ABRUPT CLIMATE CHANGE Should We Be Worried? www.whoi.edu For more information, contact Shelley Dawicki Robert B. Gagosian Woods Hole Oceanographic Institution President and Director, Woods Hole Oceanographic Institution Woods Hole, MA 02543 Prepared for a panel on abrupt climate change at the (508) 289-2270 • [email protected] World Economic Forum www.whoi.edu/institutes/occi/hottopics_climatechange.html Davos, Switzerland, January 27, 2003 The Global Ocean Conveyor The global ocean circulation system, often called the Ocean Conveyor, transports heat worldwide. White sections represent warm surface cur- rents. Purple sections represent cold deep currents. Jayne Doucette, WHOI Graphic Services Are we overlooking potential abrupt climate shifts? and atmosphere scientists at many institutions worldwide. But Most of the studies and debates on potential climate the concept remains little known and scarcely appreciated in change, along with its ecological and economic impacts, have the wider community of scientists, economists, policy mak- focused on the ongoing buildup of industrial greenhouse gases ers, and world political and business leaders. Thus, world lead- in the atmosphere and a gradual increase in global tempera- ers may be planning for climate scenarios of global warming tures. This line of thinking, however, fails to consider another that are opposite to what might actually occur.1 potentially disruptive climate scenario. It ignores recent and It is important to clarify that we are not contemplating a rapidly advancing evidence that Earth’s climate repeatedly has situation of either abrupt cooling or global warming. Rather, shifted abruptly and dramatically in the past, and is capable of abrupt regional cooling and gradual global warming can un- doing so in the future. fold simultaneously. Indeed, greenhouse warming is a desta- Fossil evidence clearly demonstrates that Earth’s climate bilizing factor that makes abrupt climate change more prob- can shift gears within a decade, establishing new and different able. A 2002 report by the US National Academy of Sciences patterns that can persist for decades to centuries. In addition, (NAS) said, “available evidence suggests that abrupt climate these climate shifts do not necessarily have universal, global changes are not only possible but likely in the future, poten- effects. They can generate a counterintuitive scenario: Even tially with large impacts on ecosystems and societies.”2 as the earth as a whole continues to warm gradually, large The timing of any abrupt regional cooling in the future also regions may experience a precipitous and disruptive shift into has critical policy implications. An abrupt cooling that hap- colder climates. pens within the next two decades would produce different This new paradigm of abrupt climate change has been well climate effects than one that occurs after another century of established over the last decade by research of ocean, earth continuing greenhouse warming. 2 3 The Conveyor’s Achilles’ Heel? Today With additional fresh water N O R T H A M E R I C A N O R T H A M E R I C A ADDITIONAL FRESH W ATER S P A I N S P A I N A F R I C A A F R I C A The Ocean Conveyor is propelled by the sinking of cold, salty (and If too much fresh water enters the North Atlantic, its waters could therefore denser) waters in the North Atlantic Ocean (blue lines). stop sinking. The Conveyor would cease. Heat-bearing Gulf Stream This creates a void that helps draw warm, salty surface waters waters (red lines) would no longer flow into the North Atlantic, and northward (red lines). The ocean gives up heat to the atmosphere European and North American winters would become more severe. above the North Atlantic Ocean, and prevailing winds (large red ar- (See computer animation at www.whoi.edu/institutes/occi/ rows) carry the heat eastward to warm Europe. climatechange_wef.html) Jack Cook, WHOI Graphic Services Are we ignoring the oceans’ role in climate change? edge of ocean dynamics does not match our knowledge of atmo- Fossil evidence and computer models demonstrate that Earth’s spheric processes. The oceans’ essential role is too often neglected complex and dynamic climate system has more than one mode in our calculations. of operation. Each mode produces different climate patterns. The evidence also shows that Earth’s climate system has sen- Does Earth’s climate system have an ‘Achilles’ heel’? sitive thresholds. Pushed past a threshold, the system can jump Here is a simplified description of some basic ocean-atmo- quickly from one stable operating mode to a completely differ- sphere dynamics that regulate Earth’s climate: ent one—“just as the slowly increasing pressure of a finger even- The equatorial sun warms the ocean surface and enhances tually flips a switch and turns on a light,” the NAS report said. evaporation in the tropics. This leaves the tropical ocean saltier. Scientists have so far identified only one viable mechanism The Gulf Stream, a limb of the Ocean Conveyor, carries an enor- to induce large, global, abrupt climate changes: a swift reorgani- mous volume of heat-laden, salty water up the East Coast of the zation of the ocean currents circulating around the earth. These United States, and then northeast toward Europe. currents, collectively known as the Ocean Conveyor, distribute This oceanic heat pump is an important mechanism for re- vast quantities of heat around our planet, and thus play a funda- ducing equator-to-pole temperature differences. It moderates mental role in governing Earth’s climate. Earth’s climate, particularly in the North Atlantic region. Con- The oceans also play a pivotal role in the distribution and veyor circulation increases the northward transport of warmer availability of life-sustaining water throughout our planet. The waters in the Gulf Stream by about 50 percent. At colder north- oceans are, by far, the planet’s largest reservoir of water. Evapo- ern latitudes, the ocean releases this heat to the atmosphere— ration from the ocean transfers huge amounts of water vapor to especially in winter when the atmosphere is colder than the ocean the atmosphere, where it travels aloft until it cools, condenses, and ocean-atmosphere temperature gradients increase. The Con- and eventually precipitates in the form of rain or snow. Changes veyor warms North Atlantic regions by as much as 5° Celsius in ocean circulation or water properties can disrupt this hydro- and significantly tempers average winter temperatures. logical cycle on a global scale, causing flooding and long-term But records of past climates—from a variety of sources such as droughts in various regions. The El Niño phenomenon is but a deep-sea sediments and ice-sheet cores—show that the Conveyor hint of how oceanic changes can dramatically affect where and has slowed and shut down several times in the past. This shutdown how much precipitation falls throughout the planet. curtailed heat delivery to the North Atlantic and caused substan- Thus, the oceans and the atmosphere constitute intertwined tial cooling throughout the region. One earth scientist has called components of Earth’s climate system. But our present knowl- the Conveyor “the Achilles’ heel of our climate system.”3 4 5 Dramatic Changes in the North Atlantic 2500 Denmark 1500 Greenland Strait East Irminger Sea 34.93 Denmark Strait 00 34.96 500 5 Iceland 34.92 15 Faroe- 00 500 34.95 Shetland500 2500 Channel 34.91 0 Channel Irminger 0 34.94 Irminger 15 Sea 34.90 00 34.93 15 500 34.89 0 0 Labrador 25 Scotland 34.92 Sea 34.88 500 0 0 25 500 0 Ireland 34.95 Labrador Sea Faroe-Shetland Channel 3500 350 34.93 Labrador 2500 3500 34.94 500 34.93 34.92 4500 Atlantic Ocean 34.92 Salinity (parts per thousand) 34.91 34.91 Subpolar seas bordering the North Atlantic have become noticeably 34.90 34.90 less salty since the mid-1960s, especially in the last decade. This is 34.89 34.89 , April 2002 34.88 Nature the largest and most dramatic oceanic change ever measured in the 34.88 era of modern instruments. This has resulted in a freshening of the 34.87 deep ocean in the North Atlantic, which in the past disrupted the 1965 1970 1975 1980 1985 1990 1995 2000 1965 1970 1975 1980 1985 1990 1995 2000 Ocean Conveyor and caused abrupt climate changes. Year Year B. Dickson, et al., in What can disrupt the Ocean Conveyor? Computer models simulating ocean-atmosphere climate dy- Solving this puzzle requires an understanding of what launches namics indicate that the North Atlantic region would cool 3° to and drives the Conveyor in the first place. The answer, to a large 5° Celsius if Conveyor circulation were totally disrupted. It would degree, is salt. produce winters twice as cold as the worst winters on record in For a variety of reasons, North Atlantic waters are relatively the eastern United States in the past century. In addition, previ- salty compared with other parts of the world ocean. Salty water ous Conveyor shutdowns have been linked with widespread is denser than fresh water. Cold water is denser than warm wa- droughts throughout the globe. ter. When the warm, salty waters of the North Atlantic release It is crucial to remember two points: 1) If thermohaline circu- heat to the atmosphere, they become colder and begin to sink. lation shuts down and induces a climate transition, severe win- In the seas that ring the northern fringe of the Atlantic—the ters in the North Atlantic region would likely persist for decades Labrador, Irminger, and Greenland Seas—the ocean releases large to centuries—until conditions reached another threshold at which amounts of heat to the atmosphere and then a great volume of thermohaline circulation might resume.