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The Science of Abrupt Climate Change
We generally consider climate changes as taking place on the scale of hundreds or even thousands of years. However, since the early 1990s, a radical shift in the scientific understanding of Earth's climate history has occurred. We now know that that major regional and global climate shifts have occurred in just a few decades or even a single year. The most recent of these shifts occurred just 8200 years ago. If an abrupt climate change of similar magnitude happened today, it would have severe consequences for humans and natural ecosystems. Although scientists consider an abrupt climate change unlikely in the next 100 years, their understanding of the phenomena is still a work-in-progress, and Earth's history shows that such a change could be triggered in just a year or two by natural processes or by human-caused global warming with little warning.
The National Academy of Sciences--the board of scientists established by Congress in 1863 to advise the federal government on scientific matters--compiled a comprehensive report in 2002 entitled, Abrupt Climate Change: Inevitable Surprises. The 244-page report, which contains over 500 references, was written by a team of 59 of the top researchers in climate, and represents the most authoritative source of information about abrupt climate change available. Most of the material that follows was taken from this report.
Ice cores hold an amazingly detailed record of Earth's climate. Each year, snow falling on glacial areas accumulates, piling on top of thousands of years of past snow, compressing the snow into yearly layers of ice, like rings inside a tree trunk. Preserved in the ice are tiny bubbles of ancient air that tell us the composition of the atmosphere at that time. The amount of dust in the snow tells us how windy the climate was. The thickness of the layer tells how much precipitation fell that year. Most importantly, the amount of the "heavy" isotope of oxygen, 18O, lets us infer the average atmospheric temperature, since water vapor with "heavy" 18O molecules condenses out of clouds more readily at cold temperatures.
Accessing this treasure-trove of climatic information is a huge undertaking--cores of ice must be drilled miles deep in some of the most inhospitable places on Earth. In 1989 the National Science Foundation funded the $25 million Greenland Ice Sheet Project II (GISP2) to drill an ice core through the entire two mile depth of the Greenland ice sheet. At the same time, a separate European project (GRIP), drilled through the ice just 20 miles away, providing a crucial independent check of the GISP2 data. By 1993, both the GRIP and GISP2 drills had hit bedrock, and two miles of ice cores, preserving 110,000 years of climate history in year-by-year layers, were taken to laboratories for analysis.
What the scientists found was surprising and unnerving. They had known from previous ice core and ocean sediment core data that Earth's climate had fluctuated significantly in the past. But what astonished them was the rapidity with which these changes occurred.
Ocean and lake sediment data from places such as California, Venezuela, and Antarctica have confirmed that these sudden climate changes affected not just Greenland, but the entire world. During the past 110,000 years, there have been at least 20 such abrupt climate changes. Only one period of stable climate has existed during the past 110,000 years--the 11,000 years of modern climate (the "Holocene" era). "Normal" climate for Earth is the climate of sudden extreme jumps--like a light switch flicking on and off.
As seen in Figure 1, the ice core record showed frequent sudden warmings and coolings of 15° F (8°C) or more. Many of these changes happened in less than 10 years. In one case 11,600 years ago, when Earth emerged from the final phase of the most recent ice age (an event called the Younger Dryas), the Greenland ice core data showed that a 15°F (8°C) warming occurred in less than a decade, accompanied by a doubling of snow accumulation in 3 years. Most of this doubling occurred in a single year.
Current theories on the cause of abrupt climatic change focus on sudden shut downs and start-ups of the Meridional Overturning Circulation (MOC) (also referred to as the thermohaline circulation), which is a global network of density-driven ocean currents. The Meridional Overturning Circulation transports a tremendous amount of heat northward, keeping the North Atlantic and much of Europe up to 9°F (5°C) warmer, particularly in the winter. A sudden shut down of this current would have a ripple effect throughout the ocean-atmosphere system, forcing worldwide changes in ocean currents, and in the path of the atmospheric jet stream. Studies of North Atlantic Ocean sediments have revealed that the Meridional Overturning Circulation has shut down many times in the past, and that many of these shut downs coincide with the abrupt climate change events noted in the Greenland ice cores.
How does one shut down the Meridional Overturning Circulation? First, one must examine the MOC itself. The MOC, or Great Ocean Conveyor Belt (Figure 2), is a system of interconnected ocean currents that girdle the planet.
At the surface, warmer ocean currents (shown here in orange) are driven by the winds, and so move parallel to the wind direction, except where continental land masses block the way. Water can also move vertically in the ocean. High density water sinks, and low density water rises. Salty water is more dense than fresh water, and cold water is more dense than warm water, so that wherever we find cold, salty water, it tends to sink. Colder currents (shown here in blue) are deeper and have higher salinity.
In the tropical Atlantic, the sun's heat evaporates large amounts of water, creating relatively warm, salty ocean water. This warm, salty water flows westward toward North America, then up the East Coast of the U.S., then northeastward toward Europe, forming the mighty Gulf Stream current. As this warm, salty water reaches the ocean regions on either side of Greenland, cold winds blowing off of Canada and Greenland cool the water substantially (in Figure 2, these regions are marked with white circles labeled, "Heat release to the atmosphere.") These cool, salty waters are now very dense compared to the surrounding waters, and sink to the bottom of the ocean. Thus, the oceanic areas by Greenland where this sinking occurs are called "deep-water formation areas". This North Atlantic deep water flows southward toward Antarctica, eventually making it all the way to the Pacific Ocean, where it rises back to the surface to complete the Great Ocean Conveyor Belt. It takes about 1000 years for the water to make a complete circuit around the globe.
Since the Great Ocean Conveyor Belt is driven in part by differences in ocean water density, if one can pump enough fresh water into the ocean in the key areas on either side of Greenland where the Gulf Stream waters cool and sink, this will lower the ocean's salinity (and therefore its density) enough so that the waters can no longer sink. As a result, the Atlantic conveyor belt and Gulf Stream current would shut down in just a few years, dramatically altering the climate.
It is unknown precisely how much fresh water is needed to shut down the MOC. Scientists are fairly certain that the last two abrupt coolings seen the Greenland ice core, the "Younger Dryas" event and the "8200 years before present" event (Figure 1), both occurred when huge North American glacial melt-water lakes flooded down the St. Lawrence River into the North Atlantic when the ice dams restraining the lakes broke. The sudden addition of low-density fresh water presumably partially or totally stopped the sinking of ocean waters in the North Atlantic, slowing or completely stopping the Meridional Overturning Circulation. Once the fresh water got into the North Atlantic, it stayed, puddling on top of the ocean and freezing in winter. The Meridional Overturning Circulation stayed shut off for about 1100 years during the Younger Dryas event, then suddenly restarted, for reasons scientists don't understand. Current computer models of the climate cannot reproduce the observed sudden shut-down or start-up of the Meridional Overturning Circulation at the beginning and end of the Younger Dryas period.
Other sudden shut downs of the Meridional Overturning Circulation observed in ice core and ocean sediment records are not thought to be due to sudden melt-water floods into the North Atlantic. These events may have happened simply because Earth's climate system is chaotic, or perhaps because some critical threshold was crossed when increases in precipitation, river run-off, and ice melt put enough fresh water into the ocean to shut down the Meridional Overturning Circulation.
Global warming will increase precipitation, river run-off, melting of the Greenland ice sheet, and melting of polar sea ice, all of which will increase the amount of fresh water flowing into the critical deep-water formation areas by Greenland. In the 2007 IPCC Fourth Assessment Report Summary for Policymakers it states that, based on current model simulations, it is very likely (90-99% confidence) that the meridional overturning circulation (MOC) of the Atlantic Ocean will slow down during the 21st century. It also confirms the scientific consensus that is very unlikely the MOC will undergo a large abrupt transition during this century. Today's science is such that any longer-term assessments of the MOC cannot be made with confidence.
As of 2010, there is no evidence that the MOC is slowing down. In fact, a study published in the March 25, 2010 issue of Geophysical Research Letters by NASA oceanographer Josh Willis found that the circulation may have even sped up slightly in the recent past. Until recently, the only direct measurements of the circulation's strength have been from ship-based surveys and a set of moorings anchored to the ocean floor in the mid-latitudes. Willis' new technique is based on data from NASA satellite altimeters, which measure changes in the height of the sea surface, as well as data from Argo profiling floats. The international Argo array, supported in part by the National Oceanic and Atmospheric Administration, includes approximately 3,000 robotic floats that measure temperature, salinity and velocity across the world's ocean. With this new technique, Willis was able to calculate changes in the northward-flowing part of the circulation at about 41 degrees latitude, roughly between New York and northern Portugal. Combining satellite and float measurements, he found no change in the strength of the circulation overturning from 2002 to 2009. Looking further back with satellite altimeter data alone before the float data were available, Willis found evidence that the circulation had sped up about 20 percent from 1993 to 2009.
A shut down of the Meridional Overturning Circulation would suddenly decrease the amount of heat in the North Atlantic, leading to much colder temperatures in Europe and North America. A 2003 report prepared for the Department of Defense outlines what would happen if an abrupt climatic change similar to the 8200 years before present event were to recur today:
Dr. Wally Broecker of Columbia University, the scientist who first pointed out the link between the Atlantic's conveyor circulation and abrupt climate change, wrote a letter in March 2004 to Science magazine, accusing the authors of the study of making exaggerated claims that "only intensify the existing polarization over global warming." Broecker argued that a global-warming induced abrupt climate change is not likely to occur until 100 years or so into the future, by which time Earth's temperature will have warmed sufficiently to offset much of the abrupt cooling a Meridional Overturning Circulation shut down would trigger. Broecker added: "What is needed is not more words but rather a means to shut down carbon dioxide emissions." The authors of the study defend their scenario thusly: "We have created a climate change scenario that although not the likely, is plausible, and would challenge United States national security in ways that should be considered immediately."
The possibility of the freezing of the UK and Europe will be determined by a "tug-of-war" of sorts, between the amount of greenhouse gases and the speed with which the MOC slows down. Greenhouse gases may have more of an impact than a slowing of the MOC, simply because they are more abundant today than ever in the earth's record. (CO2 levels were at 380 ppm as of 2007, and were never above 300 ppm during the 400,000 years studied in Antarctic ice cores).
Ocean experts see the MOC as having three levels: "faster," "slower," or "off." A 2005 comparison of eleven climate models showed that the MOC will likely be slowed by 10-50%, however, because the levels of carbon dioxide are so elevated, any cooling produced by the MOC slowing would be modest because the greenhouse gases would more than compensate. As a result, a net warming is still shown by these models for the UK and surrounding countries. Improving our measurements to monitor the MOC will allow for better predictions and reduce uncertainty of the amount of warming or cooling these areas of northern Europe will encounter.
The immediate obvious needs are for accurate, long-term measurements of the temperature, salinity, and flow rates of the major ocean currents in the North Atlantic Ocean. An expedition set sail from Great Britain on Feb. 13 2004, to provide just that. The voyage was part of a joint US/UK research project called Rapid Climate Change, which began in 2001. In the U.S., Senator Susan Collins (R-Maine) sponsored bill S.1164 to authorize $60 million for the National Oceanic and Atmospheric Administration (NOAA) to study abrupt climate change. The bill defined abrupt climate change as "a change in the climate that occurs so rapidly or unexpectedly that human or natural systems have difficulty adapting to the climate as changed." The bill did not pass, and there is little chance for revival. The NTSC Joint Subcommittee On Ocean Science and Technology authored an Ocean Research Priorities Plan in January 2007, providing five key elements for reducing our vulnerability to abrupt climate change. These include: daily monitoring of ocean currents, temperature, and carbon, now-casting, model development, past-climate-change reconstructions, and additional climate-impact assessments.
The historical records shows us that abrupt climate change is not only possible--it is the normal state of affairs. The present warm, stable climate is a rare anomaly. It behooves us to learn as much as we can about the climate system so that we may be able to predict when the next abrupt shift in climate will come. Until we know better when this might happen, it would be wise to stop pouring so much carbon dioxide into the air. While it currently appears that an abrupt shut down of the MOC and subsequent abrupt climate change event are not likely this century, we need to know more. In the words of Dr. Wally Broecker, "the climate system is an angry beast, and we are poking it."
NASA's "A Chilling Possibility" press release, March 2004.
"Abrupt Climate Change: Should we be worried?" Analysis by Woods Hole Oceanographical Institute, January 2003.
The Great Climate Flip-flop, a 1998 Atlantic Monthly article.
Abrupt climate change, a report prepared by Global Business Network (GBN) for the Department of Defense, October 2003.
The Two-Mile Time Machine : Ice Cores, Abrupt Climate Change, and Our Future, by Dr. Richard Alley, 2001. An excellent book on the Greenland ice cores findings.