A bipolar seesaw
How do you have an Ice Age and warming at the same time? As wacky as the climate patterns on Earth are right now, they’ve been stranger in human history.
A paper in the journal Nature this week draws attention to the Younger Dryas conundrum about 13,000 years ago, a period of abrupt climate change. The last Ice Age was ending, owing to a shift in the Earth’s orientation to the sun, bringing us into the modern, temperate period that sustained the rise of civilizations. But the global warming of then was pretty a bumpy ride. There was simultaneous warming and cooling on the planet at the same time.
A cold blast hit northern Europe and froze it again for another thousand years. Meanwhile, the polar South continued heating up and glaciers retreated. The Younger Dryas, named after a white flower that grows in the Arctic, was not a global event.
The research, undertaken by Columbia University’s Lamon-Doherty Earth Observatory, tracked the retreat of a New Zealand glacier by examining the exposed rocks, called moraines. “Whereas North Atlantic mean annual temperatures dropped drastically, by at least 15â€‰Â°C, atmospheric temperatures in the southern mid latitudes increased during this period,” the researchers write.
They go on to say that a classic explanation for the variance is the “bipolar seesaw mechanism,” essentially the shut-off of the Atlantic Gulf Stream that brought colder temperatures to the North and heat retention in the South. Another possible cause is a shift in the intertropical convergence zone and westerly wind patterns southwards, “which has been shown to increase Southern Ocean upwelling and outgassing of CO2 abruptly.”
The answer remains elusive whether rising CO2 levels alone could have caused the South to warm, or whether it needed another mechanism to move it forward, like the the bipolar seesaw.
Whatever happened, it’s a lesson that global warming is never the same everywhere.