Average temperatures in the Northern Hemisphere over a 21-year period until 2010 may have been the highest over the past 4,000 years, according to a study of air trapped in an ice sheet for several millennium by an international research group.
The results of the study, by the group that included scientists from the National Institute of Polar Research as well as China and Denmark, have been posted in Climate of the Past, a journal of the European Geosciences Union.
"We learned just how small the changes caused by nature were and just how large the changes caused by mankind were," said Takuro Kobashi, a project assistant professor at the National Institute of Polar Research, who was part of the research group.
The research group analyzed air that had been trapped in an ice sheet at a depth of about 900 meters in Greenland to unravel the climate change mechanism in this rapidly warming country, located within the Arctic Circle, over the past 4,000 years.
The group also looked at changes in insolation, or solar radiation striking the planet, as a result of volcanic and solar activity as well as changes in the Earth's orbit.
Using that data, the group reconstructed temperature changes at intervals of a decade for the entire Northern Hemisphere.
As a result, the group found that the average temperature in the Northern Hemisphere between 1990 and 2010 was 0.71 degrees higher than the average temperature for the past 4,000 years. That led to the possibility that the 21-year period may have had the highest average temperature over those four millennium.
Looking at changes over the entire 4,000 years, the average temperature for much of that period fell in a range of plus/minus 0.16 degrees.
In recent years, with global warming suspected to be the culprit, the ice sheets in Greenland have been rapidly decreasing in size. The melted ice causes sea levels to rise and has had an effect on the climate in the Northern Hemisphere.
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