Thursday, May 17th 2012, 5:49 AM EDT
The Little Ice Age, a cooler period of the Earth's recent history, may have been a global event rather than confined to the northern hemisphere, according to a paper published last week.
Some climate skeptic blogs are excited about this finding, and have taken it as a starting point to argue that the rise in global surface temperature over the last few decades is just a natural recovery from the Little Ice Age, implying that it has nothing to do with, for example, man-made emissions of carbon dioxide. We asked one of the co-authors of the paper whether this is a valid interpretation.
The Little Ice Age ( LIA) was a period of colder than average temperatures and glacier advance between 16th and mid-19th centuries. This new paper adds to evidence suggesting that the LIA was a global event, rather than a regional event - something that is still under debate by scientists.
It documents temperature on the West Antarctic Ice Sheet dipped to around half a degree Celsius cooler than average between the years 1400 and 1800, according to a borehole record. This roughly coincides with a more pronounced cooling observed in records from Greenland, suggesting that the cooling was probably global in scale.
Because it suggests a global effect, this paper lends weight to theories that waning solar activity (the LIA coincides with a 70 year period of low solar activity called the 'Maunder Minimum') and/or climate-cooling volcanic emissions caused the LIA, rather than changes to oceanic currents.
Although the paper hasn't received any media coverage that we can see, climate skeptic bloggers highlighted the research, saying that its findings "imply that the 0.7C global warming since 1850 simply represents a recovery from the Little Ice Age."
We spoke to Jeffrey Severinghaus, Professor of Geosciences at the University of California's Scripps Institution of Oceanography, to find out whether he agrees with this interpretation of his work. He calls such interpretations a "red herring", saying that whilst the warming since 1850 is "in part due to the recovery from the Little Ice Age", that doesn't hold true for warming of the last 50 years, which is a "very different animal" from the warming of the last 160 years.
Severinghaus explained to us that the LIA is thought to have ended because:
"The Sun underwent an increase in its energy output, especially in the ultraviolet part of the spectrum, at the end of the Little Ice Age, and there was a lull in major volcanic eruptions."
So the warming between 1850 to 1950 was:
"consistent with warming caused by increases in solar output and decreases in climate-cooling volcanism."
"no such increase in solar output has occurred in the past 50 years, and volcanism has not changed significantly. These facts are undisputed. So natural factors cannot explain the warming of the past 50 years."
So in recent decades, the planet has warmed in a different way:
"The warming of the past 50 years has been mostly at night, and it has been accompanied by profound stratospheric cooling, both "fingerprints" of an unnatural warming."
As we have pointed out in previous blogs, only considering the surface temperature record does not tell the whole story about how much the Earth's climate system has warmed. And, as Severinghaus explained to us, assuming scientists base global warming theory solely on the surface temperature record is not sound. He told us:
"The real basis for prediction of future warming is that basic physics tells us that carbon dioxide absorbs heat, and human emissions of this gas will cause (and are causing) a rise in its atmospheric concentration. These facts are also undisputed."
Severinghaus also cautioned against another common misconception frequently promoted by climate skeptic commentators:
"We have long known that climate can change for natural reasons. But to assert that this implies that humans cannot also change the climate, is like saying that forest fires cannot be set by careless campers because lightning also causes forest fires. It is simply illogical. Climate change has multiple causes."
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