Conventional wisdom has held that climate fluctuations during the Holocene have been fairly stable and consistent around the globe. Previous work by Schaefer, published in Science in 2006, showed that before the end of the last ice age, about 17,000 years ago, most of the world began warming more or less simultaneously, with the exception of the North Atlantic.* His research nearly three years later, however, paints a much more complicated picture: The rates at which the glaciers were retreating in each hemisphere fell out of synch in the past 7,000 years, and underwent rapid periods of growth and decline. Ice sheets in New Zealand appeared to reach their maximum post–ice age advance some 6,500 years ago, whereas in the Swiss Alps glaciers didn't peak until sometime between 700 years and 150 years ago—during the Little Ice Age.
This surprising difference didn't fit with either of the prevailing theories for planetary or hemispheric climate drivers. Cyclical variations in Earth's axial tilt and orbit relative to the sun are thought to control global climate changes, including the advance and retreat of major ice ages. And inverse climatic patterns between the hemispheres are thought to be triggered by ocean currents: As more fresh water is deposited in the north with melting glaciers, the "great ocean conveyor belt" slows down its delivery of warm water to the Southern Ocean. But neither this phasing nor anti-phasing fit the data. "In New Zealand, the influence is from neither an orbit nor the ocean, so we have to come up with something else," Schaefer says. One new theory points to the influence of regional shifts in wind patterns, like those that blow in with the Interdecadal Pacific Oscillation.
After scientists chiseled into moraines in the northern and southern latitudes, the next obvious site was around Earth's equator. The question there: Do tropical glaciers act like those in northern or southern regions? According to a new study in the tropics led by a team at the University of New Hampshire, past fluctuations of tropical glaciers near Machu Picchu in Peru match up very closely with behavior in the Northern Hemisphere. The Little Ice Age, which was known to have expanded glaciers in North America as well as Europe, appeared to chill the climate at least as far south as Peru. Schaefer is a co-author on this cosmogenic dating paper published in the September 25 issue of Science.
This finding sparks even more questions: What are the mechanisms for this link? Where, between Peru and New Zealand, is the link lost? The research does at least offer hope for finding answers: "These two studies [from New Zealand and Peru] primarily prove that we now have the tools in hand to decipher an almost untapped climate record: glacier moraines," Schaefer says.
Implications from the ice
Penn State's Alley, too, is optimistic that scientists can use these perfected tools to fill in this global puzzle, glacier by glacier. "With enough of this work, we should get a full understanding of the times of warming that forced glacier retreat from the moraines. The pattern of this across the planet will tell us a lot about the mechanisms that caused the warmings of the past," Alley says. "By sorting out the causes of climate changes in the past, we should gain a better understanding of what has happened, a better ability to test our models, and ultimately better and more confident projections of what future climate might hold for us."
Back in Central Park, Schaefer has been working with his team to finish sorting data from remnants of the Laurentide Ice Sheet as well as take more measurements on far-reaching continents; he also continues to think about the consequences for humanity: "This is more than an academic problem, it's a society problem," Schaefer says, highlighting many regions' dependencies on glaciers for agriculture, water and energy.
He would like to see a greater focus on adaptations that prepare societies for the impacts of glacier fluctuations—especially as the growing global map of cosmogenic chronologies more confidently projects severe impacts. "Climate drivers are tiny, tiny, tiny," Schaefer says, also pointing out that even the most conservative warming estimate from the United Nations's Intergovernmental Panel on Climate Change, a group of leading climate scientists, is bigger than anything the Holocene has ever seen. "There's no reason to believe that glaciers aren't going to do the same thing they always do: If you make it warm they are gone."
But with a growing lineup of hard-hitting players in this field, he'll let the rocks make the final call.
*Correction (9/25/09): This sentence has been changed since posting. It originally stated that the warming began about 14,000 years ago.