"The past few decades have been unique in the past 200,000 years in terms of the changes we see in the biology and chemistry recorded in the cores," University of Colorado glaciologist Yarrow Axford said in the study by Canadian and US researchers.

"We see clear evidence for warming in one of the most remote places on Earth at a time when the Arctic should be cooling because of natural processes," added the chief author of the study published in the Proceedings of the National Academy of Sciences.

For thousands of years, environmental changes in a remote lake on Canada's Baffin Island closely matched natural, cyclical climate changes such as those caused by the Earth's periodic wobble as it swings around the sun, the researchers said.

However, lake sediment cores dating from 1950 show that expected climate cooling was overridden by human activity like greenhouse gas emissions, the study said.

Researchers were able to reconstruct the local climate over the past 200,000 years by analyzing algae, insect fossils and geochemical traces in sediment cores extracted from the 100-acre (40 hectare) lake.

The cores stretch back 80,000 years further than existing Greenland ice cores, revealing environmental conditions prevalent during two earlier ice ages and three interglacial periods.

Researchers found that several types of mosquito-like midges that for many thousands of years thrived in cold climate surrounding the lake suddenly began declining at around 1950; two midge species adapted to the coldest weather disappeared altogether.

And they further discovered that a species diatom, a lake aUlgae, that was relatively rare before the 20th century, has made unprecedented gains in recent decades, possibly due to the thinning ice cover on the lake.

"Our results show that the human footprint is overpowering long-standing natural processes even in remote Arctic regions," said study co-author John Smol, of Canada's Queen's University in Kingston, Ontario.

Another study published September in Science magazine that reconstructed 2,000 years of Arctic temperatures from ice and lake sediment cores and tree rings, found that the recent global warming trend is overriding a natural cooling trend caused by Earth's periodic wobble.

The Earth is now some 600,000 miles (966,000 kilometers) further from the sun during the Northern Hemisphere summer solstice than it was at the time of Jesus Christ, causing an overall cooling of the Arctic until recently, explained the researchers.

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