Le petit age glacière pour demain !

A perspective on potential climate changes presented by Dr. Robert B. Gagosian, President and Director of Woods Hole Oceanographic Institution

Over the past two decades, we have heard about greenhouse gases and the idea that our planet is gradually warming. I’d like to throw a curveball into that thinking—specifically the “gradually warming” part.

This new thinking is little known and scarcely appreciated by policymakers and world and business leaders—and even by the wider community of natural and social scientists. But evidence from several sources has amassed and coalesced over the past 10 to 15 years. It points to a completely different—almost counterintuitive—scenario.

Global warming could actually lead to a big chill in some parts of the world. If the atmosphere continues to warm, it could soon trigger a dramatic and abrupt cooling throughout the North Atlantic region—where, not incidentally, some 60 percent of the world’s economy is based.

When I say “dramatic,” I mean: Average winter temperatures could drop by 5 degrees Fahrenheit over much of the United States, and by 10 degrees in the northeastern United States and in Europe. That’s enough to send mountain glaciers advancing down from the Alps. To freeze rivers and harbors and bind North Atlantic shipping lanes in ice. To disrupt the operation of ground and air transportation. To cause energy needs to soar exponentially. To force wholesale changes in agricultural practices and fisheries. To change the way we feed our populations. In short, the world, and the world economy, would be drastically different.

And when I say “abrupt,” I mean: These changes could happen within a decade, and they could persist for hundreds of years. You could see the changes in your lifetime, and your grandchildren’s grandchildren will still be confronting them.

And when I say “soon,” I mean: In just the past year, we have seen ominous signs that we may be headed toward a potentially dangerous threshold. If we cross it, Earth’s climate could switch gears and jump very rapidly—not gradually— into a completely different mode of operation.


	A schematic of the ocean circulation system, often called the Great Ocean Conveyor, that transports heat throughout the world oceans. Red arrows indicate warm surface currents. Blue arrows indicate deep cold currents. (Animation by Jack Cook, WHOI)

This is not something new under the Sun. It has happened throughout Earth’s history, and it could happen again.

The key to these climate shifts is that Earth’s climate is created and maintained by a dynamic system of moving, interacting parts. Earth’s climate system has two main components. The first one you are all familiar with by watching your local TV meteorologist or The Weather Channel. It is the atmosphere, which circulates heat and moisture around the globe. But, in fact, the atmosphere redistributes only about half of the energy that the earth receives from the Sun.

The other half is transported around our planet by a circulation system that is equally important, but far less understood—the ocean. The ocean isn’t a stagnant bathtub. It circulates heat around the planet like the heating and cooling system in your house. The atmosphere and oceans are equal partners in creating Earth’s climate. The atmosphere is a rabbit. It moves fast. Rapid changes in atmospheric circulation cause storms, cold spells, or heat waves that play out over several days.

The ocean, on the other hand, is a turtle. It may take years or decades or even millennia for similar “disturbances” to circulate through the ocean. But the ocean is a big turtle. It stores about 1,000 times more heat than the atmosphere. So changes in ocean circulation can set the stage for large-scale, long-term climate changes.

One example that you may be familiar with is El Niño. Every few years, oceanic conditions shift, and surface water temperatures in the eastern tropical Pacific get warmer. The atmosphere above the ocean shifts, too. El Niño rearranges worldwide wind and rainfall patterns, causing destructive droughts, floods, storms, and forest fires. Not to downplay El Niño in the least, because it causes grave human suffering and billions of dollars in damage—but El Niño is relatively short-lived. It lasts only a year or two.

The climate changes I’m concerned about last longer and involve the ocean circulation system that spans the entire globe—which we often call the Great Ocean Conveyor.

The Great Ocean Conveyor is the ocean’s major heat-circulating system. The ocean keeps our planet from overheating by transporting heat north and south, from the equator to the poles, in currents traveling near the ocean surface. In the Atlantic, the Conveyor removes heat from the Southern Hemisphere and releases it to the Northern Hemisphere.

The Great Ocean Conveyor is propelled by the sinking of cold, salty (and therefore denser) waters in the North Atlantic Ocean (blue arrows). That creates a void that pulls warm, salty Gulf Stream waters northward (red arrows). The Gulf Stream gives up its heat to the atmosphere above the North Atlantic Ocean, and prevailing winds (large red arrows) carry the heat eastward to warm Europe. (Animation by Jack Cook, WHOI)

The most famous and most important of these currents is the Gulf Stream. The vast Gulf Stream transports the equivalent volume of 75 Amazon Rivers. It carries heat absorbed in the tropics, and moves up the East Coast of the United States, then northeastward toward Europe.

When the Gulf Stream’s warm, salty waters reach colder latitudes, they give up their heat to the atmosphere. The atmosphere in the North Atlantic region warms by as much as 10 degrees Fahrenheit. Prevailing winds carry the heat eastward into Europe. That’s one reason London, England—which is located at the same latitude as Calgary and Edmonton—has warmer winters than New York—which is hundreds of miles farther south.

When the Gulf Stream’s waters reach the Labrador, Greenland and other northern seas, and lose their heat to the atmosphere, they become colder—and hence denser. The waters are also relatively salty. Salty water is denser than fresher water, so the whole salty mass begins to sink to great depths.

When this sinking mass of cold water reaches the abyss, it then flows at deep levels of the ocean, from the North Atlantic southward into the South Atlantic. The plunge of this great volume of cold, salty water propels the Great Ocean Conveyor. And on the back end, it creates a void that actively pulls the Gulf Stream northward to replace the waters that are sinking.

It’s a pretty neat system. We have been operating under the climate conditions created by this beneficent oceanic heating, ventilation, and cooling system for centuries.

But what if this system weren’t operating today? What if cold North Atlantic waters didn’t sink and warm equatorial waters weren’t drawn in to replace the sinking waters? Then the North Atlantic region would be a much different, much colder place.

The vast majority of us go about our business thinking that Earth’s climate system has always been this way, and always will be. But that is just not the case. In fact, we know that the Ocean Conveyor has indeed shut down—stopped operating—in the past.

We know this by examining truly interesting and valuable natural archives that record past changes in Earth’s climate. For example, we can see layers within ice sheets in Greenland or in high elevations in the Andes Mountains in South America. This layering is caused by variations in the amount of snow that fell on the top of the ice sheet in the past.



	Top: Ice sheets reveal annual layers, which scientists can analyze to reconstruct the history of precipitation and air temperatures 100,000 years in the past. (Photo by Lonnie Thompson, Ohio State University) Bottom: Cores of seafloor sediments reveal the climate history of the ocean. (Photo by Ken Buesseler, WHOI)

The light and dark bands within the ice sheet show an annual cycle of dust in the atmosphere, so each couplet reflects the amount of snow that fell in one year. By analyzing the chemistry of the ice itself, scientists can determine the temperature of the atmosphere when the snow was produced.

Thus from a single ice sheet record, it is possible to reconstruct the past history of precipitation and air temperature in a region. For very old ice sheets, like the glaciers on Greenland, it is possible to reconstruct this climate history 100,000 years into the past.

The ocean has similar archives of past climate. By taking cores from the ocean floor, it is possible to reconstruct the history of ocean climate back many thousands of years. We have used our ships to collect samples of sediments from the seafloor. Preserved in the sediments are the fossil remains of microscopic organisms that settle to the seafloor. They accumulate over time in layers—similar to the ice core—that delineate many important aspects of past climate.

For instance, certain organisms are found only in colder, polar waters and never live in warmer waters. They can reveal where and when cold surface waters existed—and didn’t exist—in the past.

From records like these, we know that about 12,800 years ago, North Atlantic waters cooled dramatically—and so did the North Atlantic region. This large cooling in Earth’s climate occurred in about a decade. And the cold spell lasted for about 1,300 years.

This period is called the Younger Dryas, and it is just one of several periods when Earth’s climate changed very rapidly from warm to cold conditions, and then back to warm again. So these long-term cold snaps are not unusual.

These shifts almost certainly involved changes in the ocean’s circulation. There were shutdowns and restartings of the Ocean Conveyor. These warm-to-cold transitions happen in about 3 to 10 years. The cold periods lasted for 500 to 1,000 years. Such oscillations in temperature and ocean circulation have occurred on a regular basis.

About 1,000 years ago, during a period of unusually warm temperatures in the North Atlantic, the Norse established settlements and vineyards in Greenland that would not be possible today. Those settlements were abandoned about 500 years ago, when we believe the most recent shutdown of the North Atlantic Ocean circulation system occurred.

During that era, called the Little Ice Age, northern Europe was much colder than it is today. Glaciers spread outward and downward in the Alps. Winters, on average, were more severe. Farming was affected. Famine was frequent.

In the 1730s and 1740s, abrupt European cooling caused famine across western Europe, especially in Ireland and France, where farmers depended on wheat and potatoes. In Ireland, this is known as the “forgotten famine.” As many people died during the forgotten famine as died during the famed potato famine of the 1840s.

The 16th-century Flemish artist Bruegel couldn’t have painted his famous frozen landscapes today, because now canals in the Netherlands rarely freeze, as they regularly did back then. And likewise, the winter in Valley Forge might not have been so cold, and Washington’s crossing of the ice-bound Delaware River wouldn’t have been so dramatic, if he had done it a century later—because our climate conditions have shifted since then, and today, the Delaware River rarely freezes.

If you read David McCullough’s biography of John Adams, you will remember that the British were about to set Boston on fire when George Washington was able to bring the cannons of Fort Ticonderoga down from upstate New York in record time. He was able to do it because the ground was frozen solid and they could slide the cannons to the Dorchester hills of Boston in time to persuade the British to retreat from Boston and to change the course of history.

So we have solid evidence that the Great Ocean Conveyor has slowed down or shut down in the past. And we have seen dire impacts on our climate. It begs the question: Could something throw a wrench into the Great Ocean Conveyor in the near future? And could that trigger abrupt, dramatic climate changes throughout our planet? The answers to those questions are, indisputably, “Yes and yes.”


Like most dynamic systems, Earth’s climate seeks a stable mode (top tier). If the system is pushed, it will stagger for a while unitl it recovers (middle tier). If it is pushed past a threshold, it will shift into another mode of operation (bottom tier). (Illustration by Jack Cook, WHOI, adapted from Abrupt Climate Change, Inevitable Suprises National Research Council)

Another way to look at Earth’s climate system is a simple mode of balances (see diagram, left). Like most dynamic systems, Earth’s climate seeks a stable mode. And it will tend to stay in that mode if nothing causes it to change. That’s the top tier of the diagram. The ball remains ensconced in its cup.

The middle tier shows what happens if you push on the system. The ball will rattle and roll around in different directions for a while until it settles back down in its cup. We humans would definitely notice that rattling until the climate system returned to equilibrium. The balance, forced to move, staggers but recovers.

Then there is a third situation— in which a strong enough push at the right time could shove the system past a threshold and into a completely different mode of operation. In terms of our climate system, that means that a small or temporary forcing could produce a sudden, large, and long-lasting change. That begs the next question: What could do that to our climate system today?

One answer to that is fresh water. If you simply add too much fresh water to the North Atlantic, the waters there will become less salty and less dense. They will stop sinking. Then the Gulf Stream slows down or is deflected southward. Winters in the North Atlantic region get significantly colder.

Now here’s the predicament. In the past year, oceanographers monitoring and analyzing water conditions in the North Atlantic, have concluded that the North Atlantic has been freshening dramatically—especially in the past decade. New data —from Ruth Curry at Woods Hole Oceanographic Institution and her colleague Robert Dickson at the British Centre for Environment, Fisheries, and Aquaculture Science—chronicles salinity changes in the western North Atlantic since 1960.

The Great Ocean Conveyor transports fresh surface water down into the depths. The depths can absorb a lot of fresh water like a sponge. But since 1970, the equivalent of an extra 20 feet of fresh water across the surface of the northern North Atlantic has been transported down into the ocean depths, most of that since 1990.

A sponge that is three-quarters saturated can still absorb more water. But the moment that sponge is fully saturated, it can absorb no more water.


	If too much fresh water enters the North Atlantic, its waters could stop sinking. The Great Conveyor would cease. Heat-bearing Gulf Stream waters (red arrows would no longer flow into the North Atlantic, and winters would become more severe. (Animation by Jack Cook, WHOI)

At some point, the North Atlantic will no longer absorb any more fresh water. It will begin to pile up on the surface. When that happens, the Great Ocean Conveyor will be clogged. It will back up and cease functioning.

The very recent freshening signal in the North Atlantic is arguably the biggest and most dramatic change in ocean property that has ever been measured in the global ocean. Already, surface waters in the Greenland Sea are sinking at a rate 20 percent slower than in the 1970s.

At what percent will the Ocean Conveyor stop? 25 percent? 40 percent? 60 percent? This is not like a dimmer switch, but more like a light switch. It probably goes from “on” to “off.”

We can’t yet determine the precise source or sources of this additional fresh water. Global warming may be melting glaciers, or Arctic sea ice. In recent decades, the volume of Arctic sea ice has decreased by 40 percent. And if North Atlantic sinking slows down, less salty Gulf Stream waters flow northward—which exacerbates the situation.

In February 2002, at a worldwide meeting of oceanographers, new data on North Atlantic freshening prompted many scientists to say that salinity levels in the North Atlantic are approaching a density very close to the critical point at which the waters will stop sinking.

One of my colleagues at Woods Hole, Terry Joyce, put it this way: “I’m in the dark as to how close to an edge or transition to a new ocean and climate regime we might be,” he said. “But I know which way we are walking. We are walking toward the cliff.”

To that sentiment, I would add this: We are walking toward the edge of a cliff—blindfolded. Our ability to understand the potential for future abrupt changes in climate is limited by our lack of understanding of the processes that control them.

New data shows that North Atlantic waters at depths between 1,000 and 4,000 meters are becoming dramatically less salty, especially in the last decade. Red indicates saltier-than-normal waters. Blue indicates fresher waters. Oceanographers say we may be approaching a threshold that would shut down the Great Ocean Conveyor and cause abrupt climate changes. (Data from Ruth Curry, WHOI, Bob Dickson, Centre for Environment, Fisheries, and Aquaculture Science and Igor Yashayaev, Bedford Institute of Oceanography)

The Heat Before the Cold
By Terrence Joyce
Published in The New York Times
April 18, 2002

This week’s unexpected heat wave across much of the Northeast and Midwest, couple with recent reports about the surprisingly fast collapse of an Antarctic ice shelf the size of Rhode Island, has heightened fears of long-term rise in temperatures brought about by global warming. But this fear may be misguided. In fact, paradoxically, global warming could actually bring colder temperatures to some highly populated areas like Eastern North America and Western Europe.

Here’s what might happen: In the North Atlantic, a 10-foot layer of fresh water - some of which may be coming from melting ice in the Arctic - has been accumulating and lowering the salinity of the ocean to depths of more than a mile for the past 30 years. Fresh water in the ocean may not sound cataclysmic, but it can upset the ocean currents that are the key to our planet’s climate control system.

In February, oceanographers presented new evidence that this northern freshwater buildup could alter currents in a way that would cause an abrupt drop in average winter temperatures of about 5 degrees Fahrenheit over much of the United States and 10 degrees in the Northeast. That may not sound like much, but recall the coldest winters in the Northeast, like those of 1936 and 1978, and then imagine recurring winters that are even colder, and you’ll have an idea of what this would be like. This change could happen within a decade and persist for hundreds of years.

Under normal circumstances, the famous warm waters of the Gulf Stream, carrying heat absorbed in the tropics, move up the East Coast of the United States and southeastern Canada and then angle toward Europe, warming the overlying atmosphere and surrounding land as they go. As the Gulf Stream system carries warm, salty water north, the atmosphere cools it, making it dense enough to sink to great depths. The plunge of that great volume of water helps propel a global system of currents sometimes called the great ocean conveyor. But add too much fresh water, and North Atlantic waters become less salty and less dense. They stop sinking. The Gulf Stream slows or is redirected southward. Winters in the North Atlantic region get significantly colder.

Changes in the conveyor were responsible for some of the most noticeable climate changes in scientific history. About 12,000 years ago, as the earth emerged from the most recent Ice Age and the North Atlantic region warmed, an influx of fresh water - perhaps from melting ice sheets - shut down the great conveyor and plunged much of the Northern Hemisphere back into ice-age conditions that lasted 1,000 years. About 500 years ago a reduction of the ocean conveyors may have turned the climate in northern Europe and the northeastern United States much colder, during what became known as the Little Ice Age, which lasted for about 300 years. In America, the Little Ice Age coincided with the notorious winter at Valley Forge.

There is not enough evidence for scientists to know for sure whether the influx of fresh water in the North Atlantic has come from an already altered ocean circulation, changes in rainfall patterns or rivers, or glacial and Arctic Ocean ice. We don’t know the exact threshold at which sinking, and the great ocean conveyor, could stop. A global ocean-observing system would greatly enhance our ability to monitor changes that can spawn major, long-lasting climate shifts like these and lead to reliable predictions of what may follow. But the evidence we do have suggests that global warming could actually lead to a big chill.

Terrence Joyce is a senior scientist and chairman of the department of physical oceanography at Woods Hole Oceanographic Institution.