People have asked how scientists know that today’s climate situation is unusual. Hasn’t the Earth undergone many cold and warm cycles before? Could this just be another? Buried in the world’s ice sheets is a long story of climate on Earth–and the contribution of atmospheric CO2 to warming or cooling. Scientists can access a unique and detailed look at the history of the Earth’s atmosphere through ice cores and start to understand the recent climate in context of past ones.
Looking into layers
Scientists analyze this ice record to help reconstruct Earth’s atmosphere. As snow falls and begins to settle on an ice sheet, it traps small pockets of the atmosphere. Over time, the snow gradually compresses after each snowfall until the trapped air is pressed into the structure of the ice itself. The snow layers also collect soot and dust that settle on the snow. As time passes, the ice sheet encases a year-by-year record of climates from the past.
By drilling down deep into the ice sheet, scientists access the ice and snow records in the form of cylindrical ice cores. The length of the cores varies based on where they are drilled. Ice cores taken from glaciers typically are relatively shallow, sometimes only a few yards deep but usually no more than 200 yards deep. Cores taken from ice sheets can extend for thousands of feet and must be cut into smaller pieces to make them easier to handle.
In order to cut through a myriad of ice layers, scientist need highly specialized drills. Different drilling depths require different types of drills. Scientists drill for ice cores through solid ice in some of the harshest terrains on Earth. To retrieve the best and longest samples, scientist journey to places like Greenland and Antarctica where snow accumulates with little melting and the ice remains cold enough to preserve a layered record that stretches back thousands of years.
Preserving the Past
After drilling, researchers store the sample in polyethylene bags inside insulated boxes until they can be put in a freezer that holds ice core archives. Once the cores reach a lab, scientists look for key components in the ice to help analyze its contents. The layers in the ice can help indicate its age, similar to rings in a tree. Trace amounts of impurities and particles like soot and organic debris tell scientists about past conditions, including volcanic and oceanic activity. Links to specific historic climate events can also help date the ice in the core. Certain chemical compounds trapped in air bubbles provide clues about previous atmospheric compositions, and reveal how warm or cool previous climates may have been.
Not all ice cores are created equal, and some give a longer record than others. Some cores from Greenland boast more than 60,000 annual layers. The longest ice core ever drilled came from Antarctica, reaching nearly 2.3 miles down to Lake Vostok, a lake sandwiched between the ice sheet and the underlying bedrock. Comparisons of the ice cores to other ice core or other types of cores yield information about the global climate at a certain time in history.
Even with a myriad of ice cores already drilled, scientists continue to take samples in remote locations in Greenland, Antarctica, and Alaska to determine how the Earth’s past atmosphere responded to changes in temperature as well as atmospheric composition. Scientists use similarities between past circumstances and current ones to understand how our atmosphere and climate might be changing now. The ice still has stories to tell.