Glaciers in the Southern Hemisphere have not shown the same climate history as glaciers in the Northern Hemispere.
Both Northern and Southern hemispheres are heating up with global warming.
Yet scientists see differences in climate patterns between the hemispheres in past ages.
Their research, presented at the recent Comer Conference on abrupt climate change, underscores the unprecedented nature of our current climate change.
Rapid warming is causing the unusual match of conditions.
The climate of the hemispheres have not matched “on long time scales,” said Aaron Putnam, a post-doc at Lamont-Doherty Earth Observatory of Columbia University who studies glaciers in New Zealand.
“So the only time where we actually see synchrony between the hemispheres over at least the last 10,000 years – if not much farther back than that – is during the last century.”
And that synchrony consists of rapid warming.
Researchers showed how the Northern Hemisphere experienced climate fluctuations from the last ice age, about 12,000 years ago, to the generally warm period of the Holocene era, roughly the past 10,000 years. Scientists are now ramping up research on climate change in the Southern Hemisphere during the same period.
“One of the big questions was, have these changes that we've noticed in the Northern hemisphere” been global changes? Putnam said. “And that's really important to understand.”
Climate scientists focus on glaciers in their research as glaciers are key indicators of climate.
“We're looking to see how the glaciers have changed in the past, and we use that to learn how the climate's changed in the past because the climate controls the glaciers,” said Mike Kaplan, a glacial geologist at the Lamont-Doherty Earth Observatory at Columbia University. He studies the glaciers of Patagonia, a region in southern South America including parts of Argentina and Chile, and maps glacial retreats with cosmogenic dating.
“Glaciers leave a record on the landscape of where they've been,” Kaplan explained, helping scientists to measure their more dramatic retreat in past decades.
When glaciers retreat, they leave a ring of sediment and rocks called a moraine. An analogy can be made to the rings on a bathtub as the water slowly drains. Scientists measure the ages of those rings using cosmogenic and radiocarbon dating to determine when the glacier advanced and retreated. Mapping out when a glacier advances or retreats gives scientists a good idea of the climate change during a time period.
“It's like a proxy for what past climate has done,” Kaplan noted.
With the knowledge of past climate behavior, scientists can better predict future climate behavior.
Climate in the Northern Hemisphere of the past thousand years can be described as a “hockey stick,” with temperatures gradually going down, with some fluctuations, into the cool period of the Little Ice Age, roughly 1400 – 1850 AD, and then suddenly shooting up in the last 100 years.
In the Northern Hemisphere, glaciers have remained relatively small throughout the Holocene, though they spread during the Little Ice Age.
“In the Southern Hemisphere, in South America – Argentina and Chile – and New Zealand, we don't see that pattern,” Kaplan said.
“We see a different pattern.”
Kaplan found that in Patagonia and Lago Argentino, the period that glaciers stretched to their largest during the Holocene was between 6,000 and 4,000 years ago, not during the Little Ice Age.
Glaciers advance and retreat on a regular basis, but in the Southern Hemisphere glaciers have retreated farther in each cycle, while in the Northern Hemisphere glaciers have in general steadily advanced.
John Mercer, a pioneer glacial geologist at Ohio State University, proposed this hypothesis back in 1968. But it wasn't until the last 10 years that an improved form of dating using an isotope called beryllium-10 allowed scientists to accurately date the glacial changes and confirm the hypothesis.
Aaron Putnam is one of those scientists and confirmed the hemispheric differences when he studied the Southern Alps of New Zealand.
In Europe, the Medieval Warm Period allowed Vikings to sail across the Atlantic Ocean and settle in Greenland, while during the Little Ice Age people could safely skate across the River Thames in England. However, the Southern Hemisphere showed a very different picture.
“[The New Zealand glaciers] were more extensive during Medieval times implying colder conditions, and they were retreating during the Little Ice Age, implying warming conditions,” Putnam said. “This asynchrony seems to go back through the last 10,000 years.”
Putnam stresses the importance of the evidence that the hemisphere climates haven't synchronized. “One of the main arguments against the present warming being due to human cause is people think it's related, it's just part of the cycle,” Putnam said.
But with these new findings, scientists see that the natural cycle consists of contrasting climate between the hemispheres – not synchrony. The present trend of simultaneous warming between the hemispheres is a break from observed pattern, and that is what is alarming.
“We think it's most likely due to atmospheric CO2 rise,” Putnam concludes.
But what accounts for this past difference in climate patterns between the hemispheres?
“That, we're still trying to figure out,” Kaplan said.
Scientists are still in the process of collecting data to see what exactly the differences are between the hemispheres.
Ideas about possible causes include differences in ocean currents and differences in insolation, which is the amount of energy received from the sun. The hemispheres are anti-phase in terms of insolation: because of the tilt of Earth's axis, when one hemisphere receives more energy from the sun, the other hemisphere receives less.
The insolation-based theory works well in part. However, it runs into some problems in southern South America. By its predictions, glaciers in the region should have been at their maximum around 10,000 years ago. However, they were at their maximum 6,000-4,000 years ago.
An influential factor could be that southern South America is much closer to Antarctica, a significant mass of ice, which could exert a significant force on its climate.
“There are reasons to suspect Antarctic climate to be different than European climate,” Kaplan said. “It's affected by different natural climate variability, [such as] different ocean processes.”
With man-induced global warming, it may be that the days of natural climate variability are numbered.