A lock-in puts a new spin on climate change.© PhotodiscThe processes that produce El Niño events may also cause abrupt shifts in global climate, according to new research. This could explain why the thaw after the last ice age was interrupted by a frigid spell lasting hundreds of years1.
Amy Clement and colleagues of the Lamont-Doherty Earth Observatory in Palisades, New York, are challenging conventional wisdom about the reasons for the Younger Dryas event, a return to near-ice-age conditions that took place about 11,500 years ago. Clement's group provide a new explanation for the sudden shifts in temperature that can occur across the globe.
Evidence for global cooling during the Younger Dryas, when the Earth was warming up after the last ice age, has been found throughout the world. The prevailing explanation for the cold snap invokes the melting of the great ice sheets in the Northern Hemisphere.
Fresh water from the melting ice sheets was discharged into the North Atlantic. Being less dense than salt water, it stopped the ocean water from sinking as it flowed northwards. Sinking water in the North Atlantic drives a giant circulation of ocean water, bringing heat from the tropical regions towards the poles. If this circulation shuts down, the North Atlantic region cools, affecting global climate.
There is some evidence that the Younger Dryas might have been triggered in this way; but it is not incontrovertible. Clement and her co-workers point out that some studies of climate patterns at that time don't fit the 'ocean circulation' explanation.
They now have an alternative explanation: El Niño events. These events bring warm, wet weather to South America and drought to Australia and Indonesia. They are manifestations of the El Niño/Southern Oscillation (ENSO), a coupled ocean-atmosphere phenomenon.
ENSO, Clement's team says, can interact with changes in Earth's orbit to trigger rapid changes in climate. ENSO occurs roughly every two to seven years as a result of heat and energy moving between the atmosphere and the oceans.
Over tens of thousands of years, periodic changes in the shape of Earth's path around the Sun and the tilt of its axis are thought to cause climate shifts such as the ice ages. These orbital variations are very gradual, but they trigger feedbacks in Earth's climate system that lead to global temperature shifts.
Orbital variations can alter ENSO's pulse, say the researchers, locking it in step with the yearly cycle of seasons. El Niño events can then virtually disappear for centuries. During such a deadlock, average temperatures across the globe fall significantly.
The changeover from normal to locked ENSO behaviour happens in a matter of decades. Very few climate phenomena are known to be able to produce such a big change so quickly. The current ENSO cycle seems to be in a phase close to that in which these sudden switches can occur.
The researchers support their claims with computer simulations of the global climate system that show the cooling effect. But it will be difficult to distinguish this effect in climate records from the effect of the ocean-circulation system.
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