Aug 14, 2017

Core questions: An introduction to ice cores

Jessica Stoller-Conrad
Climate Change: Vital Signs of the Planet
An ice core. Photo: Ludovic Brucker, NASA's Goddard Space Flight Center
An ice core. Photo: Ludovic Brucker, NASA's Goddard Space Flight Center

When archaeologists want to learn about the history of an ancient civilization, they dig deeply into the soil, searching for tools and artifacts to complete the story. Scientists who study Earth’s past climates, called paleoclimatologists, take a similar approach. However, instead of digging into the soil, they look for clues about our planet’s climate history by studying coral reefs, digging into ocean and lake floor sediment and drilling deeply into glaciers and ice sheets. The samples they collect from the ice, called ice cores, hold a record of what our planet was like hundreds of thousands of years ago.

But where do ice cores come from, and what do they tell us about climate change? Climate scientist Allegra LeGrande of the NASA Goddard Institute for Space Studies (GISS) and physical scientist Kimberly Casey of the U.S. Geological Survey and Cryospheric Sciences Lab at NASA’s Goddard Space Flight Center helped answer these questions and more.

Where do ice cores come from?

Ice sheets and glaciers near Earth’s North and South Poles formed from years and years of accumulating snowfall. The weight of each year’s snowfall compresses down the previous layers of snow, and after many years, all of this pressure helps to form glacial ice. In some areas, these layers result in ice sheets that are several miles (several kilometers) thick.

Researchers drill ice cores from deep (sometimes more than a mile, or more than 1.6 kilometers) inside the polar ice sheets in Greenland and Antarctica, as well as some high-latitude ice caps and mountain glaciers. They collect ice cores in many locations around Earth to study regional climate variability and compare and differentiate that variability from global climate signals.

What can the ice tell us about past climates?

Each layer of ice tells a story about what Earth was like when that layer of snow fell. For example, LeGrande says, as snow deposits onto a growing glacier, the temperature of the air imprints onto the water molecules.

The icy layers also hold particles—aerosols such as dust, ash, pollen, trace elements and sea salts—that were in the atmosphere at that time. These particles remain in the ice thousands of years later, providing physical evidence of past global events, such as major volcanic eruptions.

Additionally, as the ice compacts over time, tiny bubbles of the atmosphere—including greenhouse gases like carbon dioxide and methane—press inside the ice. These air pocket “fossils” provide samples of what the atmosphere was like when that layer of ice formed, LeGrande said. “Scientists can directly measure the amount of greenhouse gases that were in the atmosphere at that time by sampling these bubbles,” she added.