BY
VIRGINIA MORELL
"One. Two. Three. Lift! " barks Cathy Whitlock, a fossil pollen expert and paleoclimatologist at the University of Oregon. She and the three of us – two of her students and I – tighten our grips on the cold metal tube of a lake-bed drilling rig and heave, "Again" she commands. Slowly, inch by inch and groan by groan, the coring barrel that
Whitlock and her students had manhandled into the
marshy shore of Little Lake, a blue jewel of water in Oregon's central
Coast Range, emerges from the mud.
"Once more,"
orders Whitlock. We bend to the task and at last free the barrel from the muck.
Whitlock
has extracted a couple hundred similar cores from the deep sediments of this
lake, but she beams like a kid getting her first bike as she slides her latest
sample of old mud, five centimeters thick and a meter long, out of the barrel.
"Oh, that's a
lovely core," she says. To me it looks
about as interesting as a Tootsie Roll. But to Whitlock's trained eye
even the chocolate hue of the mud holds a story.
"That rich brown color tells you it's full of organic matter – especially
pollen," she says, slicing the core in half lengthwise with her
pocketknife. "You can't see the pollen without a microscope, but it's
there."
And in that pollen He clues to one of the greatest puzzles facing
researchers like Whit-lock: What has caused – and will cause again – the sudden
climate changes that our Earth periodically undergoes? Not the 100,000-year
fluctuations between a glaciated and a warmer Earth that have occurred for the
past million years or so, but the more rapid shifts that scientists have
recently identified when the Earth switched suddenly from frozen ice age to
picnic-warm and back again. How often and how quickly have such dramatic
changes happened? Perhaps most important, what do these past abrupt reversals
tell us about the direction of Earth's climate today and in the future?
To answer such questions, scientists are busy unearthing signs of ancient climate in a surprising array of sources: glacial ice and moraines, stalagmites from caverns, tree rings and corals, dust and sand dunes, and the microscopic shells of organisms buried in deep-ocean sediments. Others, hoping to piece together the climate of the more recent past, turn to human records, using archaeological inscriptions, vintner and gardening diaries, and ship captains' logs. "We need both human and natural records," explains Ohio State University glaciologist Lonnie Thompson, who specializes in retrieving ice cores from the dwindling glaciers on tropical mountains. "We want to understand how the climate worked before and after people appeared. That's the only way we'll figure out what impact people have on climate, how much we're responsible for the way it's changing now."
Just how swiftly climate changes can occur is clear from Whitlock's study of her Little Lake cores. Those like the ones we drilled are stored at her university lab. Each meter of mud contains about 2,300 years of pollen grains from trees, grasses, and flowering plants. To find the pollen in the mud, Whitlock takes smudges from every core at set intervals, then puts the mud in a chemical bath that eats away everything but the thousands of previously invisible pollen grains. She places a droplet of the pollen residue on a slide and then "reads" about 300 grains, identifying the species of each one—a process that allows her to trace how the vegetation in the Coast Range changed during the climatic variations of the past.
"You hit bedrock at the lake at about 18.25 meters," Whitlock
says, placing a sample slide beneath her microscope. "The pollen at that
level dates to about 42,000 years ago."
Very few mountain lakes have such a continuous record, she adds, since
they are often formed when glaciers retreat. But a landslide that blocked a
small stream before the last ice age made Little Lake. The pollen in its muddy
sediments "tells us what the coastal Oregon environment was like before
and at the height of that ice age and how it changed as the climate warmed
about 13,000 years ago," says Whitlock.
"It was a big change " she continues. "Here's what the
forest looked like 21,000 years ago at the height of the last ice age. And, oh
man, was it a different world."
I take
her place at the scope, and she guides me from grain to grain. It's a
surprisingly easy tour, since there are really only two types of pollen on this
slide: the large, kidney-shaped grains of Engelmann spuce tree, and the smaller
grains of mountain hemlock, which look like ovals with two small ears.
"Now think about this," Whitlock says. "Engelmann spruce
doesn't grow in the Coast Range today. Instead, you find Douglas fir; that's
the dominant conifer. But there isn't any Doug fir pollen on that slide. Doug
fir doesn't show up until close to the end of the last ice age, and then –
suddenly boom! – it's there and the spruce forest is gone. And that happens in
200 to 500 years: A whole forest vanishes and another one takes its
place."
Whitlock
pauses. "So we want to know how that happened and why. What caused the
forest and the climate to change so dramatically and abruptly? And what happens
if the climate shifts in the other direction, toward an ice age again or toward
even warmer conditions? How are we – people – going to respond?"
Ice cores from Greenland1 first obtained and analyzed in the
1960s, gave scientists early clues to rapid climate change. Because the ice
there has accumulated undisturbed for over 100,000 years, it holds some of the
best records for such things as past temperatures, amount of precipitation, and
atmospheric conditions. The Greenland cores, combined with even older ice cores
from Antarctica's Vostok Station, showed the expected long periods of gradually
increasing cold followed by shorter warm periods. But the Greenland ice also
revealed that within the long, cold stretches there were short periods of
warming and cooling. These shorter changes came in bursts, causing the climate
to jump from cold to hot to cold again, sometimes in mere decades. The past
climate had behaved like "an impish three-year-old" flicking a light
switch, as Richard B. Alley, one of the scientists on the early 1990s Greenland
drilling project, wrote in his book The Two-Mile Time Machine. And that
raised a new question, one that remains unsolved: What caused – and may cause
again – all those flickerings?
"Some
of the ice we have here is already gone from the mountains."
Sudden climate flips occurred throughout the last ice age – from about
70,000 to 11,500 years ago. At the height of this glaciation, vast ice sheets
blanketed much of North America, Europe, parts
of Russia, and Antarctica. Periodically the ice melted, then advanced again,
until the final melting – an event that marks the beginning of the modern warm
(and more climatically stable) epoch known as the Holocene.
But getting to the Holocene was a start-stop affair. It began with an
abrupt warming – probably the cause of Whitlock's suddenly altered forest. Then
there was another switch, back to cold times, and yet another warming at 11,500
years. In that jump, Greenland's surface temperature increased by 15°F in a
single decade. England warmed suddenly too, becoming a haven for certain
beetles that can only live in balmier climes. And on both sides of the North
Atlantic, the sudden warmth melted terrestrial glaciers thousands of years old
in just a few hundred years.
"All those events happened essentially overnight," says Oregon
State University's Peter Clark, who is tracking climate changes in Ireland's
glacial geology. "We'd like to understand why the sudden retreats happened
– what triggered them and if something like that could happen today," says
Clark. "But to get those answers, we first need to know as precisely as
possible when the ice melted."
In an effort to answer that question, Clark and fellow geologist
Marshall McCabe from Ireland's University of Ulster don their rain gear and
knee-high rubber boots, grab shovels and plastic bags, and make their way to a
muddy cliff in a farmer's pasture above Ireland's Atlantic coast. Along the
way, McCabe points his shovel at a small palm tree planted outside the farmer's
house. "You know, we're at the same latitude here as southern Alaska. And
that palm shows that our friend, the North Atlantic Ocean conveyor, is
working," he says, referring to the ocean currents that pull warm water
from the tropics to the Irish coast, keeping its temperature mild. "Otherwise,
the palm would be dead." From their studies of coral reefs and marine
sediments, paleoclima-tologists have shown just how important this ocean
circulation system – the North Atlantic conveyor – is to the climate of the
entire planet. During the ice ages it weakened and even occasionally stopped,
triggering a cascade of events that ultimately led to warmer temperatures in
the Southern Hemisphere and colder temperatures in the north.
"That conveyor sits right offshore," McCabe adds, this time
aiming his shovel toward the sea. "So Ireland is particularly sensitive to
any big changes in what it's doing; they're felt here immediately."
In the last ice age, with the conveyor slowed down, Ireland was much
more like Alaska. Glaciers covered its mountains and pushed across the land and
into the sea. But whenever the climate switch was flipped and the deep freeze
momentarily ended, Ireland's glaciers began to retreat – rapidly. Meltwater
coursed over the land, cutting deep river-size channels and pouring a slurry of
mud into the sea. "These were high-energy events," says McCabe.
As the mud settled, tiny organisms called zooplankton were buried in the
sediments. Today, with relative sea level far lower than in the past because
the land is no longer weighted with ice, those muddy deposits are up to several
hundred feet above the ocean, and a geologist who knows where to look can find
in them the fossils of the shell-covered zooplankton called forami-nifera
(forams for short). Forams are an integral part of paleoclimatological
research because their calcareous shells can be dated. And that's why McCabe
and Clark have come to this pasture: to dig about 50 pounds of foram-filled mud
for dating. With precise dates for the rapid retreat of the ice in hand, the
two will be able to link Ireland's glacial history with that of North America
and Scandinavia.
By dating forams from mud on the Irish Sea coast, McCabe and Clark found
evidence for a rapid 35-foot rise in global sea level about 19,000 years ago.
"That was a Northern Hemisphere melting, a pulling back of the entire ice
margin," says Clark. "It wasn't just a little local event. We figure
that the equivalent of two ice sheets the size of Greenland's today must have
melted within a few hundred years."
What could have triggered such a large-scale event? McCabe and Clark
argue that it could have been the weight of the ice itself. As the ice sheets
grew, their increasing weight pushed down on the underlying land. Where the
glaciers sank far enough to reach sea level, the ice began to float, breaking
up into icebergs. "That would have added more fresh water to the ocean,
changing its salinity and deepwater currents," says Clark.
"What happens if the climate shifts in the other direction, toward an ice age again or toward even warmer conditions?"
More fresh water in the North Atlantic would have slowed the conveyor and decreased the amount of warm water pulled from the tropics, changing ocean circulation dynamics and temperatures as far south as Antarctica, Computer models that simulate the Earth's climate show that what happens in the North Atlantic very quickly affects the rest of the planet, "As the water gets cooler here, the ocean gets warmer in the Southern Hemisphere," says Clark. "It's a seesaw effect. That warming could have caused an ice sheet in Antarctica to melt."
And that additional cold fresh water from Antarctica would, in turn,
have caused the tropical warm currents to flow back toward the north, starting
up the North Atlantic conveyor. Once again the Northern Hemisphere ice sheets
would have begun to melt.
"You essentially would have ended up with ice sheets melting at
both ends of the Earth at slightly different times," says Clark.
"Today we have two big ice sheets: Greenland and Antarctica. And the
climate is changing because of the high amount of carbon dioxide we've put in
the atmosphere. How will it affect those ice sheets? If they melt, how will
that affect us?"
Not everyone is convinced that the North Atlantic Ocean conveyor is the
only switch for the Earth's sudden climate changes. "Maybe that's true for
the higher latitudes, but it's not for the tropics," says Lonnie Thompson,
whom many credit with retrieving the best paleoclimate records from the torrid
zone – the latitudes between the Tropic of Cancer and Tropic of Capricorn.
Indeed, until research by Thompson and others showed something different, most
scientists regarded the tropics as a place where little climate change had ever
taken place – not even during the ice ages.
"There's a bias in our view of climate change that sees events in
the Northern Hemisphere as the most important," Thompson explains as we
gear up to enter his ice-core storage room in the basement of Scott Hall on the
Ohio State University campus. "But it's a data-collecting bias: That's
where we have the most records from." Behind a nondescript, beige door
marked 089-B He 6,000 meters of ice cores that give Thompson the data to
challenge that interpretation. The cores come from glaciers crowning summits
in the Andes, the Himalaya, and Alaska, and from Mount
Kilimanjaro. I'm glad for the down-filled parka, gloves, and snow boots when
the first blast of arctic-cold air from Thompson's ice room hits my face.
The cores are kept in silvery, cardboard cylinders and lie in stacks on
frost-covered shelves. A temperature gauge reads minus 30°C (minus 22°F), and I
shiver in spite of the down. But the numbing cold is necessary to preserve what
has or will soon disappear: the climatic history of the tropics. "The
sources for these records – the glaciers on the highest mountains – are melting
because of the increasing greenhouse gases in the atmosphere," Thompson
says. "Some of the ice we've collected and have here is already gone from
the mountains."
Greenhouse gases, such as carbon dioxide and methane, are released by a
variety of human activities. Over the past 150 years, the amount of these gases
has increased enormously in the Earth's atmosphere, trapping more heat and causing
temperatures to rise – and glaciers worldwide to melt. And as the ice melts
away so do the records that Thompson and other scientists deem vital to a
better understanding of Earth's climate.
Thompson pulls down one of the cardboard containers and carries it to a
table, handling it as carefully as if it were a tome from a library's rare-book
room. "We forget that the Earth is a globe and that 50 percent of the
surface of the planet is in the tropics. That's a major heat source, and I
think it has a much bigger role in driving climate change than we've
realized."
Thompson opens the cylinder and pulls out a meter-long ice core that's
wrapped in plastic.
"This is a core we drilled on Sajama mountain in Bolivia," he
says. It is dense and white, yet as Thompson points out, it also has slight
variations, the faintest ringlike bands, indicating the annual accumulations of
snowfall. By counting the bands, he can estimate the age of a core. And this
one, Sajama's final core, the last one Thompson pulled from the mountain's ice
before hitting rock, dates to 25,000 years ago, making it the oldest core
Thompson has found in his high-altitude work in the tropics.
"This core shows that there actually were climate shifts in the
tropics of the same magnitude that Greenland experienced during the ice
ages," he says. Near the Equator, Earth's climate had switched rapidly
back and forth from cold to warm just as it had in Greenland. And that makes Thompson think that the North Atlantic isn't the only
driving mechanism for these abrupt changes. There may be a second driver in the
Pacific Ocean.
Other anomalies in this high-mountain ice suggest that the past 10,000
years, which is often characterized as a stable climate period, was in fact
also given to climate swings. Thompson opens another cylinder and produces a
core from the ancient snows of Mount Kilimanjaro. Like the Sajama core, it is
dense and white – except for a two-centimeter-thick band, which is black.
"That's dust," says Thompson. "It dates to 4,200 years
ago when there was a terrible 200-year drought in North and East Africa. The
upper atmosphere must have been full of sand, dirt, and dust, all of which
mixed with the snow as it fell on Kilimanjaro."
Hieroglyphic inscriptions from the period describe how the annual Nile
flood failed for about 50 years. The Egyptians suffered in a drought, and
people died from famine. At about this time Egypt's Old Kingdom ended, and a
period of social and political upheaval began. Thompson believes that the dry
spell contributed to the collapse of the Old Kingdom. Some archaeologists also
think that the drought extended north into the eastern Mediterranean and
contributed to the decline of the Akkadian empire in Mesopotamia.
"It shows what climate change can do," says Thompson.
"That was an abrupt, but natural, occurrence, when there were only 250
million people on the planet. Now there are 6.3 billion of us, and we're
changing the climate."
Every paleoclimatologist I'd spoken to had said much the same thing.
Some were certain that we had already succeeded in flipping one of Earth's
climate switches and had triggered a new abrupt change. Others were more
cautious, saying only that given the steady emission of carbon dioxide and
other gases, the climate was bound to be different. All were alarmed by our
collective refusal to slow down our use of fossil fuels. One wryly summed up
our behavior as a "remarkable experiment" a quip I pass on to
Thompson as we leave his ice-core room.
"He forgot one word," Thompson says, ready as ever to add to
the record what is missing. "It's a remarkable, uncontrolled experiment."
The climate of the past is our anchor for looking at the future,"
Cathy Whitlock told me when explaining the importance of her fossil pollen
studies. "If we can understand the past linkages between the ocean,
atmosphere, and biosphere, and determine which parts were the really big
players in past sudden change, then maybe we can better deal with future
surprises."
That's the dream, the goal of paleoclimatology. And although all the
connections among the disparate parts of the Earth's climate have yet to be
fully untangled, computer modelers have made big steps in predicting what the
weather will be in the near future. One of the best models runs on a
supercomputer at the United Kingdom's Hadley Centre for Climate Prediction and
Research. Simon Tett, a Hadley Centre climate specialist sets up his laptop at
my London hotel and calls up a map of the world. Superimposed on it are swirls
and colors representing ocean and atmospheric currents – essentially a model of
Earth's climate. Plug in different factors, like a big spike in CO2
and methane levels, and you can sit back and watch the weather change.
"Right. So here is what the world's climate could be in 2080," Tett
says. A red hue settles over most of North America and Europe, indicating
higher temperatures, while the Arctic turns from white to blue as the summer
ice cap melts.
"People don't realize how dramatic these changes will be,"
says Tett. "But we expect to see a two- to five-degree [Celsius] warming
over the next hundred years. It will be higher over land, but the oceans will
also warm."
The warming doesn't mean that every place will suddenly become like
Miami. Some areas, like the interior of the United States, are likely to grow
hotter and drier. Others, like China, Southeast Asia, and the western U.S.,
may get more precipitation but less snowfall, jeopardizing the drinking water
of people in cities like Los Angeles. Sea levels around the world are projected
to rise as the last of the glaciers melt and the
warmer oceans expand. Intense hurricanes may occur more frequently, and storm
surges coupled with the higher sea level could severely damage cities like New
York. Heat waves, like the one Europe experienced in the summer of last year,
may become the summer norm.
Can we do anything to stop the change?
"No," says Tett. "We'd need to get to zero emissions to
stabilize the CO2 that's already in the atmosphere. And that's not
the path we, as societies, have chosen. Even if we were to stop CO2 emissions
now, we are committed to warming.
"We'll have a better idea of the actual changes in 30 years. But it's going to be a very different world."
"Ultimately there will be an effect on the ocean's thermohaline
circulation – the conveyor belt," he continues. "Climate models show
that circulation will slow, but it's possible that it could collapse. One
result of that would be cooler winter temperatures in Europe."
Tett turns off his laptop. "We'll have a better idea of the actual
changes in 30 years, because some of us will have lived through them. But it's
going to be a very different world."
Outside, the light of a cold winter sun spills over the London streets.
It's a week before Christmas and shoppers bustle by. There's the whoosh and
honking of traffic, and the smell of diesel and gasoline fumes rising in the
air. I hail a cab and set off for the airport.
"The weather's going to change," the cabbie tells me.
"It's fine now, but that's the end of it; it's turning rough tomorrow."
I nod in agreement. He is more right than he knows.