Snow is a simple affair to most of us – part recreation, part inconvenience, always temporary. But at the poles of our planet, snow acts as a precise and tireless historian, archiving snapshots of bygone climates kept in frozen file cabinets several times taller than the Empire State Building.
On the Greenland and Antarctic ice sheets and some mountains, fallen snow does not fully melt in the summer. Instead, it deposits layers of data year after year, building an invaluable resource for scientists who, over the past few decades, have sought to place industrial civilization in a larger climatic context. Our deepest polar file cabinet, from an Antarctic summit, reaches back 800,000 years – several times older than our species. That’s still only a sliver of Earth’s 4.5 billion year history, but it goes far beyond the scope of our technology for measuring the climate, which has captured only the past 160 years of global temperatures and 60 years of carbon dioxide levels.
Paleoclimatologists extract the files, compressed into hard ice by the weight of newer snow, by drilling a metal tube into a glacier and pulling up an ice core in sections a few meters long and 5 inches wide. A complete core from the ice sheets can be nearly two miles long. Ice cores from mountain glaciers closer to the equator are much shorter.
Some of the data are immediately apparent. At the younger end of the ice core, the seasons show up in layers that can be counted like tree rings. A volcanic explosion 55,000 years ago appears in a Greenland ice core as a brown slice of ash punctuating the translucent cylinder.
In the laboratory, scientists break open the tiny bubbles that populate the ice cores and measure the chemical composition of their contents. It’s due to these capsules of ancient air that we know current CO2 concentrations in the atmosphere are now higher, and rising faster, than at any point in the last 800,000 years. Because air can still move through snow until the layers become compressed enough to solidify, the climate captured in those bubbles is younger than ice that surrounds them. Paleoclimatologists adjust their calculations accordingly, accounting for the rate of snow accumulation at the drilling site.
The ice cores offer even more information to those who can read messages hidden in chemical code. Since the ratio of light and heavy hydrogen and oxygen isotopes in water molecules depends in part on temperature, scientists can build a temperature record from the ice. This record is then joined to ice core data from other parts of the planet, along with information from tree rings, corals and sediments, to form a global picture of the past.
An important piece of that picture is the ice cores from mountains in subpolar and mid-latitude regions, including the Andes and the Himalayas. After all, most living things reside far from the poles. But these stores of ice are shrinking under the pressure of rapid global warming. With the decline of mountain glaciers, we face not only irrevocably altered ecosystems and higher seas, but also the loss of a treasure trove of natural archives.