An analysis by Scripps Institution of Oceanography, UC San Diego researchers contradicts a common assumption among scientists that the Little Ice Age was strictly a Eurocentric affair.
The period between 1400 and 1850 was marked by an average temperature drop of just less than 1 degree C (1.8 degrees F), but not just in the Northern Hemisphere. Scripps graduate student Anais Orsi and colleagues found evidence of the same cooling trend in samples of ice from the West Antarctic Ice Sheet. That contradicts prevailing theories that the Little Ice Age was not globally synchronized, but a regional cooling possibly triggered by changes in ocean circulation that created a temperature see-saw effect between the hemispheres.
The authors wrote in the journal Geophysical Research Letters that reduced solar activity and atmosphere-dimming volcanic activity are more likely sources of the cooling that appears now to have been more synchronized.
Orsi analyzed data from the West Antarctic Ice Sheet Divide as part of a project that is giving researchers a chance to study 80,000 years of climate history preserved in cores of ice taken from as deep as 3,500 meters (11,480 feet) below the ice sheet surface. With Scripps oceanographer Bruce Cornuelle and geoscientist Jeff Severinghaus, she measured temperature down a 300-meter (984-foot) long borehole containing ice formed in the last millennium.
The ice is a good insulator, and does not adjust its temperature very quickly. Down deep in the ice sheet, the ice still retains information about the temperature it was at when the snow fell. It enabled Orsi to simply take direct temperature readings in the hole left over by a drilling project, and create a record of hundreds of years of climate.
The team found that temperatures fell an average of 0.52 degrees C (0.94 degrees F) between 1400 and 1800.
The study has set off a flurry of interpretations in the climate change blogosphere, since its May 9 publication. Some climate change skeptics have seized on the Scripps temperature record as support for the idea that increases in temperature over the last 150 years are merely evidence of a recovery from the cold Little Ice Age and not of a human-caused global warming. Others have countered that assertion, pointing to the portion of Orsi’s study that notes warming in the West Antarctic Ice Sheet has accelerated to 0.8 degrees C (1.4 degrees F) per decade in the past 20 years.
For her own part, Orsi said her study does not address or take a position on debates about the causes of global warming. To her, the more important takeaway message is that there is a need for more global temperature records. Historically, she said, temperature records have had a parochial character, most having been taken in the proximity of Northern Hemisphere population centers. Data from the Southern Hemisphere are also necessary for scientists to find the fingerprint of climate changes throughout history and understand what has caused those changes. Without global data, the task of distinguishing causes of observed climate changes is much more difficult, she said.
“It’s really important to have records of the history of climate everywhere in the world, not just where you live,” she said. “The rest of the world needs a little attention too.”
— Robert Monroe
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