Climatologist Darrell S. Kaufman from Northern Arizona University used tree rings, lake sediments, ice cores and other samples to reveal what temperatures were like on a decade-by-decade basis for the past 2,000 years.

The report, published in the journal Science, says that even though changes in Earth’s orbit have caused the Arctic to receive progressively less energy from the summer sun over the past 8,000 years, temperatures reversed a cooling trend in the mid-1800s and began to climb as the Industrial Age began.

Study results document that the warming accelerated in 1950 as industrial production of greenhouse gases expanded worldwide.

All three natural sources of climate information point to the conclusion that had such pollution not been mixed into the atmosphere, the Arctic would have continued to cool as the North Pole moved farther away from the sun during the summer months.

A 21,000-year cycle in Earth’s orbit has caused the Arctic to be 600,000 miles farther from the sun at that time of year than it was 2,000 years ago. It will be another 4,000 years before the trend reverses.

"The amount of energy we're getting from the sun in the 20th century continued to go down, but the temperature went up higher than anything we've seen in the last 2,000 years," said team member Nicholas P. McKay of The University of Arizona in Tucson.

Photo: Alexei Novikov - Fotolia