ALDA Hello and welcome to Scientific American Frontiers. I'm Alan
Alda. This week we're going to talk about the weather. You may have
seen last summer's blockbuster movie about a catastrophic change
in the global climate -- New York gets flash-frozen, it snows in
Delhi and enormous hailstones batter Tokyo. It all made for great
entertainment, but was the science any good? When the movie came
out, scientists were unanimous in saying it couldn't happen -- or
at least that it couldn't happen like that. A few, though, acknowledged
that there was a grain of truth behind the hype, and behind the
hype is where we're going with this program. We're going to meet
scientists who are figuring out how the enormous streams in the
world's oceans circulate, and how changes in them might cool us
down. We're going to link those changes to a warming climate. And
we're going to see what the reality of a changing climate can look
like, in Alaska. That's coming up on tonight's episode, Hot Planet-Cold
SEA'S GREATEST RIVER
ALDA (NARRATION) We're at the Woods Hole Oceanographic Institution
on Cape Cod, Massachusetts, and we're with Ruth Curry. She studies
the oceans, but she doesn't have to leave her chair to do it.
CURRY Here in this office I have the world's oceans at my fingertips,
basically. I have been collating millions and millions of oceanographic
profiles from people who have been out observing the ocean for the
last hundred years, or so. I have them all here stored in my computer,
and I've got analysis software that I use, and I explore the oceans.
ALDA (NARRATION) For the last 10 years, Ruth Curry has been looking
closely at the saltiness of the oceans. As we'll see, ocean saltiness
helps drive enormous currents which move water around the globe
especially the one that Jules Verne called "the sea's greatest
river," the Gulf Stream. The Gulf Stream is like a giant heat pump,
bringing warm water up north.
CURRY The warm surface waters come up from the tropics, through
the Caribbean, and Gulf of Mexico and around Florida.
ALDA (NARRATION) Satellite images, based on sea temperature, show
that the Gulf Stream is not a river. It's a broad mass of swirling
CURRY When they reach the coast of North Carolina they start to
move offshore. And the Gulf Stream flows northward and eastward.
A portion of that then branches southward, like this. But another
portion of it branches northward, and it brings those warm waters
eastward across to Europe, up by Great Britain. It flows into the
Nordic Seas on the coast of Norway.
ALDA (NARRATION) Here in the Norwegian and Greenland Seas, a key
process takes place. The warm surface waters are cooled by Arctic
winds. That makes them sink into the deep ocean basin here, but
in the process, cold water is pushed over an ocean ridge that runs
between Scotland and Greenland. It's an enormous pump, which drives
the Gulf Stream. The cooled water overflows down the ridge, back
into the Atlantic.
CURRY They flow along the coast of Greenland. They follow the western
ALDA (NARRATION) The cold southward flows are now deep they're
underneath the warm Gulf Stream that's flowing north on the surface.
CURRY They cross underneath the Gulf Stream and then they continue
their flow pattern towards the equator.
ALDA (NARRATION) In the 1980s a Columbia University scientist called
this a "great conveyor belt." It brings a third of all the sun's
energy that falls on the Atlantic, up to the northeast US, Europe
and Scandinavia. It's the largest heat pump in a global ocean conveyor
that redistributes heat around the planet. Oceanographers have been
sampling the world's seas for a long time. One of the basic things
they look at is salinity how much salt is in the water. Salinity
varies -- it's high in the tropics, where the sun evaporates moisture
into the atmosphere, and low towards the poles, where precipitation
adds fresh water. Saltwater is heavy, while freshwater is light.
We'll see how vitally important that is, in a moment. At Wood's
Hole, Ruth Curry has been tracking the salinity in the Greenland
and Norwegian Seas the region where the ocean conveyor cools,
sinks and heads back south. These decorative panels are not the
latest wallpaper designs. In fact they show, in an exaggerated view,
the shape of the sea floor from the Arctic and the Greenland Sea
on the left, down through the Denmark Strait between Greenland and
Iceland, and on into the Atlantic. The upper view shows how heavy,
or dense, the water is, the lower view how salty. In the 1950s the
Greenland Sea was dense and salty. This is the ocean conveyor at
work the water sinking as it cools, then spilling out through
the Denmark Strait into the Atlantic. Twenty years later things
were changing. Freshwater the purple color was washing in from
the north, on the left. RUTH CURRY We're now starting to see in
the seventies the incursion of this fresh water. It's beginning
to come down from the Arctic. As we continue on into the seventies
and eighties, we see that freshwater washed all the down to the
bottom of the ocean. In the nineties we see this very, very fresh
water -- it's beginning to accumulate at the surface. It's filled
in the sub-polar basins and the density, that mountain of density
has basically collapsed. The red colors are now only found in the
deepest part of the Greenland Sea. Here's the situation in the last
five years or so, and we see that this dark blue, or purplish waters,
indicates that a tremendous layer of freshening has spread across
the Nordic Seas and into the sub-polar basins.
ALDA (NARRATION) The heart of the ocean conveyor its pump is
saltwater cooling, and sinking. But freshwater is lighter than saltwater
it floats on top. So if the Nordic Seas get fresh enough, the
pump could be threatened.
CURRY No matter how cold the waters get, they cannot sink down into
the deep ocean. Dense waters will not be formed, and that will slow,
and perhaps even stop the ocean conveyor belt.
ALDA (NARRATION) Remember what the conveyor belt does it brings
an enormous amount of heat from south to north. So if it does ever
stop, a lot of people will feel the consequences.
CURRY Places like northern Europe, Greenland, Iceland and the eastern
United States would especially see changes, on the order of two
to five degrees centigrade changes. And what that translates to
is difficult to describe exactly, but if you can remember some of
the coldest winters you might have experienced on the east coast
of the United States, that's what we could be seeing again.
ALDA (NARRATION) In our next story we're going to ask where all
that freshwater in the Nordic Seas is coming from.
ALDA (NARRATION) We've come across town to another Woods Hole research
organization. It's the Marine Biological Laboratory. There's a group
here that specializes in freshwater research.
PETERSON They're coming up with calculations...
ALDA (NARRATION) For the last 10 years, Bruce Peterson has been
leading a team studying how freshwater works at the top of the globe,
in the Arctic Ocean.
PETERSON Much of the freshwater that enters the Arctic Ocean doesn't
necessarily come as precipitation directly on the Ocean, it comes
as runoff from this watershed. For orientation, you have Greenland
here, Pacific Ocean here, the Atlantic Ocean here, and for reference
here is Alaska, this would be Canada, and now you have Eurasia over
on this side. Now two thirds of the watershed that actually contributes
freshwater to the Arctic Ocean is on this Eurasian side.
CHILDREN Hello. Hi. Hello.
ALDA (NARRATION) We're in Zhigansk, in Siberia. There's excitement
in town because school's out. Spring's around the corner, and the
ice on the river is breaking up. Way out there, the river is flowing
fast north towards the Arctic Ocean. This is the Lena River
it's as big as the Mississippi. Russia's Arctic rivers are closely
monitored, with their heights and flow rates recorded throughout
the year a practice that began in Soviet days. The records for
the six largest Russian Arctic rivers go back 70 years twice as
long as any of the other rivers that flow into the Arctic Ocean.
That's long enough for Bruce Peterson's team to reliably pick up
trends in the amount of freshwater being discharged.
PETERSON The amazing thing to us, when we looked at these records,
is that -- two, three, four, five of those six watersheds were ramping
up in discharge, especially in the last 30 to 40 years.
ALDA (NARRATION) River flow is simply a reflection of the amount
of rain and snow falling on the land that the river drains. The
global pattern in which moisture evaporates in the tropics, and
precipitates near the poles, has been ramping up in recent decades
-- just like the Russian rivers. Tropical waters are getting measurably
saltier, and polar waters are getting noticeably fresher, as Ruth
Curry found. For climate scientists and oceanographers, there's
a simple explanation for this.
CURRY The warming of our planet and especially the warming of our
surface oceans is removing freshwater through increased evaporation
from the low latitudes. That freshwater is being precipitated at
ALDA (NARRATION) This is what happens on a warmer earth there's
just more heat driving weather systems. There's no doubt increased
river flows are freshening the Arctic Ocean. But that might not
be the only cause of its freshening. We're in the St. Elias Mountains,
in southern Alaska. The region has been called North America's Himalayas
a 600-mile stretch of mountain ranges. In front of us is part
of Jeffries Glacier, an immense river of snow and ice.
CLAUS Ready to roll? Looks like a beautiful day.
ARENDT Yeah. Looks great.
ALDA (NARRATION) Today, Anthony Arendt and Paul Claus are going
glacier flying. PAUL CLAUS Are you good to go?
CLAUS Power's on. Everybody's clear?
ARENDT OK, the gyro is zeroed and we're ready for takeoff.
ALDA (NARRATION) They're flying out of the strip at Paul's remote
hunting lodge. He's an expert bush pilot, who works with the glacier
research group from the University of Alaska, Fairbanks. They head
over to Jeffries Glacier, about 40 miles away and 6,000 feet up.
These mountains have some of the largest glaciers in the world.
For the last 10 years the Fairbanks group has been keeping track
of a hundred representative glaciers in Alaska and Canada. They
do it with precision flying. Year after year, they've flown the
same tracks down the glaciers, bouncing a laser altimeter off the
surface as they go. They end up with the exact heights of the glaciers,
accurate to within about 12 inches.
ARENDT I'll start logging right when you cross the pass.
ALDA (NARRATION) Paul banks the plane round to the center line of
the glacier, while Anthony prepares to log their position in relation
to GPS satellites, along with their exact height above the glacier.
ARENDT OK, laser's in range right now, 200 meters. Looking good.
And we're logging. 50 meters. 25 meters. It looks good. We'll just
fly right down the center line.
ARENDT 50 meters.
ALDA (NARRATION) They also plot the terminus point in the valley
where the glacier transforms into a river.
ARENDT Pretty distinct terminus on this one.
CLAUS Oh, I guess. Ice ends right now.
ALDA (NARRATION) Back at the lab there's a lot of number crunching
to come up with the latest dimensions of each of their hundred glaciers.
ECHELMEYER This is our glaciology lab É
ALDA (NARRATION) Keith Echelmeyer started the glacier project, and
used to do all the precision flying himself. They found that, compared
to the maps of 50 years ago, the glaciers have shrunk dramatically.
And that's not all.
ALDA Is it really accelerating? What do you mean, accelerating?
VALENTINE We find that, on average, every year, that the glaciers
are thinning by a half a meter. Every year.
ALDA Every year.
VALENTINE So. You know. So...
ALDA You mean, from the '50s up through the '90s every year they
lost an average of a half a meter.
VALENTINE Half a meter.
VALENTINE Then, if you look at the period from the early 1990s to
2000, 2002, that rate goes up to 1.8 meters per year. So nearly
two meters every year of ice is lost on average, over the entire
surface of the glacier.
ALDA So that really is zipping ahead, isn't it?
ECHELMEYER Oh, it's zipping ahead, really a lot. And if that continues
in the future, that's what I was saying, a lot of these glaciers
won't be here.
ALDA (NARRATION) The findings translate into staggering volumes
of water added to the oceans 900 trillion gallons just from Alaska
and western Canada in the last 50 years, enough to raise sea levels
by about a quarter of an inch, worldwide. Only a fraction of that
fresh water finds its way into the Arctic Ocean, but the point is
this -- these are the most closely studied glaciers in the world,
and there is no doubt they are melting. The planet's largest accumulation
of ice outside Antarctica is in Greenland. If it's behaving in the
same way, that could dump a lot of freshwater precisely at the ocean
conveyor belt's most vulnerable point. The outlook for the conveyor
the thermohaline circulation, as scientists call it could change
CURRY At the present rate of freshening, the system could go on
for decades, or maybe even hundreds of years and not alter the thermohaline
circulation. On the other hand, if we had a sudden release event
-- something like a big chunk of the Greenland ice sheet breaking
off and entering the ocean and melting, or else a pool of glacial
meltwater forming a lake and then an ice dam breaking and having
a sudden release of that water into the Nordic Seas -- then that's
the sort of thing that might precipitate an actual change in the
ALDA (NARRATION) In our final story, we'll see that sudden changes
like this have happened before. We'll trace how scientists reconstruct
past climate events. And we'll ask what a stalled ocean conveyor
could do to the weather.
A LITTLE ICE AGE
ALDA (NARRATION) We're in medieval Europe, and it's bitterly cold.
For several centuries, ending around 1850, Europe was gripped by
what's called the Little Ice Age. In its coldest years it was catastrophic.
In 1693 the harvest failed and millions died. In 1709 a French priest
wrote, "Most of the hens have died of cold, and the beasts in their
stables." In fact this was a global phenomenon New York harbor
froze over in 1780, and there were mass migrations of Native Americans
seeking warmer areas. It's not clear what caused the Little Ice
Age perhaps volcanic dust shading out sunlight, combined with
changes in the sun itself. But what it shows is how disruptive even
small changes in temperature can be. The average cooling in the
Little Ice Age was around three degrees. But averages are misleading
they hide much larger swings from day to day, or year to year.
Reconstructing the climate of the past is a major scientific preoccupation,
now that there is so much attention on what we might be doing to
our climate today. One of the best places to look for traces of
past climate is at the bottom of the ocean in the thick layers
of sediment that accumulate over the centuries, and millennia. This
crew is collecting long sediment core samples off the coast of Brazil.
They're brought up in plastic pipes that have been rammed into the
sea floor. Teasing out what the sediment cores have to say about
past climate is a long and painstaking process. First the core is
split. One half is for analysis, the other is held for reference.
Oceanographers have collected thousands of sediment cores from all
the world's oceans. They're stored in enormous, warehouse-sized
libraries like this one at the Woods Hole Oceanographic Institution.
Unlocking the knowledge in the library starts like this. Most of
the sediment is just silt, washed out from the continents in rivers.
What they're looking for is these tiny white grains. The grains
are actually shells skeletons of plankton that lived in the ocean,
died and sank to the bottom. There's a wealth of information in
the shells. Some plankton like cold water, some warm so that's
a kind of thermometer. The shells are made of calcium carbonate,
and its component atoms can be analyzed. The carbon can be radiocarbon
dated, so you know when the plankton were living. This setup is
used to extract and purify carbon from the shells. It's part of
a US national radiocarbon dating lab at Woods Hole. Then the carbon
samples go next door to the nuclear accelerator, which counts the
different carbon atoms and figures out their age. You can also analyze
the oxygen from the shells, to reveal the temperature of the water
they lived in. A parallel set of measurements has been made on air
bubbles trapped in ice cores drilled from the Greenland ice sheet.
The result has been startling a classic scientific paradigm shift.
Rather than what we'd thought, since the end of the last Ice Age,
12,000 years ago, earth's climate has not been stable. It's cooled
abruptly, several times.
KEIGWIN There's on the order of six or eight events that we see
in sediment cores, and they correlate to events that we see in Greenland
ice cores as well. The most recent of them would be the Little Ice
ALDA (NARRATION) The Little Ice Age had been regarded as a curious
anomaly, but now here was a whole bunch of mysterious events. In
a lucky coincidence, the ocean sediments and Greenland ice cores
were giving up their secrets just as, in the 1980s and 90s, our
understanding of the ocean conveyor belt was growing. Scientists
are now putting the two together. Take the cooling event that occurred
eighty two hundred years ago. At the time, there were still remnants
of ice sheets around Hudson Bay. An enormous lake of meltwater had
accumulated behind the ice. Then suddenly it burst through the blockage,
flooded into the Bay and out to the ocean.
KEIGWIN Very abruptly, in as little as one year around eighty two
hundred years ago, the water finally bored its way through the ice
and the whole system collapsed. There's a very widespread cooling
known all around the North Atlantic region that's exactly coincident,
as nearly as we can tell, with this massive discharge of ice and
water out of Hudson Bay. So there's a mechanism.
ALDA (NARRATION) The cooling mechanism is simply that the flood
of freshwater shut down the ocean conveyor the thermohaline circulation.
At least one other cooling event was probably triggered this way.
Could this happen today? The ice in our warming planet is definitely
on the move. Over the last few years thirteen hundred square miles
of Antarctic sea ice have broken off, and drifted away. The Greenland
ice sheet is more than a million cubic miles of ice, sitting right
next to the thermohaline circulation pump that drives the ocean
conveyor. Scientists like Ruth Curry are well aware of the potential.
CURRY We know that Greenland is melting. We know that it is changing
its characteristics -- the amount of slippage is increasing as a
result of the accumulation of freshwater at the base of the glaciers.
I think that the Greenland ice sheet is probably the biggest wild
card in this equation. If a large chunk of that should fall off
into the ocean, then that could be enough to produce the amount
of freshwater that would then have an impact on the thermohaline
ALDA (NARRATION) We're back on the Lena River in Siberia. The next
step for Bruce Peterson's group is to analyze the waters of the
big Russian Arctic rivers, and define chemical fingerprints for
them. Then they can track the water once it's out in the ocean,
and begin to tie down how different sources rivers or melting
glaciers are contributing to the overall freshening.
HOLMES There it is.
ALDA (NARRATION) At the same time, Ruth Curry's colleagues are installing
a system of deep moored instruments to directly monitor the ocean
conveyor, as it flows north, and south, off the east coast of the
US. If the ocean conveyor shuts down, it could happen gradually
-- as a result of the freshening of the Arctic Ocean; or abruptly
-- if there's a big ice event in Greenland. Either way, scientists
will be watching.