October 1, 2011
With Deaths of Forests, a Loss of Key
Climate Protectors
By JUSTIN GILLIS
WISE RIVER, Mont. — The
trees spanning many of the mountainsides of western Montana glow an earthy red,
like a broadleaf forest at the beginning of autumn. But these trees are not
supposed to turn red. They are evergreens, falling victim to beetles that used
to be controlled in part by bitterly cold winters. As the climate warms,
scientists say, that control is no longer happening.
Across millions of acres, the pines of the
northern and central Rockies are dying, just one among many types of forests
that are showing signs of distress these days.
From the mountainous Southwest deep 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.
Scientists have figured out — with the
precise numbers deduced only recently — that 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.
“At the same time that we’re recognizing
the potential great value of trees and forests in helping us deal with the excess
carbon we’re generating, we’re starting to lose forests,” said Thomas W. Swetnam, an expert on forest history at the University of
Arizona.
While some of the forests that died
recently are expected to grow back, scientists say others are not, because of
climate change.
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.
Scientists are not sure how likely this
feedback loop is, and they are not eager to find out the hard way.
“It would be a very different world than
the world we’re in,” said Christopher B. Field, an ecologist at the Carnegie
Institution for Science.
It is clear that the point of no return
has not been reached yet — and it may never be. Despite the troubles of recent
years, forests continue to take up a large amount of carbon, with some regions,
including the Eastern United States, being especially important as global
carbon absorbers.
“I think we have a situation where both
the ‘forces of growth’ and the ‘forces of death’ are strengthening, and have
been for some time,” said Oliver L. Phillips, a prominent tropical forest
researcher with the University of Leeds in England. “The latter are more
eye-catching, but the former have in fact been more important so far.”
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.
“We’re not completely blind, but we’re not
in good shape,” said William R. L. Anderegg, a
researcher at Stanford University.
Many scientists say that ensuring the
health of the world’s forests requires slowing human emissions of greenhouse
gases. Most nations committed to doing so in a global environmental treaty in
1992, yet two decades of negotiations have yielded scant progress.
In the near term, experts say, 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.
But now even that plan is at risk, for
lack of money. Other strategies, like thinning overgrown forests in the
American West to make them more resistant to fire and insect damage, are also
going begging in straitened times. With growing economic problems and a
Congress skeptical of both climate science and new spending, chances for
additional funding appear remote.
So, even as potential solutions to forest
problems languish, signs of trouble build.
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.
Many scientists had hoped that serious
forest damage would not set in before the middle of the 21st century, and that
people would have time to get emissions of heat-trapping gases under control
before then. Some of them have been shocked in recent years by what they are
seeing.
“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.
As best researchers can tell, the oceans
are taking up about a quarter of the carbon emissions arising from human
activities. That is causing the sea to become more acidic and is expected to
damage marine life over the long run, perhaps catastrophically. But the
chemistry is at least somewhat predictable, and scientists are reasonably
confident the oceans will continue absorbing carbon for many decades.
Trees are taking up a similar amount of
carbon, 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.
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.
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.
Destruction by fires and insects is a part
of the natural history of forests, and in isolation, such events would be no
cause for alarm. Indeed, despite the recent problems, 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.
One major reason is that forests, like
other types of plants, appear to be responding to the rise of carbon dioxide in
the atmosphere by growing more vigorously. The gas is, after all, the main food
supply for plants. Scientists have been surprised in recent years to learn that
this factor is causing a growth spurt even in mature forests, a finding that
overturned decades of ecological dogma.
Climate-change contrarians tend to focus
on this “fertilization effect,” hailing it as a boon for forests and the food
supply. “The ongoing rise of the air’s CO2 content is causing a great greening
of the Earth,” one advocate of this position, Craig D. Idso,
said at a contrarian meeting in Washington in July.
Dr. Idso and
others assert that this effect is likely to continue for the foreseeable
future, ameliorating any negative impacts on plant growth from rising
temperatures. More mainstream scientists, while stating that CO2 fertilization
is real, are much less certain about the long-term effects, saying that the
heat and water stress associated with climate change seem to be making forests
vulnerable to insect attack, fires and many other problems.
“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.”
It is possible the recent die-backs will
prove transitory — a coincidence, perhaps, that they all occurred at roughly
the same time. The more troubling possibility, experts said, is that the
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
So far, humanity has been lucky. While
some forests are starting to release more carbon than they take up, that effect
continues to be outweighed by forests that pack carbon away. Whether those
healthy forests will predominate over coming decades, or will become sick
themselves, is simply unclear.
The other day, deep in a healthy New
England thicket of oaks, maples and hemlocks, two young men scrambled around on
their hands and knees measuring twigs and sticks that had fallen from the
trees.
“What was the diameter on that?” asked Jakob Lindaas, a Harvard student
holding a pencil and clipboard.
Leland K. Werden,
a researcher at the university, called out a metric measurement, and they moved
to the next twig. It was one of thousands they would eventually have to measure
as part of an effort to tell how fast the wood, knocked off the trees in an ice
storm in 2008, was decaying.
The debris they were cataloging would not
have struck a hiker as anything to notice, much less measure, but the Harvard
Forest, 3,000 acres near Petersham, Mass., is one of
the world’s most intensively studied patches of woods. The work the men were
doing will become a small contribution toward solving one of the biggest
accounting problems of modern science.
In every forest, carbon is constantly
being absorbed as trees and other organisms grow, then released as they die or
go dormant. These carbon fluxes, as they are called, vary through the day. They
vary with seasons, with climate and weather extremes, with the health of the
forests and with many other factors. Across the world, scientists are
struggling to track and understand this 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.
Meticulous measurements over the decades
have established that the Harvest Forest is gaining weight, roughly two tons
per acre per year, on average. It is characteristic of a type of forest that is
playing a big role in limiting the damage from human carbon emissions: a
recovering forest.
Not so long ago, the land was not a forest
at all. Close to where the men were working stood an old stone fence, a
telltale sign of the land’s history.
“When the European colonists came to
America, they saw trees, and they wanted fields and pastures,” explained J.
William Munger, a Harvard research fellow who was
supervising the measurements. So the colonists chopped down the original forest
and built farmhouses, barns, paddocks and sturdy stone fences.
By the mid-19th century, the Erie Canal
and the railroads had opened the interior of the country, and farmers plowing
the thin, stony soils of New England could not compete with produce from the
rich fields of the Midwest. So the old fields were abandoned, and trees have
returned.
Today, the re-growing forests of the
Eastern United States are among the most important carbon sponges in the world.
In the Harvard Forest, the rate of carbon storage accelerated about a decade
ago. As in much of the world, the temperature is warming there — by an average
of 2.3 degrees Fahrenheit in the last 40 years — and that has led to longer
growing seasons, benefiting this particular forest more than hurting it, at
least so far.
“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
Stripping the bark of a tree with a
hatchet, Diana L. Six, a University of Montana insect scientist, pointed out
the telltale signs of infestation by pine beetles: channels drilled by the
creatures as they chewed their way through the juicy part of the tree.
The tree she was pointing out was already
dead. Its needles, which should have been deep green, displayed the sickly red
that has become so commonplace in the mountainous West. Because the beetles had
cut off the tree’s nutrients, the chlorophyll that made the needles green was
breaking down, leaving only reddish compounds.
Pine beetles are a natural part of the
life cycle in Western forests, but this outbreak, under way for more than a
decade in some areas, is by far the most extensive ever recorded. Scientists
say winter temperatures used to fall to 40 degrees below zero in the mountains
every few years, killing off many beetles. “It just doesn’t happen anymore,”
said a leading climate scientist from the University of Montana, Steven W.
Running, who was surveying the scene with Dr. Six one recent day.
As the 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.
The species Dr. Six was pointing out, the
mountain pine beetle, has pushed farther north into Canada than ever recorded.
The beetles have jumped the Rocky Mountains into Alberta, and fears are rising
that they could spread across the continent as temperatures rise in coming
decades. Standing on a mountain plateau south of Missoula, Dr. Six and Dr.
Running pointed to the devastation the beetles had wrought in the forest around
them, consisting of a high-elevation species called whitebark
pine.
“We were going to try to do like an
eight-year study up here. But within three years, all this has happened,” Dr.
Six said sadly.
“It’s game over,” Dr. Running said.
Later, flying in a small plane over the
Montana wilderness, Dr. Running said beetles were not the only problem
confronting the forests of the West.
Warmer temperatures are causing mountain
snowpack, on which so much of the life in the region depends, to melt earlier
in most years, he said. That is causing more severe water deficits in the
summer, just as the higher temperatures cause trees to need extra water to
survive. The whole landscape dries out, creating the conditions for intense
fires. Even if the landscape does not burn, the trees become so stressed they
are easy prey for beetles.
From the plane, Dr. Running pointed out
huge scars where fires had destroyed stands of trees in recent years. “Nothing
can stop the wildfires when they get to this magnitude,” he said. Some of the
fire scars stood adjacent to stands of lodgepole pine
destroyed by beetles.
At the moment, the most severe problems in
the nation’s forests are being seen in the Southwestern United States, in
states like Arizona, New Mexico and Texas. The region has been so dry that
huge, explosive fires consumed millions of acres of vegetation and thousands of
homes and other buildings this summer.
This year’s 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.
“A lot of 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.”
Stalled Efforts
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.
Some steps have already been taken in
recent years, with millions of acres of public and private forest land being
designated as conservation reserves, for instance. But other ideas are
essentially stymied for lack of money.
Widespread areas of pine forest in the
Western United States are a prime example. A scientific consensus has emerged
that people mismanaged those particular forests over the past century, in part
by suppressing the mild ground fires that used to clear out underbrush and
limit tree density.
As a consequence, these overgrown forests
have become tinderboxes that can be destroyed by high-intensity fires sweeping
through the crowns. The government stance is that many forests throughout the
West need to be thinned, and some environmental groups have come to
agree. economy. With little money available to
subsidize the thinning, the Forest Service is reduced to treating only small
sections of forest that pose the biggest threat to life and property.
On an even larger scale, experts cite a
lack of money as endangering a program to slow or halt the destruction of
tropical forests at human hands.
Deforestation, usually to make way for
agriculture, has been under way for decades, with Brazil and Indonesia being
hotspots. The burning of tropical forests not only ends their ability to absorb
carbon, it also produces an immediate flow of carbon back to the atmosphere,
making it one of the leading sources of greenhouse gas emissions.
Rich countries agreed in principle in
recent years to pay poorer countries large amounts of money if they would
protect their forests.
The wealthy countries have pledged nearly
$5 billion, enough to get the program started, but far more money was
eventually supposed to become available. The idea was that the rich countries
would create ways to charge their companies for emissions of carbon dioxide,
and some of this money would flow abroad for forest preservation.
Climate legislation stalled in the United
States amid opposition from lawmakers worried about the economic effects, and
some European countries have also balked at sending money abroad. That means it
is not clear the forest program will ever get rolling in a substantial way.
“Like any other scheme to improve the
human condition, it’s quite precarious because it is so grand in its
ambitions,” said William Boyd, a University of Colorado law professor working
to salvage the plan.
The best hope for the program now is that
California, which is intent on battling global warming, will allow industries
to comply with its rules partly by financing efforts to slow tropical
deforestation. The idea is that other states or countries would eventually
follow suit.
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.