Reprinted with permission of the author.
The following was written for The Global Warming Desk Reference published by
Greenwood Press.
------------------------------------------------------------------------
Biotic Feedbacks:
Will Global Warming Feed Upon Itself?
Bruce E. Johansen
Human-induced warming of the Earth may be working in tandem
with several natural feedback mechanisms to accelerate climate
change through biotic feedbacks. The possibility that
human-induced warming may feed upon itself produces a special
sense of urgency in many climate scientists public statements.
Along with a sense of urgency, the possibility of biotic
"surprises" infuses a high degree of uncertainty into all
forecasts of global warming's possible effects.
George M. Woodwell, one of the world's most respected experts
on biotic-feedback mechanisms, has written
A significant body of experience...suggests that there
are mechanisms entrained by a change in global climate
that tend to increase the trend of temperature
change.... [T]here is a possibility that the warming
itself may cause a series of further changes in the
earth that will speed the warming.... The most serious
questions have to do with the potential for...surprises,
especially surprises which lead to positive feedbacks.
The potential appears significant.... (Woodwell, 393,
406)
Woodwell explains how global warming could feed powerfully
upon itself:
Disruptions of forests globally, especially in the
higher latitudes of the northern hemisphere will lead to
a significant increase in the release of carbon into the
atmosphere. That release can easily be in the range of
1-2 billion tons per year. It means that allowing the
warming to progress leads to a potential for surprises.
That's only one of the surprises.
If that were to occur it would mean that correcting
the problem would be even more difficult than it is at
the moment. The releases from the combustion of fossil
fuels at the moment (1996) are about 6 billion tons a
year. The annual accumulation is 3-4 billion tons a
year. Stabilizing the composition of the atmosphere
would require removing from current releases something
of the order of 3 billion tons, perhaps a little more,
immediately. That's a half or more of the current
releases of fossil fuels -- a very important challenge.
Warmer temperatures speed the decay of organic
matter in soils. Forests...and tundra soils of the
Northern Hemisphere contain sufficient carbon to add
significantly to the annual emissions, thereby speeding
the accumulation of carbon dioxide in the atmosphere.
Such a positive feedback has not been incorporated into
current estimates of the warming. It is one of several
potential surprises lurking in the wings as warming
proceeds. (Epstein, Current Effects)
In cold water, for example, methane clathrates form crystal
structures which are somewhat similar to water ice. Warming
temperatures could destabilize the clathrates, and release some of
their stored methane. Roughly 10 trillion tons of methane is
trapped under pressure in crystal structures in permafrost or on
the edges of the oceans' continental shelves, "the Earth's largest
fossil-fuel reservoir", according to Gerald Dickens, a geologist
at James Cook University in Townsville, Australia. (Pearce) The
greenhouse potential of all the methane stored in clathrates on
the continental shelves and in permafrost worldwide is roughly
equal to that of all the world's coal reserves. (Cline, 34) Some
of the land masses which host these deposits already have warmed 2
to 4 degrees C. (4 to 7 degrees F.) during the twentieth century.
(Lachenbruch)
Atmospheric scientist Roger Revelle has estimated that, with a
3 degree C. rise in global average temperature, methane emissions
from clathrates would increase half a gigaton per year. Over a
century, this rate could be enough to double the amount of methane
in the atmosphere. Add to this another twelve gigatons of methane
that could be released by clathrates liberated from ocean bottoms
under the Arctic Ocean once the ice cap now covering them melts.
"It is possible," writes Jonathan Weiner, "that [this]...feedback
effect is already underway and the rise in Earth temperatures in
the last hundred years has already sprung many gigatons of methane
from their molecular prisons at the bottom of the sea." (Weiner,
118)
Woodwell writes that,
If, for instance, a sufficient decline in the water
table occurs in the boreal and tundra peatlands,
subterranean fires could speed oxidation of the peat in
the vast, remote peatlands of Canada and Russia, spewing
forth smoke, CO2, and CH4 [methane], throughout the
northern hemisphere for years.... [I]f water tables
remain high, these peatlands might shift toward the
production of CH4 at high rates." (Woodwell, 406-407)
Woodwell and colleagues, writing in Climatic Change, contend
that while terrestrial ecosystems may have absorbed some of the
increased carbon generated by human activity during most of the
twentieth century, "The recent rate of increase in
temperature...leads to concern that we are entering a new phase in
climate, one in which the enhanced greenhouse effect is emerging
as the dominant influence on the temperature of the Earth."
(Woodwell, et al., 495)
Biotic feedbacks were discussed in Paris during early
December, 1998, at a conference on climate variability organized
by the World Meteorological Organization. At this conference,
Stephen Schneider warned that the permafrost of Siberia and Arctic
North America may already be melting and releasing methane into
the air because global warming is occurring as quickly in Siberia
as anywhere else on the planet.
Scientists who study biotic feedbacks sometimes remind
themselves that while models are linear, nature can behave in
random ways which confound linear analysis. The speed and
geographic variability of ozone depletion surprised many
scientists who had studied the anticipated effects of
chloroflourocarbons (CFCs) in theory. It is believed that the role
of biotic feedbacks in global warming could be similarly
surprising. In its Second Assessment (1995), the IPCC stated that
non-linear systems "when rapidly forced," are particularly subject
to unexpected behavior ("surprises"). Examples of such
"surprises," according to Schneider, may "include rapid decrease
in the thermohaline circulation in the North Atlantic Ocean,
excitation of certain dynamical modes of response of the climate
system, rapid decarbonization of terrestrial ecosystems (e.g.,
forest die-back in fires or insect outbreaks), [and] catastrophic
deglaciation of ice shelves in the West Antarctic." (Schneider,
Talk Abstract)
Atmospheric levels of carbon dioxide, which increased between
1.5 and 2.4 parts per million per year during the later years of
the twentieth century, builds like a bank account compounding
interest. Every five years, the total from which the range of
increase is calculated rises by about 8 per cent. Add to this the
fact that soils tend to release more carbon dioxide and methane
naturally under warmer conditions. In scientific language, "Any
changes that increase temperature or reduce pressure may liberate
CH4 from hydrate.... The major potential dangers include massive
emission from Arctic hydrate, especially in western Siberia."
(Nisbit, 193, 212) Higher temperatures accelerate the oxidation
rates of sulfur dioxide and nitrogen oxide to sulfuric and nitric
acids, the precursors of acid rain.
Warming temperatures may change the behavior of the Earth's
hydrological cycle. Warmer ocean water removes less carbon dioxide
from the atmosphere than cooler water, so warming of the oceans
may feed upon itself in coming years. Water vapor is also a potent
absorber of heat in the atmosphere. It has been estimated that a
doubling of carbon-dioxide in the atmosphere would increase its
water content by about 30 per cent, raising temperatures an
additional 1.4 degrees C. (Hansen, 957) Many models project a rise
in cloudiness, and attendant atmospheric moisture, in a warmer,
more humid world. George M. Woodwell raises the possibility of a
rapid surge in global warming beyond any possibility of human
control:
The possibility exists that the warming will proceed to
the point where biotic releases from the warming will
exceed in magnitude those controlled directly by human
activity. If so, the warming will be beyond control by
any steps now considered reasonable. We don't know how
far we are from that point because we do not know
sufficient detail about the circulation of carbon
dioxide among the pools of the carbon cycle. We are not
going to be able to resolve those questions definitely
soon. Meanwhile, the concentration of heat-trapping
gases in the atmosphere rises..." (Woodwell, 130)
Given Woodwell's expectations, the peoples of the Earth in the
year 2000 are rapidly approaching a point of no return with regard
to biotic feedbacks. Deforestation is accelerating around the
world due to growing populations and levels of material affluence.
Use of fossil fuels, which has increased at an annual rate of
roughly five per cent during most of this century, shows no signs
of stabilizing, much less falling by half in the next thirty
years. China, alone, projects burning enough fossil fuel (mainly
coal) by 2025 to account for about half the present consumption of
fossil fuels by everyone on Earth. (Legget, 27)
The "Methane Burp" Hypothesis
Researchers who have drilled into sediment layers near the
east coast of Florida found evidence that melting methane
clathrates thawed suddenly (over the course of a few thousand
years) about 55 million years ago, initiating a sudden episode of
global warming which ended with crocodiles and palm trees in the
Arctic. At the peak of this episode, greenhouse-gas levels in the
atmosphere were between two and six times as high as at present.
Lisa Sloan, a paleoclimatologist at the University of California
(Santa Cruz) and Gerald Dickens, a paleoceanographer at James Cook
University in Queensland, Australia (two of several scientists who
conducted the study) reported the findings at a meeting of the
American Geophysical Union late in 1999.
The study of methane clathrates has become more popular in
recent years, as evidence accumulates that their release,
especially from oceans, may be a major driving force in Earth's
climate cycles. James P. Kennett, et al. studied climate records
for the last 60,000 years off Santa Barbara, California, and parts
of Greenland, finding that "surface and bottom temperatures change
in concert." (Kennett, et al.; Blunier, 68) This findings support
assertions by E.G. Nisbet [Canadian Journal of Earth Sciences
27(1990):148] that massive release of oceanic methane from
clathrates have played a significant role in rapid warmings during
the past, even without added forcings by human industry.
Scientists have yet to reach any sort of consensus on causes
of the Earth's "methane burps." No one yet knows why a trillion
tons of methane may be released so suddenly from solid methane
hydrates around the world. This chemical reaction provoked a
sudden (in geologic time) global warming of 4 to 8 degrees C.
James Cook and Gerald Dickens theorize that "The methane probably
oxidized to form carbon dioxide. which eventually reached the
atmosphere, driving greenhouse warming." (Kerr, 1999, 1465)
The sediment cores drilled by Katz, et al. contained remnants
of small marine organisms called foraminifera, which preserve a
record in their shells of carbon levels in the ocean. The shells
tell a story of an extreme warming (possibly more than 10 degrees
F.) in the ocean over a short time, which killed more than half of
the foraminifera. The sediment core also contains evidence of an
underwater landslide which scientists believe took place as
melting methane clathrates "warmed dramatically, breaking apart
into water and methane gas, and bubbl[ed] ferociously out of the
sea floor." (Witze, 4)
This line of reasoning was supported by Richard Norris of the
Woods Hole Research Center, and Ursula Rohl of Germany's
University of Bremen, who wrote in Nature that the "methane burp"
occurred when an as-yet-unknown natural provocation pumped
greenhouse gases into the atmosphere, causing a sudden bout of
global warming. "Our results suggest that large natural
perturbations to the global carbon cycle have occurred in the
past...at rates that are similar to those induced today by human
activity," Norris and Rohl wrote. (p. 775) Miriam E. Katz, Dorothy
K. Pak, Gerald R. Dickens, and Kenneth G. Miller assert that this
surge in global temperatures may have played a crucial role in the
evolution of warm-blooded mammals as the Earth's dominant species
10 million years after a cataclysmic event (probably the impact,
on the Earth, of a very large asteroid) ended dominance by the
dinosaurs. Katz, et al. contend that "elevated temperatures
quickly opened high latitude migration routes for the widespread
dispersal of mammals." (Katz, 1531)
Stephen P. Hesselbo and colleagues reported in Nature (July
27, 2000) that roughly 200-140 million years ago large quantities
of methane were liberated from ocean floors, possibly because of
warming global temperatures. This "methane pulse" -- a "voluminous
and extremely rapid release of methane from gas hydrate contained
in marine continental-margin sediments" (Hesselbo, et al., 392) --
combined with oxygen in the oceans to form carbon dioxide,
accelerating the worldwide warming. Along the way, a large
proportion of oceanic animal life (perhaps 80 per cent) died for
lack of oxygen. "One of the important questions that is debated a
lot today is the stability of this methane hydrate reservoir, and
how easy it is to release the methane," said Stephen P. Hesselbo,
lead author of the paper. "The extinction and the association with
lack of oxygen has been fairly well established, but the
association with methane release is something that hadn't been
realized before," he said. (Prehistoric, 9)
New Forecasts of Temperature Rise
A report on global warming issued by the Pew Center on Global
Climate Change in 1999 asserts that temperatures will rise
somewhat more by the year 2100 than forecast in 1995 by the
Intergovernmental Panel on Climate Change's Second Assessment. The
study, titled The Science of Climate Change: Global and U.S.
Perspectives, was researched and written by Tom Wigley of the
National Center for Atmospheric Research for the Pew Center.
The Pew Center study projects global-mean temperature
increases ranging from 1.3 to 4.0 degrees C (2.3 to 7.2 degrees
F), as compared with the IPCC's projections of 0.8 to 3.5 degrees
C (1.4 to 6.3 degrees F). The Pew Center study also forecasts a
sea-level rise of 17 to 99 centimeters (7 to 39 inches) by the
year 2100, compared to a previous IPCC projection of 13 to 94
centimeters (5 to 37 inches).
The study suggests that the rate of warming in the United
States may be "noticeably faster than the global-mean rate." (Pew)
The study expects temperatures in the southeastern and
southwestern sections of the United States to warm slightly less
than the global mean. The northernmost states, from North Dakota
eastward to Maine, are expected to warm as much as twice the
global mean during winter months, according to Wigley's
projections. The study also forecasts that "The frequency of
high-precipitation events is likely to increase, bringing
increased chances of flooding." (Pew) The Pew Center study also
estimates that about one-third of the expected global warming
during the next century may be attributed to changes in the sun's
radiative output.
A report by the United Kingdom's Hadley Center for Climate
Change, incorporating improved representations of ocean currents
into models of the climate system, suggests that a "runaway"
greenhouse effect is possible by the end of the twenty-first
century. (Brown, 1998) The study contends that as lack of rainfall
turns large swaths of the Amazon, the Eastern United States,
Southern Europe and other areas into near-deserts, the ability of
plants and trees to absorb greater amounts of carbon dioxide will
be reduced, resulting in higher atmospheric concentrations and
rapid global temperature increases.
By 2050, this report projects that agricultural output in
central and southern Africa will be severely reduced, and North
America's agricultural heartland could see wheat and corn yields
fall by as much as 10 per cent. Extreme water shortages will
affect 170 million people, according to this study, which was
presented at the Buenos Aires climate conference in 1998. The
study forecasts that temperatures on land will rise an average of
6 degrees C. by the year 2100, subjecting about 100 million people
to annual hazards of coastal flooding from rising sea levels.
The Hadley Center study also projects that the Gulf Stream,
which is an important warming influence on much of Europe during
the winter, will be 20 per cent less strong in the future, but
that Europe still will warm considerably. This study, unlike some
others (See also: Chapter Four, Icemelt), does not foresee a
weakening of the Gulf Stream as portending colder winters for
Europe while most of the rest of the world warms. Instead, the
Hadley Center model forecasts that Western Europe, including
Scotland, will gain the ability to grow extra grain, and that
European storms will become more severe, especially during the
winter.
The Hadley Center study anticipates that the benefits for
plants of a carbon-enhanced atmosphere will be outweighed by lack
of rainfall in many important agricultural areas. The study also
asserts that many tropical grasslands will be transformed into
deserts, leading to widespread extinction of wildlife. Michael
Meacher, British environment minister, told The Guardian that
"These are sobering findings. Millions of people will have life
made miserable by climate change, with increased risk of hunger,
water shortages and extreme events like flooding. Combating
climate change is the greatest challenge of human history."
(Brown, 1998) The Hadley Center study also anticipates that much
of central and southern Africa will experience a reduced ability
to grow staple crops. While the agricultural heartland of the
United States will suffer production reductions (as previously
noted) because of drought and heat, the study projects that Canada
will experience a wheat-production increase of about 2.5 per cent.
Temperature readings during the late 1990s indicated that a
steep rise in temperatures seemed to be underway. On March 9, 2000
the National Oceanic and Atmospheric Administration (NOAA) said
the winter of 1999-2000 was the warmest such season in the United
States since the government began keeping records 105 years
earlier. This marked the third year in a row that record warmth
was recorded in the United States during the winter months. Since
1980, more than two-thirds of U.S. winters have been warmer than
average, NOAA said.
The average temperature in the United States between December,
1999 and February, 2000 was 38.4 degrees F., six-tenths of a
degree warmer than the record set the previous year. Scientists at
NOAA reported that every state in the continental United States
was warmer than its long-term average, with 21 states from
California to the Midwest ranked as much above average. As the
report was being released, a winter carnival was being canceled
near Wausau, Wisconsin, for lack of snow. Other casualties of the
warmth included North America's largest cross-county ski race and
an ice fishing derby in International Falls, Minn.
A week later, the Great Lakes, the world's largest body of
fresh water, were measured at their lowest level in recorded
history, because of scant snowfall during the winter, after
several years of declining water levels. Consequences have
included dry wells, landlocked docks, obstacle courses for
commercial shipping and pleasure boaters, and smelly drinking
water in some areas. Many docks have become useless, while
emergency dredging has been required for others. At the same time,
the temperature of lakes water has been climbing. During 2000,
Buffalo, New York reported that its harbor's water temperature at
the end of March had equaled the record warmest (39 degrees F.)
set in 1998.
During 1990, Congress commissioned a study of how global
warming would affect various regions by the year 2100. The report
("Climate Change Impacts on the United States;"
[www.nacc.usgcrp.gov] was issued in draft for public comment
during the summer of 2000. The report, which involved 5,000 people
in nine federal agencies, projected that average temperatures will
rise 5 to 10 degrees F. by the end of the century. This report
analyzed possible climate changes in eight regions of the United
States, "based on a pair of state-of-the-art climate models -- one
from the Canadian Climate Center and one from the United Kingdom
Hadley Centre for Climate Research and Prediction" (Kerr, June 23,
2000, 2113). The entire study was coordinated by Thomas Karl,
director of the NOAA National Climate Center in Asheville, North
Carolina, who said that the report illustrated a "range of our
uncertainties." To Karl, the report also indicated that "The past
isn't going to be a very good guide to future climate." (Kerr,
June 23, 2000, 2113)
The two climate models used in the study sometimes contrast
vividly. The Canadian model, for example, indicates frequent
severe drought in the United States' agricultural heartland, while
the Hadley model suggests plentiful rainfall in the same area.
Generally, however, the report supports a 5 to 10 degree F. rise
in temperatures during the century, in line with the models of the
Intergovernmental Panel on Climate Change. The report was largely
a compilation of data from existing sources because when Congress
mandated the study no funds were provided to pay for it.
According to this report, water levels in the Great Lakes are
expected to drop five feet by century's end. By the year 2000,
Lakes Eire, Michigan, Huron had dropped three feet in three years,
while Lakes Superior and Ontario were down about eighteen inches
during the same period. Todd Thompson of the Indiana Geological
Survey was quoted (Flesher, B-1) as saying that the levels of the
Great Lakes ebb and flow in thirty-year cycles. The lakes receded
during the drought years of the 1930s, then again in the 1960s.
Coming years will reveal whether present lake-level declines are
merely cyclical, or part of a new trend related to global warming.
In the meantime, the Toledo Beach Marina was spending $1 million
during the year 2000 to add three and a half feet of draft to its
docking facilities. (Flesher, B-3) Lake Eire averages only 70 feet
deep, and in some places even a few inches make a big difference
for cargo shipping. At about the same time, the National
Environmental Trust released a report describing global warming's
anticipated effects on the Great Lakes. Philip Clapp, president of
the organization, said dredging of shipping lanes caused by
declining lake levels could cost billions of dollars. (Fauber,
1-A)
The same federal report also anticipates some beneficial
effects, such as reduced costs for snow removal in many Midwestern
cities, and an opportunity for farmers to profit by planting more
than one crop a year. (Fialka, A-24) This 700-page report contains
regional summaries of global warming's possible effects, many of
which cite previous research cited in this volume.
As news of recent temperature rises arrived during the year
2000, the IPCC strengthened its previous statement affirming human
modification of the Earth's climate. In 1995 the IPCC had
concluded that "the balance of evidence suggests a discernible
human influence." In its 2000 assessment, this language reads:
"There has been a discernible human influence on global climate."
(Kerr, 2000, 590, emphasis added) The temperature records of the
last 1,000 years now leave very little doubt that the upward spike
in temperatures since 1980 has been influenced in large part by
human greenhouse-gas emissions. The IPCC's new forecasts for
global temperatures in 2100 changed little in 2000 from the 1990
or 1995 assessments -- 1 degree C. to 5 degrees C., according to
several sets of assumptions about how human numbers, societies,
economies, and technologies may change during that time.
The Possible Speed of Climate Change
During May of 1997, twenty-one nationally prominent ecologists
warned President Clinton that rapid climate change due to global
warming could ruin ecosystems on which human societies depend. The
signers, including Stephen H. Schneider and three colleagues from
Stanford University, urged Clinton to take a "prudent course" in
the then-upcoming global climate-change negotiations in Kyoto,
Japan. (Basu) The scientists warned that the warming would happen
so quickly that many plant and animal species will not be able to
adapt. The resulting breakdown of ecosystems could lead to
disturbances with major effects on human populations, the
scientists warned. These may include increasing numbers of fires,
floods, droughts, and storms, as well as erosion and outbreaks of
pests and pathogens. The letter says that if present (1997) levels
of greenhouse-gas emissions continue to rise, the climate will
change more quickly during the coming century than at any time
during the past 10,000 years.
"The signers include the leading international experts on many
particular dimensions of this problem," said Harold Mooney,
Stanford professor of biological sciences and the organizer of the
effort. "As you will read in the letter, they all have deep
concerns about the ecological consequences of rapid climatic
change." (Basu) Among the signers are Mooney, as well as Paul
Ehrlich of Stanford (who is considered to be an international
leader in ecological research), and Jane Lubchenco of Oregon State
University, a past president of the American Association for the
Advancement of Science. Seven of the signers are members of the
National Academy of Sciences and five are past presidents of the
Ecological Society of America.
In the United States, the scientists said,
[R]apid climate change could mean the widespread death
of trees, followed by wildfires and...replacement of
forests by grasslands. National parks and forests could
become inhospitable to the rare plants and animals that
are preserved there -- and where the parks are close to
developed or agricultural land, the species themselves
may disappear for lack of another safe haven. Worldwide,
fast-rising sea levels could inundate the marshes and
mangrove forests that protect coastlines from erosion
and serve as filters for pollutants and nurseries for
ocean fisheries. "The more rapid the rate [of change]
the more vulnerable to damage ecosystems will be," the
scientists told the president. "We are performing a
global experiment [with] little information to guide us.
(Basu)
The ecologists warned that in some United States
temperate-zone forests, rapid climate change could lead to
"widespread tree mortality, wildfires and replacement of the
forests by grasslands. Species that are long-lived, rare, or
endangered will be severely disadvantaged." (Basu) "It would be
difficult to imagine, for example," the scientists wrote, "how the
imperiled species of Everglades National Park, such as the Cape
Sable Sparrow and American Crocodile, could migrate north into the
urban and agricultural landscapes of coastal and central Florida
and successfully re-establish themselves." (Basu)
The scientists' letter seemed to have an impact at the White
House, judging from presidential rhetoric. In his 1999 State of
the Union speech, President Clinton called global warming "our
most fateful new challenge," as he recalled 1998 as the warmest
year ever recorded, with heat waves, floods and storms which "are
but a hint of what future generations may endure if we do not act
now." Clinton proposed creation of a new "clean air fund to help
communities reduce greenhouse and other pollutions." Clinton also
said he "want[ed] to work with members of Congress in both parties
to reward companies that take early, voluntary action to reduce
greenhouse gases. (President Clinton) Many of Clinton's proposals
were repeated a year later in his 2000 (and last) State of the
Union speech. (Clinton)
"We know from ice-core records and deep-sea sediment records
that the earth's climate is capable of changing much more quickly
than we had previously thought," said Jeff Severinghaus of the
University of California. (Webb, 1998c) "In some cases," said
Severinghaus, "The climate warmed abruptly in less than 10
years...up to possibly 10 degrees centigrade." (Webb, 1998c)
Severinghaus' findings were presented at the 1998 climate-change
conference in Buenos Aires.
Severinghaus continued:
It is possible that by increasing greenhouse gases, we
will induce such a change and that, instead of the
smooth warming that's being anticipated over the next 50
years, that we'll instead go along for a while with very
little warming and then all of a sudden in a matter of
three or five or ten years we'll have a very large
catastrophic warming. (Webb. 1998c)
At the National Ice Core Laboratory in Denver, thousands of
meter-long tubes are arrayed on shelves, holing ice cores from the
Arctic and Antarctic at minus 36 degrees C. These ice cores
contain records of the Earth's changing climate for the last
420,000 years. From studies of ice cores taken in Greenland,
scientists have assembled a climatic record which indicates that
during the last 8,000 years the earth's climate has been
relatively warm and stable. At the end of ice ages (the most
recent one, which ended about 12,000 years ago is an example),
temperatures tend to swing wildly in cycles of five to twenty
years. According to Gale E. Christianson, "Temperatures rose by an
astonishing 10 degrees C. within the lifespan of a Paleolithic
hunter, and some scientists now think that even that figure is too
low by half." (Christianson, 128)
Richard B. Alley, also writing in the Proceedings of the
National Academy of Sciences, reviews ice-core evidence from the
last 110,000 years which indicates that climate may vary very
little over long periods, then undergo changes as large as those
between glacial and interglacial conditions, sometimes within a
few years or decades. Alley points out that the development of
complex human civilization has taken place during a period without
such rapid changes. (Alley, 1331)
Thomas V. Lowell of the University of Cincinnati's Geology
Department, writing in the Proceedings of the National Academy of
Sciences, used changes in glacial mass to track of climatic
change. Using such measures, Lowell provides graphic evidence
(from glacial samples near coastal Alaska's Prince William Sound
and Western Greenland) that average temperatures declined slowly
from about 1450 A.D. to almost 1900 A.D. (the so-called "Little
Ice Age"). At that time, temperatures began to climb rapidly, at a
pace of about 0.8 degrees C. per century, four times the rate of
change during the previous 900 years. The temperature curve rises
at an increasingly steep angle toward the end of the century.
(Lowell, 1351)
Jonathan Overpeck, director of the paleoclimatology program
for the National Oceanic and Atmospheric Administration (NOAA),
said at the end of 1998 that "There is no period that we can
recognize in the last 1,200 years that was as warm on a global
basis [as the present]." Overpeck presented his findings at a
meeting of the American Geophysical Union in San Francisco. "That
makes what we're now seeing more unusual, and more difficult to
explain without turning to a 'greenhouse gas' mechanism," said
Overpeck. (Warrick) By the 1990s, according to the IPCC's Second
Assessment (1995) the temperature was rising at the most rapid
rate in at least 10,000 years.
Until the 1990s, many climate scientists believed that the
Earth had warmed dramatically during the period of time which
Europe called the Middle Ages, roughly 900 to 1400 A.D. New
research, based on tree rings, glaciers and other "proxy"
measurements of past climate around the world, indicates that this
warming was limited mainly to northern latitudes in Europe and
North America. Evidence of a rapid warm-up during the Middle Ages
has been used as "proof" by some global-warming skeptics that
natural variations may explain rapid temperature increases
worldwide during the last quarter of the twentieth century. "Our
study of the Medieval Warm Period supports the likelihood that no
known natural phenomenon can explain the record twentieth-century
warmth," Overpeck said. "Twentieth century global warming is a
reality and should be taken seriously." (Warrick)
Dean Edwin Abrahamson confirms Overpeck's analysis:
One must go back in time 5 to 15 million years to the
late Tertiary to find a time that was 3 or 4 degrees C.
warmer than now. During periods when there was no
permanent pack ice in the Arctic, climatic and
vegetational region and boundaries were displaced as
much as 1,000 to 2,000 kilometers north of their present
position (a displacement which we may replicate during
the next 100 years). (Abrahamson, 15)
During the period Abrahamson describes, intense aridity was
the norm from present-day North and South Dakota to Missouri and
Alabama, as well as throughout Central and Southern Africa. These
changes may be similar to those which will be experienced by
generations to come. As Abrahamson comments,
Beyond the year 2050...we could be committed to a far
larger warming -- probably on the order of 6 to 10
degrees C. The climatic conditions that might be
associated with such a warming are, with few exceptions,
pure mystery.... Today's climate models have little, if
any, validity for such extreme warming.... There can be
no planned adaptation under these conditions."
(Abrahamson, 21)
By early in the year 2000, scientists working for National
Atmospheric and Oceanic Administration (NOAA) released
compilations of global temperatures for the last half of the
twentieth century which reveal a speed of warming that most
climatologists had not expected until late in the twenty-first
century. The rate of warming (1 degree F. over the entire century)
increased to a rate of 4 degrees F. during the century's last
quarter, according to calculations of Tom Karl and associates,
published in the March 1, 2000 edition of Geophysical Research
Letters. This is roughly the rate of increase which several
climate models had forecast for the second half of the
twenty-first century. "The next few years could be very
interesting," Karl told the Los Angeles Times. "It could be the
beginning of a new increase in temperatures." (Analysis, 12) Tom
Wigley, a senior scientist at the National Center for Atmospheric
Research in Boulder, Colorado, said that warming was strengthened
frequent El Niño events, which he said are not human-induced.
"Those months were unusual," he said, "but they weren't unusual
due to human influences." (Analysis, 12)
Karl and colleagues begin a statistical analysis of recent
global temperature trends with the observation that between May of
1997 and September of 1998, sixteen months in a row, global
temperatures set observational (e.g. century-scale) monthly
records. Their analysis of a century-plus of records (roughly 1880
to 2000) indicates that the rate of warming tends to surge upward,
then relent a little, and then surge anew. "The increase in global
mean temperatures is by no means constant." (Karl, et al., 719)
Karl and colleagues conclude that "The warming rate over the past
few decades [since the mid-1970s] is already comparable to that
projected during the twenty-first century based on IPCC
business-as-usual scenarios of anthropogenic climate change."
(Karl, et al., 719)
We interpret the results to indicate that the mean rate
of warming since 1976 is clearly greater than the mean
rate of warming averaged over the late nineteenth and
twentieth centuries. It is less certain whether the rate
of temperature change has been constant since 1976 or
whether the recent string of record-breaking
temperatures represents yet another increase in the rate
of temperature change.... Moreover, these results imply
that if the climate continues to warm at present rates
of change, more events like the 1997 and 1998 record
warmth can be expected. (Karl, et al., 720,721)
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