Paleo-climate data suggests region should be cooling, but greenhouse gas forcing has overpowered nature pattern
By Summit Voice
SUMMIT COUNTY — Without ever-increasing concentrations of heat-trapping greenhouse gases in the atmosphere, the Arctic would gradually be cooling instead of experiencing the rapid warming that’s been documented in the past few decades.
The long-term cooling trend, documented back to at least 2,000 years ago, is related to wobbles in the Earth’s orbit that have reduced the intensity of sunlight reaching the Arctic in summertime, when Earth is farther from the Sun, according to a recent study led by scientists from Northern Arizona University and the National Center for Atmospheric Research.
But in the middle of the 20th century, the gradual cooling ended abruptly, replaced by a sharp increase in Arctic temperatures — even though orbital cycles would suggest a continued cooling trend. The research, based on geologic records and computer models, strongly suggests that the heat-trapping effects of greenhouse gases are overpowering natural climate cycles.
“This result is particularly important because the Arctic, perhaps more than any other region on Earth, is facing dramatic impacts from climate change,” said NCAR scientist David Schneider. “This study provides us with a long-term record that reveals how greenhouse gases from human activities are overwhelming the Arctic’s natural climate system.”
In the 1990s, Arctic temperatures reached their warmest level of any decade in at least 2,000 years. The results indicate that recent warming is more anomalous than previously documented, said Northern Arizona University’s Darrell Kaufman, lead author and head of the synthesis project.
The scientists reconstructed summer temperatures across the Arctic over the last 2,000 years by decade, extending a view of climate far beyond the 400 years of Arctic-wide records previously available at that level of detail.
“Scientists have known for a while that the current period of warming was preceded by a long-term cooling trend,” Kaufman said. “But our reconstruction quantifies the cooling with greater certainty than before.”
The study quantifies a pervasive cooling across the Arctic on a decade-by-decade basis that is related to an approximately 21,000-year cyclical wobble in Earth’s tilt relative to the Sun. Over the last 7,000 years, the timing of Earth’s closest pass by the Sun has shifted from September to January.
The research team’s temperature analysis shows that summer temperatures in the Arctic, in step with the reduced energy from the Sun, cooled at an average rate of about 0.2 degrees Celsius (about .36 degrees Fahrenheit) per thousand years. The temperatures eventually bottomed out during the “Little Ice Age,” a period of widespread cooling that lasted roughly from the 16th to the mid-19th centuries.
Even though the orbital cycle that produced the cooling continued, it was overwhelmed in the 20th century by human-induced warming. The result was summer temperatures in the Arctic by the year 2000 that were about 1.4 degrees Celsius (2.5 degrees fahrenheit) higher than would have been expected from the continued cyclical cooling alone.
“If it hadn’t been for the increase in human-produced greenhouse gases, summer temperatures in the Arctic should have cooled gradually over the last century,” said Bette Otto-Bliesner, an NCAR scientist who participated in the study.
The historical 2,000-year temperature trend was reconstructed from sediment evidence in Arctic lakes that show the length of the growing season, and from glacial ice and tree-ring records.
The data from those field observations was tested with a climate model that factored in the reduction of seasonal sunlight in the Arctic due to the shift in the Earth’s orbit.
The results give scientists more confidence in computer projections of future Arctic temperatures.
“This study provides a clear example of how increased greenhouse gases are now changing our climate, ending at least 2,000 years of Arctic cooling,” said NCAR scientist Caspar Ammann, a co-author.
The new study follows previous work showing that temperatures over the last century warmed almost three times faster in the Arctic than elsewhere in the Northern Hemisphere. This phenomenon, called Arctic amplification, occurs as highly reflective Arctic ice and snow melt away, allowing dark land and exposed ocean to absorb more sunlight.
“Because we know that the processes responsible for past Arctic amplification are still operating, we can anticipate that it will continue into the next century,” says Gifford Miller of the University of Colorado at Boulder, a member of the study team. “Consequently, Arctic warming will continue to exceed temperature increases in the rest of the Northern Hemisphere, resulting in accelerated loss of land ice and an increased rate of sea level rise, with global consequences.”
The international study, led by Northern Arizona University and the National Center for Atmospheric Research (NCAR), will be published in the September 4 edition of Science. It was primarily funded by the National Science Foundation, NCAR’s sponsor.