Aptly named for its location behind a ball field in New York City's Central Park, Umpire Rock may offer a useful vantage point for calling balls and strikes. For scientists, however, it has served as a speed gun for calculating the trajectory and timing of an ancient glacier that once played an active role in global climate change.

"The Laurentide coughs and the climate will change," says Joerg Schaefer, a geochemist at Columbia University's Lamont–Doherty Earth Observatory.

Schaefer refers to the Laurentide Ice Sheet that covered the island of Manhattan, along with the northern third of the U.S. and most of Canada, until about 18,000 years ago when the great glacier finally retreated. It had spent more than 70,000 years affecting and reflecting the world's weather through periods of melting and growth. Today, only carved terrain and rocky remnants remain, including the popular leftover that lies a short walk east of West 62nd Street.

Umpire Rock is just one of many enormous boulders—from Antarctica to New Zealand—created under the weight and movement of glacial ice. With increasingly sophisticated techniques, Schaefer and other scientists are more closely studying the chemical footprints on these rocks, thereby gaining valuable insights into climate change.

"Glaciers are great climate indicators," says Richard Alley, a glaciologist at The Pennsylvania State University, in an e-mail. "Although they respond to many things (such as increased snowfall, for example), they are controlled primarily by changes in summer temperatures. Warming melts ice—and it is almost that simple."

A glacier's response to climate can be very sensitive to even the slightest temperature fluctuations: "Just one degree [Celsius] has forced glaciers to move back and forth," Schaefer says. As researchers determine precisely when and where glaciers have advanced and retreated, they can add to a global map of summer temperatures during the Holocene epoch, which spans from 10,000 years ago, after the last ice age, to today. Schaefer thinks a better understanding of variations in this era could fill in missing key predictors for Earth's future climate. "We can only evaluate how dramatic changes will be if we know this natural variability," Schaefer says. "Everything humans do to the climate now is on top of this."

Shedding new light on the climate
Schaefer and other scientists are using a combination of old stonemason tools and cutting-edge machinery, along with some basic geochemistry, to decipher and date this distribution of ancient temperatures.

When a glacier starts its retreat, it exposes the surface it had entombed to daylight. This accumulation of debris, known as a moraine, is often identified when geologists see large boulders that were formed as a result of sediment compacted over thousands of years under a glacier's weight. Cosmic rays zipping through Earth's atmosphere begin bombarding these previously protected rocks at the moment they are coughed out from under the receding ice sheet. The collisions trigger chemical reactions that form a unique radioactive element, beryllium 10. Scientists can then count these newborn nuclides, or isotopes (an element whose atoms have a different number of neutrons, giving it slightly different properties from the common variety), using a technique known as cosmogenic dating, to determine exactly when the glacier pulled back and the bombardment began. "The glory of cosmogenic dating is that you can ask a rock how long it has been sitting there being zapped by cosmic rays," Alley notes.

To get an answer from the rock, the nuclide is isolated from a chiseled sample—about the top centimeter on its surface—and then analyzed via mass spectrometry. With chemists' ability to isolate pure beryllium 10 and spectrometric sophistication growing over the last decade, Schaefer says, "tiny, tiny amounts of these nuclides can now be measured with very high precision."

The total count is then compared with the background rate at which the beryllium 10 is naturally produced in the atmosphere, and a precise age is derived. In the best cases, Schaefer says, a date can now be stamped with an error of plus or minus 10 years. (Only one mass spectrometer accelerator, located at the Lawrence Livermore National Laboratory in California, is currently powerful enough to provide these kinds of estimates.)

Cosmogenic dating has been around since the 1980s, but this improved sensitivity is allowing the possibility of dating much younger rocks that have had less time to accumulate beryllium 10. "The isotope records now overlap our historic record," Schaefer says. "This opens up a climate record that we couldn't read out before."

Tree rings, ice cores and marine life also offer glimpses into climate's past, yet they are not always available, nor always precise. "You could get lucky and run into a tree stump, but most glaciers leave deposits that are always traceable by this technique," says John Stone, a geochemist at the University of Washington in Seattle who uses cosmogenic methods for glaciers in Antarctica. "This allows a wide variety of glacial records to be obtained."

Global data set in stone
Schaefer wants to see the improved dating method taken around the world, and has been chiseling rocks himself from New York to New Zealand. He is the lead author of a study published in the May 7, 2009, issue of Science that investigated the ebbs and flows of glaciers in New Zealand's Southern Alps.

Unlike the majority of the world's glaciers, some in this region are advancing, rather than retreating. But hope that this glacial behavior means a less dire global climate predicament is arrested by Schaefer's findings: Glaciers in the Northern Hemisphere seem to behave independently from those in the south, and these "growing glaciers" have been expanding at an increasingly slower rate over the last several thousand years.

5 Comments

1. 1. pgtruspace 07:14 PM 9/24/09

   I 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.

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2. 2. kkrajick 09:55 PM 9/24/09

   The 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.
   

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3. 3. kilingtonskier 10:10 PM 9/26/09

   Climate 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.

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4. 4. guojiubiao 08:31 AM 9/28/09

   Recently, 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.

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5. 5. jtdwyer 10:26 PM 3/5/10

   If 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.
   
   Not 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...

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