Washington - Research on Arctic and North Atlantic ecosystems shows the recent warming trend counts as the most dramatic climate change since the onset of human civilisation 5 000 years ago, according to studies published on Thursday.
Researchers from Cornell University studied the increased introduction of fresh water from glacial melt, oceanic circulation, and the change in geographic range migration of oceanic plant and animal species.
The team, led by oceanographer Charles Greene, described "major ecosystem reorganisation" - or "regime shift" - in the North Atlantic, a consequence of global warming on the largest scale in five millennia.
"The rate of warming we are seeing (now) is unprecedented in human history," said Greene, whose research appears in the November 2008 issue of the journal Ecology.
In order to forecast the path of climate change, Greene and colleagues have been reconstructing major episodes of warming and cooling in the Arctic over the past 65 million years.
They have found in the paleoclimate record periods of rapid cooling, with average temperatures plunging by 10 degrees Celsius (18 degrees F) within just decades or even years.
But the rise in temperatures during the past five decades is unmatched since the onset of human civilisation, Greene said.
The paleoclimate data gives the scientists more insight into the impact of melting Arctic ice sheets and glaciers on the North Atlantic oceanic system.
They have found "extensive" shifts in the geographic range of numerous plant and animal species.
For instance, the massive Arctic fresh-water melt in the past 10 years has helped one species of microscopic algae move from the Pacific ocean to the North Atlantic.
The last time that algae appeared in the North Atlantic was 800 000 years ago, the Cornell research found.
The increase of fresh water can have a huge impact on the ecosystems of the Atlantic continental shelf, for instance extending the growing seasons of phytoplankton and microscopic drifting animals fundamental to the food chain.
"Such climate-driven changes can alter the structure of shelf ecosystems from the bottom of the food chain upwards," according to Greene.
In another example, the collapse in the last century of cod populations in the north Atlantic is partially due to overfishing, but also partly due to Arctic glacial melt adding more fresh and colder water to the ocean, which stifles cod reproduction.
At the same time, the research noted, less cod and colder water benefited shrimp and snow crab populations.
"As climate changes, there are going to be winners and losers, both in terms of biological species and different groups of people," said Greene.
The Cornell studies also focused on the way the introduction of more freshwater in the north Atlantic can disrupt circulation patterns further south.
"When Arctic climate changes, waters in the Arctic can go from storing large quantities of fresh water to exporting that fresh water to the North Atlantic in large pulses, referred to as great salinity anomalies," Greene explains.
By modelling the current changes, the Cornell researchers posited that the highly saline water of the deep North Atlantic will likely not be heavily affected by the "pulses" of fresh water during the 21st century.
"Continued exposure to such freshwater forcing, however, could disrupt global ocean circulation during the next century and lead to very abrupt changes in climate, similar to those that occurred at the onset of the last ice age," the studies said.
"If the Earth's deep ocean circulation were to be shut down, many of the atmospheric, glacial and oceanic processes that have been stable in recent times would change, and the change would likely be abrupt," said Greene.