Synthèse panoramique – Rome, 21.11.2008 – Forum (anglais)

États-Unis

Wallace Broecker

Prix Balzan 2008 pour la science du changement climatique

Par ses découvertes sur le rôle joué par les océans et leurs interactions avec l’atmosphère ainsi que par ses découvertes sur l’évolution des glaciers grâce aux informations fournies par les carottes glaciaires, Wallace Broecker a contribué de façon magistrale à notre compréhension des changements climatiques qu’ils soient abrupt ou graduels.

Panoramic Synthesis

During the mid 1980s two independent strands of research I had pursued, ever since my PhD thesis, suddenly came together. One involved the determination of the rate at which the ocean’s interior is ‘ventilated’ with newly-produced deep water and the other involved the record of the Earth’s climate for the last 100 or so thousand years. This ‘collision’ came about when I realized that the large and abrupt jumps in temperature recorded in Greenland ice cores could be explained by a turning on and off of the deep water formation in the northern Atlantic Ocean. Once formed, this water flowed down the length of the Atlantic Ocean around the tip of Africa joining the great circum Antarctic current. From there it penetrated into the deep Indian and deep Pacific and eventually upwelled to the surface. It completed its global circuit by flowing back to the Atlantic Ocean. This being the case, the disruption I proposed led to a reorganization of the way in which the ocean operated and, as a consequence, to major changes in the atmosphere as well. It was as if our planet’s climate is “quantized”. Turn off deep water production in the northern Atlantic and the climate jumps from one way of operating to another.

This idea was, of course, revolutionary and as is the case for any new paradigm, it took a while to catch on. Once it did, supporting information began to roll in from both the paleoclimate and modeling communities. It turned out that equivalents of Greenland’s temperature jumps were present in climate records from throughout the Northern Hemisphere and also the tropics. While muted in the Southern Hemisphere, records from Antarctic ice cores showed an antiphasing with those from the north. This led to the concept of a bipolar seesaw in ocean operation. Shutdowns of deep water formation in the northern Atlantic were apparently compensated by enhanced deep water formation in the south.

From the beginning, a key question involved the cause of the North Atlantic’s deep water formation shutdowns. Although the answer remains the subject of debate, it is generally agreed that the melting of large armadas of glacial ice launched from Hudson Straits into the northern Atlantic formed a fresh water lid and, under the cold conditions of glacial time, this lid allowed winter sea ice to form. This prevented the density of surface water from becoming great enough to allow deep water to form.

It took a while to understand how it could be that reorganizations of ocean circulation could have had such widespread and profound impacts on climate. The answer came when Berkeley’s John Chiang showed that the presence of expanded sea ice coverage in the northern Atlantic could do the job. This ice would not only prevent ocean heat from escaping to the atmosphere but it would also reflect sunlight back to space. This would create Siberian-like winters in Canada and northern Europe. Chiang showed that it would also push the thermal equator and its associated rain belt to the south explaining the impacts seen in records from the tropics.

Early on, I was concerned that the warming to be induced by fossil fuel CO2 might lead to a repeat of the conveyor shutdown. However, no source for a sudden fresh water flood is available. Further, a host of model studies have shown that the expected increase in rainfall and river runoff to be caused by global warming would result in only a gradual slowdown rather than an abrupt shutdown. Further, in a warmer world, the large climate feedback caused by a sea ice expansion would be absent.  

However, now that it is clear that the Earth’s climate system is capable of undergoing abrupt shifts in operation, we cannot rule out future surprises. Hence I have been keeping my eyes and ears open for candidate scenarios. Several years ago I heard a lecture by Isaac Held, one of the world’s leading atmospheric geophysicists. He made a convincing case that global warming would intensify rainfall in the tropics and, as a result, the extra tropical dry lands would expand and become even more arid. This struck me as bad news for more than a billion of the worlds poorest live in dry lands and about 40 percent of the world’s grain is grown on irrigated land.

Having on and off during my career made studies of records associated with lakes in desert areas, I realized that they offered a check on Held’s scenario. The idea is that, as these lakes never overflow, their sizes fluctuate with the amount of precipitation occurring in their drainage basins. As evaporation from the lake surface must balance input from the rivers which supply them, the greater the input, the larger the area the lakes occupy.

However as, during the last 100 or so thousand years, the Earth’s temperature has not been significantly warmer than present, I was forced to turn to the record for glacial time. The flip side of Held’s scenario would be that during colder times, the dry lands should have been smaller and less dry. Radiometric dating of material from paleo shorelines of lakes in dry lands of the western United States, of the Middle East and of Argentina demonstrate that during the peak of the last glacial period (24 to 18 thousand years ago) lakes were as much as five to eight times larger than their present-day remnants! Further, lakes in tropical eastern Africa were much smaller than now during that time. Encouraged by this preliminary look, I am currently working with a group of like-minded scientists to make sure that it was cold temperatures rather than some other aspect of glacial time that drove these changes.

In April 2002, I received a letter which led to a new dimension in my activities. It came from Gary Comer, a man I had never heard of, asking for advice. He explained that during the previous summer he had been able to sail his 175-foot aluminum-hulled yacht, Turmoil, through the Northwest Passage from Greenland to the Bering Sea with no interference from sea ice. As he was the first to be able to do this, he wondered whether global warming was already having an impact on the Arctic. A week later, Gary came to Columbia’s Lamont-Doherty campus full of questions. My answers must have pleased him for he took me under his wing and, in turn, did the same for a number of my friends who study abrupt climate change. Not only did he provide us with financial help, but he also made available transport to field locales otherwise inaccessible to us. In so doing, he created a cadre of scientists who, two years after his death, continue to work closely together.

Gary Comer’s mission went beyond his active participation in our research. He was deeply concerned about the potential adverse impacts that global warming would create and felt that the promotion of discoveries regarding the large sensitivity of the Earth’s climate system to small nudges could serve as a warning regarding the consequences of the large nudge fossil fuel CO2 is about to deliver.

One example stands out in my mind. Klaus Lackner, a colleague at Columbia University, was the first to realize that the same air streams used to drive wind turbines could be even more effectively harnessed to remove CO2 from the atmosphere. Once I got the idea straight in my head, I realized that Klaus had come up with what would not only become the key element in any strategy to stem the rise of CO2 in our atmosphere, but also the only way to bring it back down again once stabilization had been accomplished. So, I urged Gary Comer to provide venture capital for the development of a device capable of economically capturing CO2. Despite his financial advisors’ concern that such support would instead be “Adventure” capital, Gary went for it and, now five years later, a small company named General Research Technologies fueled by Lackner’s genius has succeeded. They promise a commercially available prototype within two years.

In conclusion, I have thoroughly enjoyed my 56 years as a scientist. Columbia University’s Lamont-Doherty Earth Observatory has been and remains my Eden. I plan to use the Balzan prize money to further research aimed at applying the paleoclimate record as a guide for what is to come.