according to some estimates.

Beyond the ongoing changes, scientists worry most about the future. Long-term forecasting uses mathematical models that consider carbon dioxide, circulation patterns around the globe, and much more. In the end, different models produce various predictions. Nevertheless, James Clark, a forest ecologist at Duke University says, "We've seen a growing level of confidence from the climate community in their ability to come up with temperature estimates."

Still, scientists can point only to a range of possibilities. According to the White House Initiative on Global Climate Change, "State-of-the-art climate models suggest... an increase of about 3.5 degrees F in global temperatures over the next century." An upcoming report from the National Assessment Synthesis Team predicts that temperatures could increase, on average, between 5 and 10 degrees. Some areas might get hotter than others, but major swings in peak temperatures are unlikely. In discussing peak temperatures, Malone says, "I don't think that the numbers will vary more than a few degrees--2 or 3 degrees variance from an average."

Cities already experience significant warming. A city's summer temperature can soar 6 to 8 degrees above surrounding regions. These urban heat islands arise because buildings and pavement absorb so much heat. Moreover, fewer trees in cities reduce shading and evapotranspiration, in which plants use up heat by, essentially, sweating. Furthermore, Hashem Akbari of the Lawrence Berkeley National Laboratory's Heat Island Group says, "By shading a building, a tree can reduce the air conditioning used during the summer."

As carbon dioxide builds up, precipitation patterns also will change. Most models predict rainfall increases of 5 to 20 percent. Like increases in temperature, higher rates of rainfall started some time ago. According to the White House Initiative on Global Climate Change: "Since the beginning of the century, precipitation in the United States has increased by about 6 percent, while the frequency of heavy downpours of more than 2 inches per day has increased by 20 percent." Nevertheless, many models predict drought conditions in some areas, especially in the Southeast. Either way--too much rain or not enough--will challenge forests. Wet conditions could build up fuel, and droughts can trigger fires or simply make trees die of thirst.

How will a changing climate affect trees? Increasing carbon dioxide could fertilize plants, because they can use it to make sugars. Likewise, most biological processes speed up as temperature increases, so the future climate could accelerate tree growth. Scientists put small trees in chambers and increase the carbon dioxide, and they grow faster. Nevertheless, Clark says, "It could also be that plants will acclimate and the increased growth may go away in a short amount of time."

To test the effects of carbon dioxide on a real forest, investigators at Duke University increased this gas by 50 percent in a loblolly pine forest. The trees' growth rate increased for a few years, then tapered off. Ram Oren, one of the investigators, adds: "We are not entirely sure how much of these transient responses is the result of our experiments. We sort of jerk the system with an increase in pressure of carbon dioxide and we observe the response, but the system may not have been responding the same way if the change had been gradual."

To forecast how large areas of forest might change with climate, scientists use models. The biodiversity committee of the Forest Sector of the National Assessment looked for common trends among seven different scenarios of future climate change. Committee member and bioclimatologist Ronald Neilson says, "The most consistent thing is that cold-limited boundaries of forests--those are upper-elevation boundaries and northern boundaries--uniformly tend to shift up in elevation or north." For instance, beech-maple forests in the Northeast might move toward Canada, and pines in the Southeast would move north, squeezing into oak-hickory habitat.

No one knows how fast this might happen. For forests to move, trees must migrate. Louis Pitelka, director of the Appalachian Laboratory, says, "There isn't very good information on what maximum rates of migration are." Some trees would surely move faster than others. Some require specific soil conditions that could limit migration. Others will need particular pollinating organisms to move with them, too. This information and more will be required to improve the predictions of what lies ahead for forests. As Oren says: "I would be surprised if anybody would actually stake their retirement on the accuracy of the predictions about forest changes."

How fast forests move also will depend on disturbances, including forest fires, which could speed up opportunities for species to move north. Neilson says, "In virtually all of the time-dependent simulations that we've done, in the West, we see large increases in fire." Forest pests also might create disturbances. Bob Mickler, an environmental consultant for the Forest Service, says, "Some insect models show the range of gypsy moths expanding northward through Maine and up into the east coast of Canada."

Some insects are already expanding their ranges. Camille Parmesan of the University of Texas at Austin and her colleagues studied 35 species of nonmigratory butterflies in Europe, and found that more than half shifted their range north during the last century. More movement among animals surely lies ahead.

Andy Hansen, a Montana State University ecologist, says, "The models predict that some groups, like reptiles and amphibians, would experience higher potential species richness in the U.S., due to the increases in temperature and precipitation. For birds and for mammals, the prediction was that in southern portions of the U.S. the richness would drop fairly substantially, due to the heat exceeding current tolerances."

Scientists also see signs of climate-induced changes in forests. One compelling example comes from Ranga Myneni's laboratory at Boston University. Using satellite images, he found the growing season for trees in the northern United States increased by nearly two weeks from 1981-1991.

The question is: What can be done? Foremost, conserving energy and buying more fuel-efficient devices will reduce the use of fossil fuel. Planting trees also helps. According to the Heat Island Group, strategically planted trees in cities can reduce heating and cooling costs by 10 to 20 percent. In addition, Akbari says, "If there are a lot more trees, people tend to walk more in their neighborhoods, and there is a good chance that people will walk rather than getting in their car."

Trees also fight global warming by consuming carbon. Hans Peter Schmid of Indiana University and his colleagues study the carbon cycle in real forests. He says, "We're finding that mid-latitude, northern hemisphere forests tend to sequester carbon over the years." So planting more trees would reduce the levels of carbon dioxide in the air. Nelson adds, "Any forest-management practices that enhance carbon sequestration are certainly all to the good."

Planting new trees, though, remains the best way to reduce excess atmospheric carbon dioxide. Programs such as AMERICAN FORESTS' Global ReLeaf, which has planted more than 15 million trees, are making a major difference. The average tree in a North American forest ecosystem sequesters approximately a ton of carbon dioxide in its first 40 years of growth.

To see how your lifestyle contributes to global warming, visit AMERICAN FORESTS' Personal Climate Change Calculator at www.americanforests.org. The calculator uses information such as your driving, recycling habits, and utility use to estimate the amount of [CO.sub.2] your household produces yearly. Then, with a click of your mouse, it tells you how many trees you need to plant to offset your carbon emissions.

In the end, we must watch the numbers-carbon-dioxide levels, temperatures and so on--but trees rise above purely arithmetical interpretations. As Jim Clark says, "The spiritual renewal that comes from green space is easy to overlook." If we continue to overlook the ongoing changes in our world, we will likely lose green space, our comfort, and more.

Mike May covers environmental and science issues from his home in Clinton, CT.