BBC News 26 Nov 09;
Unusually warm and cold periods in Earth's pre-industrial climate history are linked to how the oceans responded to temperature changes, say scientists.
The researchers focused particularly on intervals known as the "little ice age" and "medieval warm period".
In the journal Science, they report that these climate "anomalies" were likely caused by changes to El Nino and the North Atlantic Oscillation.
They say studying the past in this way could help refine climate models.
"We reconstructed patterns of [the Earth's] surface temperature during those two intervals," explained Professor Michael Mann from Pennsylvania State University in the US, who led the study.
He and his colleagues reconstructed 1,500 years of the Earth's climate - collecting clues from "proxies" such as ice cores, tree rings and coral. These can be used to track hundreds of years of climatic changes.
He explained that the data allowed the team to estimate how natural factors, including volcanic eruptions and changes in the Sun's output, altered the climate in the past.
"We then put these estimates into the climate models," he told BBC News.
The models revealed that these natural factors altered the Earth's surface temperature, which kick-started feedback mechanisms - El Nino or the North Atlantic Oscillation (NOA).
This produced the regional patterns in climate associated with the medieval climate era and the little ice age.
"El Nino and the NOA are dynamical patterns that can lead to shifts in rainfall and drought patterns, and influence hurricane activity," explained Professor Mann.
"They redistribute heat around the globe, leading to warming in one region [of the planet] and cooling in another."
Feeding back
The findings have allowed the team to assess which models might be missing some of the "regional mechanisms" that influence the climate.
A key thing the team discovered was that, in the past, when the planet has been warmed by natural factors it has responded with another feedback mechanism known as the La Nina effect.
This can be thought of as the opposite of El Nino - a sort of "colder phase" of El Nino phenomenon.
Professor Mann explained that a "La Nina-like climate" brings colder than normal temperatures in the eastern and central tropical Pacific and drier than normal conditions in the desert southwest of the US".
Most climate models used by the Intergovernmental Panel on Climate Change (IPCC) show that the Earth will respond in an El Nino-like way to global warming.
But a few of the models do recreate this dynamic "La Nina effect", and suggest that that when you heat the Earth's surface, the climate system tries to offset and cool.
"If the response of the Earth in the past is analogous to the temperature increase caused by greenhouse gases... it could lend credence to this counterintuitive notion of a La Nina response to global warming," said Professor Mann.
But, he added, that the Earth's response to greenhouse-gas-induced global warming might be more complex than "natural" warming.
"What this gives us is an independent reality check," said Professor Mann.
"There is still a fair amount of divergence among the various models - in terms of how El Nino changes in response to increasing greenhouse gas concentrations.
"Some of the best clues we can get are by going back to the distant past and seeing how the Earth actually responded."
