How Global Warming May
Cause the Next Ice Age...
While global warming is being officially ignored
by the political arm of the Bush administration, and Al Gore's recent conference
on the topic during one of the coldest days of recent years provided joke fodder
for conservative talk show hosts, the citizens of Europe and the Pentagon are
taking a new look at the greatest danger such climate change could produce for
the northern hemisphere - a sudden shift into a new ice age. What they're
finding is not at all comforting.
In quick summary, if enough cold, fresh water
coming from the melting polar ice caps and the melting glaciers of Greenland
flows into the northern Atlantic, it will shut down the Gulf Stream, which keeps
Europe and northeastern North America warm. The worst-case scenario would be a
full-blown return of the last ice age - in a period as short as 2 to 3 years
from its onset - and the mid-case scenario would be a period like the
"little ice age" of a few centuries ago that disrupted worldwide
weather patterns leading to extremely harsh winters, droughts, worldwide
desertification, crop failures, and wars around the world.
Here's how it works.
If you look at a globe, you'll see that the
latitude of much of Europe and Scandinavia is the same as that of Alaska and
permafrost-locked parts of northern Canada and central Siberia. Yet Europe has a
climate more similar to that of the United States than northern Canada or
It turns out that our warmth is the result of
ocean currents that bring warm surface water up from the equator into northern
regions that would otherwise be so cold that even in summer they'd be covered
with ice. The current of greatest concern is often referred to as "The
Great Conveyor Belt," which includes what we call the Gulf Stream.
The Great Conveyor Belt, while shaped by the
Coriolis effect of the Earth's rotation, is mostly driven by the greater force
created by differences in water temperatures and salinity. The North Atlantic
Ocean is saltier and colder than the Pacific, the result of it being so much
smaller and locked into place by the Northern and Southern American Hemispheres
on the west and Europe and Africa on the east.
As a result, the warm water of the Great Conveyor
Belt evaporates out of the North Atlantic leaving behind saltier waters, and the
cold continental winds off the northern parts of North America cool the waters.
Salty, cool waters settle to the bottom of the sea, most at a point a few
hundred kilometers south of the southern tip of Greenland, producing a whirlpool
of falling water that's 5 to 10 miles across. While the whirlpool rarely breaks
the surface, during certain times of year it does produce an indentation and
current in the ocean that can tilt ships and be seen from space (and may be what
we see on the maps of ancient mariners).
This falling column of cold, salt-laden water
pours itself to the bottom of the Atlantic, where it forms an undersea river
forty times larger than all the rivers on land combined, flowing south down to
and around the southern tip of Africa, where it finally reaches the Pacific.
Amazingly, the water is so deep and so dense (because of its cold and salinity)
that it often doesn't surface in the Pacific for as much as a thousand years
after it first sank in the North Atlantic off the coast of Greenland.
The out-flowing undersea river of cold, salty
water makes the level of the Atlantic slightly lower than that of the Pacific,
drawing in a strong surface current of warm, fresher water from the Pacific to
replace the outflow of the undersea river. This warmer, fresher water slides up
through the South Atlantic, loops around North America where it's known as the
Gulf Stream, and ends up off the coast of Europe. By the time it arrives near
Greenland, it's cooled off and evaporated enough water to become cold and salty
and sink to the ocean floor, providing a continuous feed for that deep-sea river
flowing to the Pacific.
These two flows - warm, fresher water in from the
Pacific, which then grows salty and cools and sinks to form an exiting deep sea
river - are known as the Great Conveyor Belt.
Amazingly, the Great Conveyor Belt is only thing
between comfortable summers and a permanent ice age for Europe and the eastern
coast of North America.
Much of this science was unknown as recently as
twenty years ago. Then an international group of scientists went to Greenland
and used newly developed drilling and sensing equipment to drill into some of
the world's most ancient accessible glaciers. Their instruments were so
sensitive that when they analyzed the ice core samples they brought up, they
were able to look at individual years of snow. The results were shocking.
Prior to the last decades, it was thought that
the periods between glaciations and warmer times in North America, Europe, and
North Asia were gradual. We knew from the fossil record that the Great Ice Age
period began a few million years ago, and during those years there were times
where for hundreds or thousands of years North America, Europe, and Siberia were
covered with thick sheets of ice year-round. In between these icy times, there
were periods when the glaciers thawed, bare land was exposed, forests grew, and
land animals (including early humans) moved into these northern regions.
Most scientists figured the transition time from
icy to warm was gradual, lasting dozens to hundreds of years, and nobody was
sure exactly what had caused it. (Variations in solar radiation were suspected,
as were volcanic activity, along with early theories about the Great Conveyor
Belt, which, until recently, was a poorly understood phenomenon.)
Looking at the ice cores, however, scientists
were shocked to discover that the transitions from ice age-like weather to
contemporary-type weather usually took only two or three years. Something was
flipping the weather of the planet back and forth with a rapidity that was
It turns out that the ice age versus temperate
weather patterns weren't part of a smooth and linear process, like a dimmer
slider for an overhead light bulb. They are part of a delicately balanced
teeter-totter, which can exist in one state or the other, but transits through
the middle stage almost overnight. They more resemble a light switch, which is
off as you gradually and slowly lift it, until it hits a mid-point threshold or
"breakover point" where suddenly the state is flipped from off to on
and the light comes on.
It appears that small (less that .1 percent)
variations in solar energy happen in roughly 1500-year cycles. This cycle, for
example, is what brought us the "Little Ice Age" that started around
the year 1400 and dramatically cooled North America and Europe (we're now in the
warming phase, recovering from that). When the ice in the Arctic Ocean is frozen
solid and locked up, and the glaciers on Greenland are relatively stable, this
variation warms and cools the Earth in a very small way, but doesn't affect the
operation of the Great Conveyor Belt that brings moderating warm water into the
In millennia past, however, before the Arctic
totally froze and locked up, and before some critical threshold amount of fresh
water was locked up in the Greenland and other glaciers, these 1500-year
variations in solar energy didn't just slightly warm up or cool down the weather
for the landmasses bracketing the North Atlantic. They flipped on and off
periods of total glaciation and periods of temperate weather.
And these changes came suddenly.
For early humans living in Europe 30,000 years
ago - when the cave paintings in France were produced - the weather would be
pretty much like it is today for well over a thousand years, giving people a
chance to build culture to the point where they could produce art and reach
across large territories.
And then a particularly hard winter would hit.
The spring would come late, and summer would
never seem to really arrive, with the winter snows appearing as early as
September. The next winter would be brutally cold, and the next spring didn't
happen at all, with above-freezing temperatures only being reached for a few
days during August and the snow never completely melting. After that, the summer
never returned: for 1500 years the snow simply accumulated and accumulated,
deeper and deeper, as the continent came to be covered with glaciers and humans
either fled or died out. (Neanderthals, who dominated Europe until the end of
these cycles, appear to have been better adapted to cold weather than Homo
What brought on this sudden "disappearance
of summer" period was that the warm-water currents of the Great Conveyor
Belt had shut down. Once the Gulf Stream was no longer flowing, it only took a
year or three for the last of the residual heat held in the North Atlantic Ocean
to dissipate into the air over Europe, and then there was no more warmth to
moderate the northern latitudes. When the summer stopped in the north, the rains
stopped around the equator: At the same time Europe was plunged into an Ice Age,
the Middle East and Africa were ravaged by drought and wind-driven firestorms. .
If the Great Conveyor Belt, which includes the
Gulf Stream, were to stop flowing today, the result would be sudden and
dramatic. Winter would set in for the eastern half of North America and all of
Europe and Siberia, and never go away. Within three years, those regions would
become uninhabitable and nearly two billion humans would starve, freeze to
death, or have to relocate. Civilization as we know it probably couldn't
withstand the impact of such a crushing blow.
And, incredibly, the Great Conveyor Belt has
hesitated a few times in the past decade. As William H. Calvin points out in one
of the best books available on this topic ("A Brain For All Seasons: human
evolution & abrupt climate change"): ".the abrupt cooling in the
last warm period shows that a flip can occur in situations much like the present
one. What could possibly halt the salt-conveyor belt that brings tropical heat
so much farther north and limits the formation of ice sheets? Oceanographers are
busy studying present-day failures of annual flushing, which give some
perspective on the catastrophic failures of the past. "In the Labrador Sea,
flushing failed during the 1970s, was strong again by 1990, and is now
declining. In the Greenland Sea over the 1980s salt sinking declined by 80
percent. Obviously, local failures can occur without catastrophe - it's a
question of how often and how widespread the failures are - but the present
state of decline is not very reassuring."
Most scientists involved in research on this
topic agree that the culprit is global warming, melting the icebergs on
Greenland and the Arctic icepack and thus flushing cold, fresh water down into
the Greenland Sea from the north. When a critical threshold is reached, the
climate will suddenly switch to an ice age that could last minimally 700 or so
years, and maximally over 100,000 years.
And when might that threshold be reached? Nobody
knows - the action of the Great Conveyor Belt in defining ice ages was
discovered only in the last decade. Preliminary computer models and scientists
willing to speculate suggest the switch could flip as early as next year, or it
may be generations from now. It may be wobbling right now, producing the
extremes of weather we've seen in the past few years.
What's almost certain is that if nothing is done
about global warming, it will happen sooner rather than later.
2004 by Thom Hartmann. This article was adapted from the new, updated edition of
Last Hours of Ancient Sunlight" by Thom Hartmann (thom at www.thomhartmann.com),
due out from Random House/Three Rivers Press in March.