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Global warming as a possible trigger for abrupt change |
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Take away ideas and understandings:
1. How global warming may trigger abrupt climate changesOur interest in abrupt climate changes in the past is not only academic. One of the important lessons we have learned from the recent geologic past is that climate can change very quickly and with a large amplitude relative to a given forcing. Some of the best examples of abrupt climate change include theHeinrich events and Dansgaard-Oeschger events during the last glacial period and the Younger Dryas cooling event (ca. 12.7-11.7 ka BP) which ushered in the warm, more stable climate of the Holocene.
The 8.2 Ka event - a case study in abrupt climate change During the Holocene, one of the best documented cooling events was the "8.2 ka event" which occured about 8,200 years ago.
The 8.2 ka event has been found over much of the northern hemisphere, but it is particularly pronounced around the North Atlantic and northern Europe region, but it has also impacted the climate of north Africa and the near east. Near 8.2 ka BP, Greenland cooled by ~5°C for a period of many decades to possibly a century in duration. The onset and end of this cool period occured within years. The 8.2 ka event has been associated with the catastrophic release of about 10^14 m^3 of freshwater from northeastern Canada spanning a period of many decades. The freshwater release was due a massive outflow of freshwater that was trapped behind the melting, remnant Laurentide ice sheets in proglacial lakes Agessiz and Ojibway.The 8.2 ka outflow of freshwater is roughly 20 times more than the amount of freshwater that has been mixed into the North Atlantic between 1970-2000. It's a big number, but not so great that it doesn't apply to the near future.
This sudden supply of freshwater to the high-latitude North Atlantic 8200 years ago is believed to have produced a freshwater cap over the deep water formation regions in the Labrador Sea, and Norwegian-Iceland- Greenland Seas region. This freshwater cap inhibited wintertime densification and convection that is required to form deep waters, so consequently deep water production was greatly reduced for a period of many decades. Hysteresis is a term that ocean dynamists use to explain the lagged response of deep water formation to changes in the supply of freshwater to the ocean surface. Essentially, deep water formation doesn't commonly respond immediately to increases in freshwater supply but rather the forcing take some time to build enough resistance to deep water production (convection). However when this limit is reached, the deep water production stops very abruptly and will stay in this "shutdown" mode for a long time even (decades) if the freshwater supply is stopped. The example shown below was taken from work by Stefan Rahmstorf. The production of North Atlantic Deep Water (NADW) decreases modestly with initial addition of freshwater. However, at point "S" the formation of NADW stops abruptly. As freshwatersupply wanes, NADW stays in its "off" mode for many decades until (d) the surface salinity can restablish itself to allow deep convection again. At that point, NADW shifts abruptly into the "on" mode.
The panel below shows how deep ocean circulation (mass overturning, measured in Sverdrups or 1,000,000 m^3 sec^-1, one Sverdrup is roughly 60 times the flow of the Mississippi River) changes between the "on" (upper panel) and "off" (lower panel) states.
Here are some model simulations of the effects of very sudden, large releases of freshwater to the North Atlantic, and some runs with more gradual additions.
Why is the 8.2 ka event so relevant to the global warming debate? What is most impressive about the 8.2 ka event is that it occured in the mid-Holocene when global temerpatures were actually a bit warmer than today. This sudden release of freshwater resulted in a short-lived (multidecade) but very abrupt reversal of the warming trend at that time. The relevance to modern society is that were such an event to take place today is would surely disrupt the social networks globally and impact nearly every aspect of our lives in a way that would be highly disproportionate to the actual climate disturbance. This is essentially because it would perturb climate (hence food and water supply, carrying capacities, political stability, trade networks) in ways that are difficult to predict. 2. The Oct. 2003 "Pentagon Report" (PDF)In 2002, the Pentagon comissioned two outside contractors Peter Schwartz and Doug Randall to assess a "what-if" climate change scenario for the near future. They were asked to "imagine the unthinkable" and to consider what might be the effects of future global warming from a scientific perspective and also from a national security perspective. The concern of course is not a gradual, linear global warming trajectory, but the possibility for abrupt changes in climate and how these might impact global security (which is the preferred mode of natural climate shifts in the past). The document was leaked to the press in Feb. 2004 and it has resulted in widespread news coverage, with particularly angry reactions from the European press. Global warming, they conclude, must "therefore be viewed as a serious threat to global stability and should be elevated beyond a scientific debate to a U.S. national security concern." An essential message of this document is that abrupt climate changes should be an expected consequence of global warming, and that these changes can be expected to fundamentally disrupt the global socioeconomic networks, which is itself a greater threat to US security than terrorism. What were their conclusions? Schwartz and Randall essentially proposed a scenario which integrated what is known about past abrupt climate change (such as the 8.2 ka event). They used current scientific projections of the possible impacts of future global warming as a basis for a possible abrupt climate change in the near future (they project that this event could happen by 2010-2020, although it is important to emphasize that this estimate is more likely a guess than anything based on scientific fact. It could happen decades later...or, less probably, sooner.) They propose that global temperatures will continue to rise over the next decades which will accelerate melting of glaciers and the strength of the hydrological cycle. The added supply of freshwater to the North Atlantic will eventually lead to a slowdown and perhaps shutdown in deep water formation which then can lead to an abrupt change in climate similar in mechanism to the 8.2 ka event. Essentially, they conclude that by 2010-2020 the North Atlantic thermohaline circulation system will shutdown and temporarily lead to a multidecadal reversal to much cooler conditions over the North Atlantic and northern Europe. The main risk is that the event is both abrupt and unpredictable, and that it can impact a huge are which is highly populated and politically vulnerable. Their "weather report" for 2010-2020 includes:
They then project the geopolitical impact of these climate changes. In short, nations become challenged to just feed their own populations and thus turn inward. Carrying capacity of the planet is altered because the climate shifts lead to unpredictable disruptions in food, water, and resource supplies. |
