To determine whether El Niño influences weather in the North Atlantic, researchers must first separate the signal of El Niño from the rest of the world's weather.
The task is not easy, as global weather patterns are, well, global. For example, during the Northern Hemisphere's winter, weather in the North Atlantic sweeps across Europe and may spark summer monsoons in Asia.
Turney and his team have found an ancient volcanic crater in northern Queensland, Australia, that is sufficiently removed from the influence of the monsoon and thus experiences weather driven only by the tropical Pacific and is sensitive to changes in the strength of El Niño.
The top 36 feet (11 meters) of the crater, called Lynch's Crater, are covered in peat. "What we've got are various things in the sediment we can measure to get a handle on environmental change when the peat itself was laid down," Turney said.
For example, peat normally forms in wet, swampy conditions, but when drier conditions prevail, such as during El Niño, the top part of the peat dries out. When the peat dries, it becomes vulnerable to bacteria, which cause the peat to decompose.
By examining the level of decomposition in the layers of peat and associated factors such as the ratio of sedge (which is indicative of wet, non-El Niño, weather) to grass (which is indicative of dry, El Niño, weather), the researchers were able to establish a climate record.
The Lynch's Crater climate record shows dry periods that last for about 1,500 years, indicating frequent El Niño events that Turney and colleagues say are correlated to the Dansgaard-Oeschger events in the North Atlantic.
The researchers hypothesize that during the last ice age these 1,500-year periods of frequent El Niño activity caused more evaporation in the Pacific and warming across northern North America. This in turn either melted the ice so that it slipped into the sea or caused more snow to fall and created a surge of glacial ice towards the sea.
Either way, the influx of melted ice disrupted the North Atlantic thermohaline circulation, which works like a giant conveyor belt transporting warm, tropical waters from South America up to Greenland, where it gets colder, saltier, and denser. This dense water sinks and drifts along the ocean bottom back to South America.
"If you can upset that by putting a whole lot of fresh water [in the North Atlantic], it forms a cap and doesn't allow salt water to sink, affecting climate around the world," Turney said.
According to Alley, the hypothesis is interesting "but far from proven."
He said that changes in methaneswamp gas from tropical wetlandsare recorded in bubbles in ice cores and occurred at the same time as Dansgaard-Oeschger events in the North Atlantic. If El Niño was the trigger, the methane would be expected to occur much earlier than the Dansgaard-Oeschger events because it takes a while for ice to melt.
Alley also said that the timing for ice sheets to surge in response to increased snow fall or melting is likely to be a bit random, making "it unlikely that one would see a direct, tight correlation in time between a tropical forcing and ice-sheet surging in the far north."
Turney, who anticipates this paper sparking debate, will continue to refine and extend Australia's historical climate record, in hopes of better preparing the world for future climate changes.
For more on the climate and the environment, scroll down for related stories and links.
SOURCES AND RELATED WEB SITES