March 2, 2009 -- A vast expansion of wetlands across North America may have invoked the "Little Ice Age," a two-centuries-long cold snap that gripped the northern hemisphere starting in the 1620's, according to a new study.
Scientists have argued for years over what caused the planet to abruptly cool during this time, invoking everything from volcanic eruptions to reduced solar activity to a pandemic that killed millions and effectively wiped out the Incan and Aztec empires in the New World.
The only known factors is that carbon dioxide levels in the atmosphere fell by around 10 parts per million at this time, mountain glaciers expanded, and global temperatures dropped by as much as 0.5 degrees Centigrade (0.9 degrees Fahrenheit).
Swamps, bogs and marshes may have contributed to the cooling. Their oxygen-poor waters are graveyards for carbon-rich plant matter, sequestering it away from the atmosphere.
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What's more, in a study of 144 wetland areas across North America, a team of researchers led by Sarah Finkelstein of the University of Toronto found that they expanded en masse across North America around the time of the Little Ice Age.
"We can't quantify exactly how much carbon was sequestered in wetlands at that time," Finkelstein said. "But we know it was a continent-wide effect. If you look in Europe and Asia, I'm sure you'll see the same thing going on."
She thinks that some large trigger event -- either solar activity or volcanic eruptions -- may have started the cooling and led to more rainy weather. As wetlands expanded they gobbled up carbon, which chilled the northern hemisphere further.
But William Ruddiman of the University of Virginia argued it's unlikely.
Computer simulations of climate at the time show that natural changes can only bring down CO2 levels by three to four parts per million. Instead, Ruddiman attributes much of the change to reforestation in Central and South America following the annihilation of indigenous people there.
"Perhaps it's partly natural, but I think it's anthropogenic (related to human activities)," Ruddiman said. "Ten parts per million is way larger than climate models suggest."
Even if that's true, Finkelstein argues that wetlands could play an important role in the future climate of the planet, making it crucial to preserve them.
"People don't like swamps and bogs generally; they get a bad rap," she said. "I don't think people realize the importance of this ecosystem, not just for biodiversity, either. These systems are influencing the global carbon cycle, and the climate."
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