Like a modern-day druid English
accent and all Ray Bradley asks trees to impart to him their knowledge
of Earth's ancient mysteries. Since Bradley is a climatologist
as well as a professor and head of geosciences at UMass the mystery
most intriguing him these days is the rapid heating up of Earth's atmosphere
known as global warming.
With a group of eight colleagues in UMass's
Climate System Research Center, which he founded in 1998, Bradley looks
not only to trees, but to corals and lake sediments, to reconstruct the
climatic history of the planet. Over the past several years his group
has brought forward startling evidence that global warming is not only
real and the evident result of human activity, but that its progress has
been swift and substantial.
In two widely cited articles, one of them
published in the journal Nature in 1998, Bradley and two other scientists
charted a timeline of yearly average temperatures on Earth over the last
1,000 years. The timeline is graphic evidence of what he calls "a
completely anomalous warming trend culminating in the 1990s." In
fact, according to the team's calculations, 1998 was the hottest year
of the millennium just past, and will likely be followed by even hotter
Bradley, whose silver hair and beard normally
frame a placid expression, is visibly piqued by those who minimize the
significance of global warming. His own sense of that significance drives
his current efforts to demonstrate how Earth's climate has changed and
why. Working from international data bases as well as his own research,
Bradley and his confederates have compiled information derived from "proxies,"
or natural archives, in Canada, Russia, Bolivia, and numerous other locations
around the globe. The proxy data represent a worldwide, collaborative
effort by climatologists who have x-rayed, chemically analyzed, and visually
scrutinized thousands of cylindrical cores taken from trees, corals, and
frozen lake sediments.
The width and the density of tree rings,
for example, vary with the interaction between species, geographic location,
and the ephemeral conditions that make up climate: temperature, light,
and the availability of water. In New England, for instance, a wide, dense
ring on an Eastern Hemlock or a White Oak both good proxy trees
suggests a warm, moist year. In the deserts of Arizona, where heat
represents stress, a similar ring on a Foxtail Pine suggests a cooler
year. And trees are affected by more than just climate, notes Bradley.
They can, for example, be stifled by pest infestation or the lack of vital
nutrients. So while each tree carries what Bradley calls a "climatic
signal," it is "the job of the analyst to determine what part
is due to climate and what part is due to other factors."
Climatic signals are also found in the rings
of corals and the laminations of lake sediments. Yearly layers of coral,
which grow atop the limestone exudate of their defunct predecessors, incorporate
different amounts of strontium, calcium, and magnesium depending on the
water temperature in which they grow. By measuring the ratios of these
minerals, climatologists can extrapolate the temperatures that correspond
to each year.
Ice cores, the annual accretions of which
can remain clearly delineated for thousands of years, convey similar messages
about temperature: for instance, in the relative amounts of two forms
of oxygen isotope in each layer. "When water evaporates, more oxygen-16
than oxygen-18 is lost," explains one of Bradley's colleagues, Mark
Abbott. "If you find a lot of oxygen-18 in a layer, the climate was
colder at that time."
"Varved," or layered, lake sediments
also convey climate history. In the Canadian high arctic, where Bradley
and his team do fieldwork, there are lakes that remain frozen for most
of the year. Each summer thaw, depending on the heat of the season, brings
a rush or a trickle of melted snow. The speed and force with which that
water moves determines the size of the particles it will wash into the
lake, and which will settle into the lake floor. Therefore, layers of
sediment with large, coarse chunks represent hot summers. Fine grains
represent cool ones.
In essence, climatologists must decipher
a manuscript which has been written in a foreign language, and of which
only the last chapter has been translated. The key to unlocking the climatic
signals of any natural archive lies in that last chapter: in the fact
that humans have been keeping explicit records of climate for the past
hundred and fifty years. Using these records, climatologists can correlate
the characteristics of modern tree, coral, and sediment with the known
climates in which they were generated, and draw conclusions about how
climate shaped them. Then, by a process called calibration, the findings
can be applied to proxy data from years when no human record of climate
The significance of Bradley's work, and
the reason it received such widespread attention, is that the research
which he published with colleagues Michael Mann, then at UMass, and Malcolm
Hughes of the University of Arizona, was the first to apply such a comprehensive,
proxy-based approach to the question of global warming.
What of those who say,
in effect, "Oh, well temperatures have always gone up and
down"? Earth's current heat wave may be unprecedented by recent standards,
but it has certainly been much hotter without the world coming to an end.
Rob DeConto, assistant professor of geosciences
and an expert on the Cretaceous period, might seem to be lending aid and
comfort to the lackadaisical when he observes that "when dinosaurs
ruled the Earth, this was a mostly ice-free planet." Antarctica,
DeConto observes, was completely forested a hundred million years ago.
"Yes, the Earth has been warmer than
it is now," says DeConto. "But the point is that it was completely
DeConto's efforts to forecast how unrecognizable
the Earth may become in the future involve the use of sophisticated computer
climate models. Of particular interest to him is how living organisms
affect climate. Until recently, he says, while climate models incorporated
such environmental factors as the amount of heat emitted by the Sun and
the chemical composition of the atmosphere, few accounted for biological
impacts. When such models were tested against the past as revealed by
proxy data, they proved inadequate.
For instance, according to the old, biology-free
models, the interiors of Earth's continents should have been very cold
during the Cretaceous. Yet paleontologists working far inland have found
fossil remains of crocodilians - crocodile ancestors - who could never
have survived such temperatures.
DeConto says that seeing Earth's surface
and biology as interactive improves such models. For example, the forests
that shrouded Earth's poles during the Cretaceous would not have reflected
the Sun's rays as the polar ice caps do today: foliage traps heat in much
the same way as a dark T-shirt on a hot summer day. Likewise, it may have
been the vegetation of the ancient continental interiors that kept them
hospitable to crocodile-types.
Among Earth's living constituents, human
beings have been among the most interactive. According to Bradley and
his colleagues, the planet's heating trend can be traced back to the industrial
revolution, when human use of fossil fuels became commonplace. Burning
coal, oil, or natural gas releases carbon dioxide and other compounds
into the air; carbon dioxide is one of several atmospheric gases which
literally blanket the Earth and prevent heat from dissipating. CO2, along
with methane, and nitrous oxide, has become increasingly abundant in the
atmosphere in the last two centuries, and Bradley, DeConto, Abbott, and
virtually all scientists in this field agree that increasing levels of
CO2, combined with deforestation, are responsible for the recent monumental
changes in temperature.
A transparency that Bradley uses in his
talks on global warming makes graphic the differences between modern and
pre-industrial amounts of atmospheric CO2. Prior to 1850, he says, "carbon
dioxide levels were at about 280 parts per million. Today they are about
360 parts per million." And if CO2 in the atmosphere continues to
increase at its current rate, it will double its pre-industrial level
in the coming century.
One of the textbooks
used by Karen Searcy '84G, curator of the UMass herbarium, to teach Introductory
Ecology supports DeConto's notion of an unrecognizable future world. If
carbon dioxide levels were to double, it says, "beech trees presently
distributed throughout all of the eastern United States and southeastern
Canada would die back in all areas except northern Maine, northern New
Brunswick, and Quebec." All of the plants and animals that populate
Massachusetts, Searcy adds, could change to suit the climate. Species
that need colder temperatures could migrate north, while others that have
never grown here may flourish.
"One of the species that is predicted
to migrate north is the sugar maple," says Searcy, "which would
take the intensity out of our autumn foliage. You would see more russets
and browns, less orange and red." And muted autumn colors, of course,
would be among the more benign consequences of global warming. DeConto
points to the melting of glaciers that could raise sea levels and devastate
flat and low-lying coastal regions. (Boston's Back Bay, not to mention
much of Bangladesh, would be at risk.) Fresh water from glaciers could
also disrupt the churning flow of sea water that normally blunts extreme
temperature changes. Perhaps most importantly, the planet is "getting
closer and closer" to the limits of its food-producing capacity,
DeConto says. "If the climate changes enough to impact an area that
supports corn - " he shrugs, leaving his sentence uncompleted.
Such predictions for the future of the planetary
environment are, of course, speculative. But in this case, they also come
from a working understanding of climate, an understanding which continues
to be deepened by studies at UMass's Climate System Research Center.
Housed in an octagonal, multi-windowed room
on the fourth floor of Hasbrouk the former home of the physics
library the center supports a multidirectional approach to climate.
It is supported in turn by funding from the National Science Foundation,
the National Oceanographic and Atmospheric Administration, and the Department
Meteorological sensors with automatic data
loggers, and tools for geologic coring and analysis, are among the resources
gathered there for use by the university's climatologists.
These resources are used to study not only
global warming, but how climate varies naturally. Abbott, especially,
is interested in "differentiating between a normal climate with natural
rhythms and what's man-made." Clearly, human use of fossil fuel is
not the only factor driving changes in climate. Consider the 1991 eruption
of Mount Pinatubo in the Philippines. Dust and ash particles from the
eruption shrouded literally shaded the islands for months,
producing an exceptionally cold summer.
More generally, says Abbott, the energy
emitted by the Sun has been increasing since the early 19th century, and
the variable distance and angle between the Sun and the wobbly Earth gradually
changes over time. His research shows that water levels in Lake Titicaca,
his study site on the border between Bolivia and Peru, have risen and
fallen with the wobbling of the Earth on its axis. "The intertropical
convergence zone - the low pressure belt that brings rain - follows the
hottest spot on the continent," he says. When changes in the angle
of Earth to Sun shift the warm zone to another part of the globe, the
rains go with it. Notably, Abbott has found elevations of water level
in Venezuelan lakes that precisely coincide with the drops in Bolivia
Regardless of the impact of natural factors
on Earth's climate, the fact remains that humans are at least partially
to blame for potentially devastating increases in temperature. What can
we do? Sign the Kyoto protocol, says Bradley. Nations that sign this international
agreement promise to reduce greenhouse gas emissions to 1990 levels by
2010. "It's not perfect," he says, "but it's a step in
the right direction."
Bradley is well-acquainted with the argument
that the protocol is a prohibitively costly approach to a threat that's
at best remote. Not surprisingly, he disagrees. "We've made huge
investments in the past to protect ourselves from fairly unlikely events
like the Russian launch," he says. "Why not guard against an
event that is slowly but undeniably creeping up on us?
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