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
Tampering with Our Stable Mode?
Human beings have made major modifications to the earth's environment in little more than a century, increasing the concentration of carbon dioxide in the atmosphere to its highest level in 260,000 years. Numerical models can be used to estimate what will happen when anthropogenic increases in the atmospheric concentration of gases such as carbon dioxide and methane block heat from leaving the earth. The increased concentration of these gases acts like a greenhouse, and the average temperature of the earth gets warmer. But the numerical models of Stocker, Rahmstorf and others suggest there may be surprises in the greenhouse.
When the greenhouse effect warms the earth, it accelerates the hydrologic cycle, more water moves around in the atmosphere, and rainfall increases in many places. Some models suggest that this will slowly decrease the salinity of the North Atlantic, making the surface water less dense. Were a critical density threshold to be crossed, ocean circulation would abruptly switch to a new stable mode.
This would be more than just a switch in ocean circulation; it would be a switch in the way tropical heat is transported to the North Atlantic. At the least, Northern Europe and Scandinavia would be 2 to 5 degrees colder on average than they are now, and the amount of precipitation would decrease dramatically. It would not necessarily be a rapid return to an ice age, but it might be a start in that direction. The orbital parameters of the earth are such that we are due for another ice age, and a cooling in the north Atlantic at a time when orbital parameters favor a return to a much colder climate could be the trigger that initiates such a change.
A switch in climate from a warm period (like the current Holocene epoch) to an ice age has happened before. Ocean and lake cores tell us that the warm Eemian period from about 131,000 to 114,000 years ago—when the distribution of ice sheets was similar to what it is today—switched to the Wisconsin ice age in no more than 400 years, the minimum time resolution of the record from these ancient sediments. Unfortunately, we have yet to recover an ice core that shows in sharp detail how the Eemian Period ended. This is old ice. It is difficult to find a place where it snowed enough to produce a high time-resolution record but not so much as to smear the record against the bedrock. An international project, led by Claus Hammer of the University of Copenhagen, has identified the most likely location in Greenland for this ice to be found and is collecting a core.
Many arctic ice cores tell us that 8,200 years ago the climate approached ice age conditions for a 400-year period before returning to conditions similar to today. This excursion was most likely caused by the one-time draining of lakes left behind by the melting of the Canadian ice sheets. This change in freshwater flux to the oceans was large but not that much different from what greenhouse-induced changes may produce in the future. The fact that it took place when climate conditions were similar to today demonstrates that large and rapid climate switches do not happen exclusively when there are extensive northern hemisphere ice sheets. It is ironic that greenhouse warming may lead to rapid cooling in eastern North America, Europe and Scandinavia, and it is possible that altered ocean circulation could lead to much larger changes. We have no experience predicting climate switches between stable modes, so it would be wise to expect surprises.
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