Recent climate change research has uncovered a disturbing feature of the Earth's climate system: it is capable of sudden, violent shifts. This is a critically important realization. Climate change will not necessarily be gradual, as assumed in most climate change projections, but may instead involve sudden jumps between very different states. This would present an enormous challenge to human societies and the global environment as a whole.
While no such violent shift has occurred over the duration of human civilization, records of prehistoric shifts are clear. By studying a range of long-term natural records—including tree rings, cave deposits, ice cores and deep-sea sediments—scientists have begun to assemble an almanac of global change spanning hundreds of thousands of years. One of the most important findings to come from these records is that the climate system has undergone many sudden shifts.
The Great Ocean Conveyor
The Great Ocean Conveyor (also called the thermohaline circulation; see image above) is one of the great unknowns in the climate system. Surface water, warmed at the equator, circulates via the Gulf Stream to high latitudes where it releases heat to the atmosphere. As a result, the water cools, becoming denser, and sinks to the deep ocean.
Computer models and paleoclimate evidence both suggest that increases in freshwater supply to the North Atlantic ocean — for example, from increased rainfall or melting glaciers -can lower the salinity of the surface waters and make them too buoyant to sink, disrupting the ocean's circulation. As a result, the northward transport of heat stops abruptly, and temperatures around the North Atlantic drop.
A distressing confirmation of the models has recently emerged. Measurements of the saltiness of the North Atlantic show that the region's waters have been growing gradually fresher over the past 40 years. The rapidity and extent of freshening came as a surprise to oceanographers. The change is equivalent to a three- to four-metre cap of freshwater appearing over a broad area of the northern North Atlantic. If this continues, the resulting loss of density could prevent sinking, and short-circuit the Conveyor within decades.
Global warming may cause severe local cooling
It is well accepted that the Earth as a whole is warming as a result of human activity. Ironically, the lessons of distant Earth history show that warming may cause a sudden drop in temperature in some regions. The heat released to the atmosphere by the North Atlantic loop of the Conveyor is largely responsible for the relatively warm temperatures enjoyed by Western Europe. If not for this, European winters would be much colder. Berlin might have the climate of Edmonton, which lies at the same latitude, while Stockholm might be more like Iqaluit.
Judging from records of the distant past, this could be disastrous for eastern North America and Western Europe. Average temperatures could plunge by five degrees C. This is about the same difference as between the global average temperature today and during the last Ice Age, when Canada lay beneath 3,000-metre-thick glaciers. The resulting colder, snowier winters would require new infrastructure, damage crops and shorten the growing season. The costs associated with such a change would be enormous.