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.
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5 Comments
Add CommentI wonder when those educated people figure out what dumb mountain people know. Glaciers are created by heavy snow falls. The ebb and flow from ice sheets is basicly a balance of variable snow fall to average melt. As temperature average changes very little and snow falls are highly variable due to changes in jet stream and ocean currents, ice sheets in one area will grow as an other nearby will shrink, or a warmer area will get rain and a snow area will be starved.
Reply | Report Abuse | Link to thisThe previous writer is working with only half the factors of the equation. Yes, glaciers grow or shrink depending on snowfall. A corollary to that is that snowfall can increase even as temperature goes up. But the writer is incorrect when he or she says that temperatures change "very little." In fact, temperatures are ascending rapidly, and this effect is especially powerful in alpine regions. This introduces the second factor in the equation: as temperatures go up the melting of snow in warm seasons increases. This decreases the amount of snow and ice in the glacier. The question is whetehr increased snowfall or increased temperature has the greater effect. At this time, and for the past century, glaciers have been retreating dramatically worldwide, because whateve the snowfall is doing, temperature is doing more: these great ice masses are melting. They are going away. About the sole exception is a small part of northern Norway, where higher temperatures have produced so much more evaporation, and thus more preciptiation (mainly in the form of snow) that glaciers there are actually growing. Everywhere else, they are shrinking. This is not an attempt at hocus-pocus: it is the way it is. Gobal warming is real, it is present, and it is having observable effects. If you choose not to believe the evidence, so be it.
Reply | Report Abuse | Link to thisClimate is an enormously complex process that has been effected by many abrupt cataclysmic events, as well as a myriad number of more subtle changes. The next major cataclysm is well underway with the 6 billion + people polluting our planet to oblivion. Our only hope is that the rest of the world will not follow the USA's consumptive example, and the USA in turn, shakes off it's political umbrella of self indulgent need for re-election rather than responsible oversight and buffering of a greedy society.
Reply | Report Abuse | Link to thisRecently, the global climate change issue is becoming an incressingly hot topic among those nation leaders and any cultured people. As a matter of fact, we human beings can no longer to ignore the effect of the even a small fluctuation of the temperature that caused by the activities took out by about six billion people on the earth or many other factors, such as radiation from outer space. It is truly that scientists can more or less trace the climate change pattern happened in the past centuries, either from tree rings or ice cores or the chemical element beryllium 10, and maybe this kind of information will give us some valuable suggestions to deal with the modern climate problem. But, in my view, only lessons can be drawm from the past temperature change pattern, it will not fit into the recent climate change situation. We are just at the starting point to investigate and study the climate change issue we faced now.
Reply | Report Abuse | Link to thisIf I read this article correctly, it seemed to indicate that, at the time of its writing, glaciers in the Northern hemisphere were retreating, but glaciers in the Southern hemisphere were advancing, but not as a result of axial movement.
Reply | Report Abuse | Link to thisNot to be picky, but if this assessment has held up it would seem that the current climate change should not technically be referred to as global warming...