By Julia Rosen
A shard of ice sits on the black surface of the lab desk, buoyed in a growing puddle. Three small heads hover above in a tight huddle. “It’s cold,” notes one of the kids. Somehow, this obvious observation always catches me off guard, as if I’ve forgotten the most fundamental quality of water’s solid phase. “That’s true,” I reply, “it’s also 10,000 years old.”
“Wow!” the students chorus, and their eyes widen as they look again with renewed awe at this innocuous specimen that could have come from an ice-cube tray in their freezer. Whether I am visiting loquacious third-graders or shyly curious middle-schoolers, I am always touched by the unjaded willingness of youth to imagine and attempt to grasp the unseen. It’s the reason every scientist falls in love with science.
I analyze ice cores in the Oregon State University Ice Core Laboratory and no longer think about their cool touch. I have learned that, like people, the most interesting things about them lie hidden inside. And, like people, it takes time and patience to understand them. When we succeed, these frozen time capsules from Greenland and Antarctica allow us to reconstruct climate far into the past so that by understanding its natural rhythms and quirks, we can predict what kind of future awaits these students.
But let’s start with the obvious: a clear, smooth cylinder of ice glittering with tiny bubbles like a flute of frozen champagne. Stunningly boring to behold, only an occasional band of volcanic ash or the subtle cloudy layers formed during dusty polar winters break its translucent monotony. However, this continuity is actually an ice core’s greatest strength. It provides a complete, unbroken record of past climates, one that is unavailable in almost any other natural archive.
As detectives of Earth’s history, geologists reconstruct stories from snapshots of ancient seas and whispers of long-dead creatures, piecing together a hazy story of our planet’s past. Ice cores are the long-lost diaries of climate. Every day, they recorded the temperature, sniffed the air and noted the snowfall. They sensed changes far from their polar homes — the amount of dust lofted from Asia, the gurgle of tropical volcanoes and much more. From the top to the bottom of a core lie flakes that witnessed every moment of geologic time that elapsed in between.
Physicists, chemists and geologists have spent 60 years learning to translate the primordial language of ice. Early pioneers of ice-core science discovered that they could estimate temperature using the chemistry of rain and snow. As the air warms, precipitation gathers more heavy molecules and fewer light molecules (known as isotopes) of water. The ratio of these isotopes thus provides a record of temperature. These scientists had the transformative idea of using old ice to reconstruct climate by exploiting this valuable relationship.
Each new analytical tool that becomes available to scientists provides another Rosetta Stone for decoding long-lost archives of the ice. Today, we can measure trace amounts of chemical impurities deposited on the ice sheets as dust and aerosols. They tell us how sea ice waxed and waned and which way the wind blew. They reveal the fingerprints of individual volcanic eruptions. While only the pristine inner core provides suitably clean ice for these highly sensitive measurements, the “snow dust” from cutting and cleaning the core does not go to waste. It can be used, for example, to reconstruct concentrations of a rare element, beryllium-10. Produced by cosmic rays high in the atmosphere, the abundance of this element reflects shifts in solar radiation.
Of all the stories that ice cores tell, however, the bubbles of air embedded within them actually contain the most impressive secrets. As snow accumulated over thousands of years, slowly hardening into solid ice and forming the massive polar ice sheets, it sealed off little breaths of ancient air between the grains of snow — the very same air we would have inhaled if we had stood on top of the ice sheet 8,000 years ago, or 80,000 or 800,000. From those microscopic samples, we can retrace the evolution of our planet’s atmosphere across almost a million years of Earth history, a period that encompasses nearly all of human existence.
In Antarctica, where extreme cold and meager snowfall limit the flow of ice, these cores stretch back across eight glacial cycles. During each, the Earth oscillated between periods of cold climate and expansive ice, including a vast glacial blanket that smothered northern North America, and a time of balmy warmth with ice sheets comparable in size to those on Earth today. Wobbles in the planet’s orbit periodically brought it closer to and farther from the sun’s furnace, setting the rhythm of the climatic metronome.
Across these dramatic changes, carbon dioxide and other greenhouse gases rose and fell with the global temperature as the Earth’s oceans and biosphere adjusted to a changing environment. These gases both responded to climate change and amplified it through their potent ability to trap the Earth’s outgoing energy. But never in the past 800,000 years did these gases reach concentrations even remotely approaching current levels, and never did they rise so quickly, or shoot up at the end of an interglacial period when the receding sun should have lulled the Earth back into an icy slumber.
At the other pole, ice cores in Greenland felt those same changes, although the records of climate before 120,000 years ago crept away through the unstoppable march of glaciers to the sea. Nonetheless, these cores tell us something else completely new. Throughout the last cold period on Earth, which our ancestors waited out in the mild climates of Africa, the Northern Hemisphere experienced a barrage of climate changes so swift and so huge that certain places on Earth warmed by 20 degrees Fahrenheit in a matter of decades. The cause of these dramatic jolts remains a mystery, but their power to radically reorganize the Earth system attests to the inherent volatility of the world in which modern civilization has only recently made a home.
We are slowly beginning to understand the anatomy of global climate and how it changes, its geographic fingerprint and its tempo. Ice cores paint a complex and sometimes surprising picture, one that generations of scientists will spend decades trying to fully understand. We now know the correct greenhouse gas concentrations to feed into our calculations as we simulate past climates in order to validate models for the future.
Ice cores have made one thing abundantly clear: Humans are in uncharted territory. In 800 millennia of records, no entries document a climate like the one we live in today. Even as you read this, we are busy writing the next page of the ice-core diaries.
Time to Listen
These observations from opposite poles forewarn a perilous future for our planet. We know without question that we’ve entered a period in geologic history for which there is no natural analog, and we know that the Earth’s climate can respond dramatically to perhaps even the smallest nudge.
However, the most terrifying lesson I learned from ice cores did not come from drilling into the past, but from just standing on the surface. At 80 degrees North, well above the Arctic Circle in the empty white wilds of the Greenland ice sheet, I watched a supply plane on skis repeatedly try to lift off. First the crew dumped cargo and then off-loaded all their fuel except what they needed to get home. Finally, on their seventh attempt, they succeeded.
The problem? The snow had warmed to the freezing point, and microscopic drops of water on the surface made the friction between the skis and the ice too great to break. Last summer, 97 percent of the surface of Greenland experienced temperatures above freezing, more than any year in NASA’s 30 years of satellite observations.
The ice cores have told us all they know, and now it’s up to us to listen.
Editor’s note: Julia Rosen is working toward her Ph.D. in the Oregon State University Ice Core Laboratory under the guidance of Ed Brook, professor in the College of Earth, Ocean, and Atmospheric Sciences and a Fellow of the American Association for the Advancement of Science. Support for the lab has come from the National Science Foundation’s Office of Polar Programs.
For more information:
Antarctic Ice Core Contains Unrivaled Detail of Past Climate, (Feb. 5, 2013)