2011 Oct. 1: NY: NYT: World’s forests dying off as global climate warms (excerpts)
Southwest into Texas, wildfires raced across parched landscapes this summer, burning millions more acres. In Colorado, at least 15 percent of that state’s spectacular aspen forests have gone into decline because of a lack of water.
The devastation extends worldwide. The great euphorbia trees of southern Africa are succumbing to heat and water stress. So are the Atlas cedars of northern Algeria. Fires fed by hot, dry weather are killing enormous stretches of Siberian forest. Eucalyptus trees are succumbing on a large scale to a heat blast in Australia, and the Amazon recently suffered two “once a century” droughts just five years apart, killing many large trees.
Experts are scrambling to understand the situation, and to predict how serious it may become.
Scientists say the future habitability of the Earth might well depend on the answer. For, while a majority of the world’s people now live in cities, they depend more than ever on forests, in a way that few of them understand.
Trees absorb vehicle emissions
forests have been absorbing more than a quarter of the carbon dioxide that people are putting into the air by burning fossil fuels and other activities. It is an amount so large that trees are effectively absorbing the emissions from all the world’s cars and trucks. Without that disposal service, the level of carbon dioxide in the atmosphere would be rising faster. The gas traps heat from the sun, and human emissions are causing the planet to warm.
Yet the forests have only been able to restrain the increase, not halt it. And some scientists are increasingly worried that as the warming accelerates, trees themselves could become climate-change victims on a massive scale.
If forests were to die on a sufficient scale, they would not only stop absorbing carbon dioxide, they might also start to burn up or decay at such a rate that they would spew huge amounts of the gas back into the air as is already happening in some regions. That, in turn, could speed the warming of the planet, unlocking yet more carbon stored in once-cold places like the Arctic.
‘A very different world’
Scientists are not sure how likely this feedback loop is, and they are not eager to find out the hard way.
Scientists acknowledge that their attempts to use computers to project the future of forests are still crude. Some of those forecasts warn that climate change could cause potentially widespread forest death in places like the Amazon, while others show forests remaining robust carbon sponges throughout the 21st century.
Wealthy countries to pay?
more modest steps could be taken to protect forests. One promising plan calls for wealthy countries to pay those in the tropics to halt the destruction of their immense forests for agriculture and logging.
In the 1990s, many of the white spruce trees of Alaska’s Kenai Peninsula were wiped out by beetles. For more than a decade, other beetle varieties have been destroying trees across millions of acres of western North America. Red-hued mountainsides have become a familiar sight in a half-dozen states, including Montana and Colorado, as well as British Columbia in Canada.
Researchers refer to events like these as forest die-offs, and they have begun to document what appears to be a rising pattern of them around the world. Only some have been directly linked to global warming by scientific studies; many have yet to be analyzed in detail. Yet it is clear that hotter weather, of the sort that science has long predicted as a consequence of human activity, is playing a large role. “The amount of area burning now in Siberia is just startling — individual years with 30 million acres burned,” Dr. Swetnam said, describing an area the size of Pennsylvania. “The big fires that are occurring in the American Southwest are extraordinary in terms of their severity, on time scales of thousands of years. If we were to continue at this rate through the century, you’re looking at the loss of at least half the forest landscape of the Southwest.”
The carbon dioxide mystery
In the 1950s, when a scientist named Charles David Keeling first obtained accurate measurements of carbon dioxide in the atmosphere, a mystery presented itself. Only about half the carbon that people were releasing into the sky seemed to be staying there. It took scientists decades to figure out where the rest was going. The most comprehensive estimates on the role of forests were published only a few weeks ago by an international team of scientists.
Trees are taking up a similar amount of carbon (as oceans), but whether this will continue is much less certain, as the recent forest damage illustrates.
Carbon dioxide is an essential part of the cycle of life on Earth, but geologic history suggests that too much can cause the climate to warm sharply. With enough time, the chemical cycles operating on the planet have a tendency to bury excess carbon. In the 19th century, humans discovered the usefulness of some forms of buried carbon — coal, oil and natural gas — as a source of energy, and have been perturbing the natural order ever since. About 10 billion tons of carbon are pouring into the atmosphere every year from the combustion of fossil fuels and the destruction of forests.
were to continue at this rate through the century, you’re looking at the loss of at least half the forest landscape of the Southwest.” The concentration of the gas in the atmosphere has jumped 40 percent since the Industrial Revolution, and scientists fear it could double or even triple this century, with profound consequences.
While all types of plants absorb carbon dioxide, known as CO2, most of them return it to the atmosphere quickly because their vegetation decays, burns or is eaten. Every year, during the Northern Hemisphere growing season, plants and other organisms inhale some 120 billion tons of carbon from the atmosphere, then exhale nearly the same amount as they decay in the winter.
Long-term carbon storage
It is mainly trees that have the ability to lock carbon into long-term storage, and they do so by making wood or transferring carbon into the soil. The wood may stand for centuries inside a living tree, and it is slow to decay even when the tree dies.
But the carbon in wood is vulnerable to rapid release. If a forest burns down, for instance, much of the carbon stored in it will re-enter the atmosphere.
the new estimate, published Aug. 19 in the journal Science, suggests that when emissions from the destruction of forests are subtracted from the carbon they absorb, they are, on balance, packing more than a billion tons of carbon into long-term storage every year.
“Forests take a century to grow to maturity,” said Werner A. Kurz, a Canadian scientist who is a leading expert on forest carbon. “It takes only a single extreme climate event, a single attack by insects, to interrupt that hundred-year uptake of carbon.”
die-offs might prove to be the leading edge of a more sweeping change.
“If this were happening in just a few places, it would be easier to deny and write off,” said David A. Cleaves, senior adviser for the United States Forest Service. “But it’s not. It’s happening all over the place. You’ve got to say, gee, what is the common element?”
Tracking an ebb and flow
A 100-foot tower stands in the middle of the Harvard Forest, studded with instruments. Put up in 1989, it was the first permanent tower of its kind in the world, built to help track the carbon fluxes. Now hundreds of them dot the planet.
Forest gaining weight
“We’re actually seeing that the leaves are falling off the trees later in the fall,” Mr. Werden said. Scientists say that something similar may be happening in other forests, particularly in cold northern regions that are warming rapidly. In some places, the higher temperatures could aid tree growth or cause forests to expand into zones previously occupied by grasslands or tundra, storing more carbon. Forests are re-growing on abandoned agricultural land across vast reaches of Europe and Russia. China, trying to slow the advance of a desert, has planted nearly 100 million acres of trees, and those forests, too, are absorbing carbon.
But, as a strategy for managing carbon emissions, these recovering forests have one big limitation: the planet simply does not have room for many more of them. To expand them significantly would require taking more farmland out of production, an unlikely prospect in a world where food demand and prices are rising.
“We’re basically running out of land,” Dr. Kurz said.
Even in forests that are relatively healthy now, like those of New England, climate risks are coming into focus. For instance, invasive insects that used to be killed off by cold winters are expected to spread north more readily as the temperature warms, attacking trees.
The Harvard Forest has already been invaded by an insect called the woolly adelgid that kills hemlock trees, and managers there fear a large die-off in coming years.
Wildfires and bugs
climate has warmed, various beetle species have marauded across the landscape, from Arizona to Alaska. The situation is worst in British Columbia, which has lost millions of trees across an area the size of Wisconsin.
Snow melting earlier
drought came against the background of an overall warming and drying of the Southwestern climate, which scientists say helps to explain the severe effects. But the role of climate change in causing the drought itself is unclear — the more immediate cause is an intermittent weather pattern called La Niña, and research is still under way on whether that cycle is being altered or intensified by global warming, as some researchers suspect. Because of the continuing climatic change, experts say some areas that are burning this year may never return as forest — they are more likely to grow back as heat-tolerant grass or shrub lands, storing far less carbon than the forests they replace.
ecologists like me are starting to think all these agents, like insects and fires, are just the proximate cause, and the real culprit is water stress caused by climate change,” said Robert L. Crabtree, head of a center studying the Yellowstone region. “It doesn’t really matter what kills the trees — they’re on their way out. The big question is, Are they going to regrow? If they don’t, we could very well catastrophically lose our forests.”
Scientists are coming to a sobering realization: There may be no such thing left on Earth as a natural forest.
However wild some of them may look, experts say, forests from the deepest Amazon to the remotest reaches of Siberia are now responding to human influences, including the rising level of carbon dioxide in the air, increasing heat and changing rainfall patterns. That raises the issue of what people can do to protect forests.
Yet, scientists emphasize that in the end, programs meant to conserve forests — or to render them more fire-resistant, as in the Western United States, or to plant new ones, as in China — are only partial measures. To ensure that forests are preserved for future generations, they say, society needs to limit the fossil-fuel burning that is altering the climate of the world.
This article, headlined “The Threats to a Crucial Canopy,” first appeared in The New York Times.