November 27, 2009
Researchers Explain Past Climate Confusion
Scientists say that unusually warm and cold periods in the history of Earth's pre-industrial climate are connected to how temperature changes affect the oceans.
The research primarily centered around time periods known as the "little ice age" and "medieval warm period".
These climate "anomalies" were likely caused by changes to El Nino and the North Atlantic Oscillation, they wrote in the journal Science.
The researchers say that way of studying the past could help refine climate models.
"We reconstructed patterns of [the Earth's] surface temperature during those two intervals," said Professor Michael Mann from Pennsylvania State University in the US, who led the study.
Mann and his colleagues reconstructed 1,500 years of the Earth's climate by collecting clues from "proxies" such as ice cores, tree rings and coral, which can be used to follow hundreds of years of climate change.
The data enabled the team to estimate how natural factors, such as volcanic eruptions and changes in the Sun's output, altered the climate in the past, explained Mann.
"We then put these estimates into the climate models," he told BBC News.
The models showed that these natural factors changed the Earth's surface temperature, which triggered feedback mechanisms like El Nino or the North Atlantic Oscillation (NAO).
This brought about the regional patterns in climate linked to the medieval climate era and the little ice age.
"El Nino and the NAO are dynamical patterns that can lead to shifts in rainfall and drought patterns, and influence hurricane activity," explained Mann.
"They redistribute heat around the globe, leading to warming in one region [of the planet] and cooling in another."
With the new findings, the team is able to determine which models might be missing some of the "regional mechanisms" that effect the climate.
An important discovery the team made is that when the planet was warmed by natural factors, it responded with another feedback mechanism known as the La Nina effect.
This is the opposite of El Nino, a kind of "colder phase" of El Nino phenomenon.
Professor Mann explained that colder temperatures in the eastern and central tropical Pacific and drier than usual conditions in the desert southwest of the U.S. are produced by "La Nina-like climate".
The majority of climate models used by the Intergovernmental Panel on Climate Change (IPCC) reveal that global warming will cause the Earth to respond in an El Nino-like fashion.
However, some of the models do recreate the dynamic "La Nina effect", and indicate that when you heat the Earth's surface, the climate system attempts to cool it down.
"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, the Earth's response to greenhouse-gas-induced global warming might be more complex than "natural" warming, he added.
"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."
On the Net: