FEATURE ARTICLE
Rapid Climate Change
New evidence shows that earth's climate can change dramatically in only a decade. Could greenhouse gases flip that switch?
Kendrick Taylor
Three Climate Modes
Because the oceans must abide by the constraints of geography and the laws of physics, there are only a few patterns in which the oceans can circulate. At a recent conference organized by Robert Webb of NOAA and Peter Clark of Oregon State University, climate scientists identified three modes of ocean circulation, each of which is associated with a different climate. The current mode produces the warmest conditions in the North Atlantic. Surface water sinks in two regions of the North Atlantic, and a large volume of surface water and heat is drawn from the tropics to replace the sinking North Atlantic water. The heat carried north by the northward-moving surface water warms eastern North America, the North Atlantic and most of Europe. The second mode of ocean circulation occurs when surface water sinks in only one area of the North Atlantic. Less surface water sinks to the bottom, so smaller amounts of warm surface water and heat are drawn north to replace the sinking water. This mode was in place during the warmest times of the Wisconsin ice age, when climate was only slightly colder than current conditions. In the coldest mode, no water sinks in the North Atlantic; hence no warm water is drawn north. This was the condition during the coldest portions of the Wisconsin ice age.
Each of these modes of ocean circulation is associated with a small range of prevailing environmental conditions. Weather anomalies such as 10-year-long droughts or wet periods, as significant as they may seem to human affairs, are a reflection of the small range of environmental conditions associated with a single mode of ocean circulation. If, however, environmental conditions are externally forced to be inconsistent with the existing mode of ocean circulation, the circulation will switch to a mode that is more consistent with those environmental conditions. An example of this forcing would be a change in the amount of solar heat reaching, and retained at, the earth's surface. Such a change could be the result of alterations either in solar output or in the way the atmosphere regulates the exchange of heat between the earth's surface and space. The transitions between different modes of ocean circulation are abrupt. The ocean-sediment cores and ice cores tell us that they frequently take only several decades or less.
Numerical models of ocean circulation developed by Thomas Stocker of the University of Bern and Syukuro Manabe of Princeton University show that each circulation mode is stable for a particular range of environmental conditions. For example, if the discharge of a river changes, altering the density of the surface water in the adjoining ocean, the ocean-circulation pattern will change only if it is unstable under this new set of conditions. As long as the climate system stays within the stable-mode range, river discharge and greenhouse-gas concentration can vary without having much influence on climate.
Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research has used numerical models to show how surprisingly sensitive ocean circulation can be to changes in freshwater discharge. His numerical models show that if the climate system is near the threshold between stable modes, a small change in the amount of freshwater entering the North Atlantic will force a large and rapid shift to a different ocean-circulation pattern. Like a coin on edge, which topples with only a breath of air, an unstable pattern quickly assumes a new position where it becomes quite stable. The climate changes recorded by the ice and ocean-sediment cores appear to have taken place when some crucial threshold was crossed, resulting in large and rapid switches—in geologic time, like the flip of a switch—in ocean circulation.
Unfortunately, no one knows yet what caused this switch to flip. We know of external forcing mechanisms, but their time periods do not match the record. For example, the distribution of solar energy reaching the earth varies according to the relative positions of the sun and earth. These variations, called Milankovitch cycles, have periods of tens of thousands of years and are thus too slow to explain the rapid changes seen every couple of thousand years during the Wisconsin ice age. The Milankovitch cycles define the big picture and determine when changes could occur, but some smaller, quicker-acting mechanism triggered the more frequent switches during the Wisconsin.
The leading idea, which has been simulated in computer models, is that increased discharges of freshwater glacial ice and river runoff into the North Atlantic reduced the density of the surface water enough that it could not sink. This slowed the ocean conveyor, forcing it to switch to another circulation pattern. Other emerging concepts place the source of the disruption of the conveyor in the tropical Pacific. Variability in the sun's output is another possible cause of the climate variations, but the record of solar output is not good enough to adequately investigate this idea. Furthermore, the dynamics of ocean circulation around Antarctica are too poorly understood to completely exclude the possibility that they may play a role. Whatever the cause may be, it is worrisome that the phenomenon that has repeatedly triggered major changes in ocean circulation and the earth's climate is so subtle that we have not been able to identify it. This emphasizes how large changes in the interaction of the oceans, atmosphere and ice sheets have been triggered by small perturbations of the environment.
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