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RECONSTRUCTING THE PAST In the 1730s and 1740s, abrupt European cooling caused The vast majority of us go about our business thinking that famine across western Europe, especially in Ireland and France, Earth's climate system has always been this way, and always where farmers depended on wheat and potatoes. In Ireland, will be. But that is just not the case. In fact, we know that the this is known as the "forgotten famine". As many people died Great Ocean Conveyor has indeed shut down—stopped during the forgotten famine as died during the famed potato operating—in the past. famine of the 1840s. We know this by examining truly interesting and valuable The 16th-century Flemish artist Bruegel couldn't have natural archives that record past changes in Earth's climate. painted his famous frozen landscapes today, because canals in For example, we can see layers within ice sheets in Greenland The Netherlands rarely freeze now, as they regularly did back or in high elevations in the Andes Mountains in South then. And likewise, the winter in Valley Forge might not have America. This layering is caused by variations in the amount been so cold, and Washington's crossing of the ice-bound of snow that fell on the top of the ice sheet in the past. Delaware River wouldn't have been so dramatic if he had done The light and dark bands within the ice sheet show an annual __it a century later—because climate conditions have shifted cycle of dust in the atmosphere, so each couplet reflects the since then, and today the Delaware River rarely freezes. amount of snow that fell in one If you've read David McCullough's year. By analysing the chemistry of biography of John Adams, you will the ice itself, scientists can deter- remember that the British were mine the temperature of the atmos- About 1,000 years ago, about to set Boston on fire when phere when the snow was produced. during a period of unusually George Washington was able to Thus, from a single ice sheet bring the cannon of Fort record, it is possible to reconstruct warm temperatures in the Ticonderoga down from upstate the history of precipitation and air North Atlantic, the Norse New York in record time. He was temperature in a region. For very able to do it because the ground was old ice sheets, like the glaciers on established settlements and frozen solid and they could slide the Greenland, it is possible to recon- vineyards in Greenland that cannon to the Dorchester Hills of struct this climate history 100,000 . Boston in time to persuade the years into the past. would not be possible today. British to retreat from Boston and The ocean has similar archives of thus change the course of history. past climate. By taking cores from So we have solid evidence that the ocean floor, it is possible to recon- the Great Ocean Conveyor has struct the history of ocean climate back many thousands of slowed down or shut down in the past. And we have seen dire years. We have used our ships to collect samples of sediments impacts on our climate. It begs the question: could something from the seafloor. Preserved in the sediments are the fossil throw a wrench into the Great Ocean Conveyor in the near remains of microscopic organisms that settle to the seafloor. future? And could that trigger abrupt, dramatic climate They accumulate over time in layers (in a similar way to ice changes throughout our planet? The answers to these ques- core layers) that delineate many important aspects of past cli- tions: indisputably mate. For instance, certain organisms are found only in colder, "Yes" and "Yes". polar waters and never live in warmer waters. They can reveal where and when cold surface waters existed—and didn't TIPPING THE exist—in the past. BALANCE From records like these, we know that about 12,800 years Another way to ago North Atlantic waters cooled dramatically—and so did the _look at Earth's cli- North Atlantic region. This large cooling in Earth's climate mate system is as a occurred in about a decade—and the cold spell lasted for about simple mode of 1,300 years. balances (see dia- This period is called the Younger Dryas, and it is just one of | gram). Like most several periods when Earth's climate changed very rapidly from dynamic systems, warm to cold conditions, and then back to warm again. So Earth's climate these long-term cold snaps are not unusual. seeks a_ stable These shifts almost certainly involved changes in the ocean's mode. And it will circulation. There were shutdowns and restartings of the Great tend to stay in that _ t Ocean Conveyor. These warm-to-cold transitions happened in mode if nothing about 3 to 10 years. The cold periods lasted for 500 to 1,000 causes it to change. years. Such oscillations in temperature and ocean circulation That's the top tier have occurred on a regular basis. of the diagram. About 1,000 years ago, during a period of unusually warm The ball remains temperatures in the North Atlantic, the Norse established settle- ensconced in its ments and vineyards in Greenland that would not be possible cup. today. Those settlements were abandoned about 500 years ago The middle tier when, we believe, the most recent shutdown of the North shows what hap- Atlantic Ocean circulation system occurred. During that era, pens if you push on called the Little Ice Age, northern Europe was much colder the system. The than it is today. Glaciers spread outward and downward in the ball will rattle and Cores of seafloor sediments reveal the climate Alps. Winters, on average, were more severe. Farming was roll around in dif- history of the ocean. (Photo by Ken Buesseler, affected. Famine was frequent. ferent directions Woods Hole Oceanographic Institution) 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. NEXUS = 13 RECONSTRUCTING THE PAST FEBRUARY — MARCH 2003 www.nexusmagazine.com