A team of researchers from the United States, Norway and Russia said the tropical plankton traveled thousands of miles on Atlantic currents and ended up above Norway because of an unusual, but naturally cyclic, pulse of warm water, not as a direct result of overall warming climate.
However, they said, such pulses are predicted to grow in frequency as global climate change causes shifts in long-distance currents.
Writing in the British Journal of Micropalaeontology, the researchers say the finding could be a preview of climate-induced changes already overtaking the oceans and land, causing redistributions of species and shifts in ecology.
"When we suddenly find tropical plankton in the arctic, the issue of global warming comes right up, and possible inferences about it can become very charged," O. Roger Anderson of Columbia University's Lamont-Doherty Earth Observatory said.
"So, it's important to examine critically the evidence to account for the observations."
The tropical plankton were apparently swept up in the warm Gulf Stream, he said, which travels from the Caribbean into the north Atlantic, but usually peters out somewhere between Greenland and Europe.
Sometimes, however, pulses of warm water penetrate along the Norwegian coast and into the arctic basin.
Such pulses have occurred in the 1920s, 1930s and 1950s, Anderson said.
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