Frozen Time Capsules

The planet is warming, and as it does, part of its history is melting away. Global warming, the increase in the average temperature of Earth’s atmosphere, is causing glaciers to disappear. These large, slowly moving masses of ice can contain water that’s been frozen for more than 5 million years. Glaciers are like time capsules, holding clues about what Earth’s climate was like long ago.

Since the mid 20th century, scientists have drilled into glaciers to collect samples called ice cores. Researchers study the chemical composition of these rods of ice. Doing so can provide information about what Earth’s atmosphere was like long ago, such as its past gas composition, temperatures, and amounts of precipitation. But “as glaciers melt, these records potentially get destroyed,” says Bradley Markle, a climate scientist at the University of Colorado Boulder.

By studying ice cores, Markle and other scientists have learned that Earth’s climate has gone through periods of warming and cooling throughout history. Understanding the conditions during these periods allows scientists to re-create a detailed picture of historical climate shifts. Researchers can compare these natural shifts to the dramatic human-made climate change we’re experiencing today. That’s why scientists are racing to collect ice cores—and the valuable data they contain—from glaciers around the world, before they’re gone for good.

The planet is warming, and part of its history is melting away. Global warming is the increase in the average temperature of Earth’s atmosphere. It’s causing glaciers to disappear. These are large, slowly moving masses of ice. Some water in glaciers has been frozen for more than 5 million years. Glaciers are like time capsules. They hold clues about Earth’s climate long ago.

Scientists began to collect samples called ice cores in the mid-20th century. They drill into glaciers to remove these rods of ice. Then researchers study their chemical composition. That provides information about Earth’s atmosphere long ago. For example, it can show gas composition, temperatures, and amounts of precipitation in the past. But “as glaciers melt, these records potentially get destroyed,” says Bradley Markle. He’s a climate scientist at the University of Colorado Boulder.

Markle and other scientists have studied ice cores and learned about changes to Earth’s climate. The planet has gone through periods of warming and cooling throughout history. Scientists want to understand the conditions during these periods. Then they can re-create a detailed picture of past climate shifts. Researchers can compare these natural shifts with today’s dramatic human-made climate change. That’s why scientists are racing to collect ice cores from glaciers around the world. They want to gather the valuable data in the ice before it’s gone for good.

Glaciers form slowly over centuries. As snow falls, gases, chemicals, and other particles in the air cling to the drifting flakes. The snow piles up on glaciers, where the airborne materials become trapped within frozen layers of ice. These layers build up year after year. Scientists can examine these layers within an ice core sample to look back at past seasons’ snowfall amounts, as well as climate conditions (see Natural Record Keepers).

Climate data recorded by people goes back only 150 years. That’s why ice cores, which are essentially archives of the past, are so important to preserve, sample, and study, says Markle. The deepest layers of a core represent its oldest ice, which can date back hundreds, thousands, or even millions of years. The longest continuous ice core record—without missing time periods—spans 850,000 years. The oldest ice samples date back about 5 million years!

Once an ice core is drilled, it’s shipped in temperature-controlled boxes to a laboratory like the National Science Foundation’s Ice Core Facility (NSF-ICF) in Colorado—the largest ice core archive in the world. As soon as new samples arrive, scientists analyze their chemical composition and age. Each core is cut into 1-meter (3.3-foot) pieces and stored inside cylindrical cardboard containers labeled with information about the core’s location and depth from which it came. Then the core goes into a -40oC (-40oF) freezer.

Why save all that ice? “We want to have an archive of ice cores so that after all the initial research is published, we can go back and do more analysis if needed,” says Richard Nunn, the assistant curator and database manager of the NSF-ICF facility. “But, also, as technology advances, we might want to revisit a core to reanalyze it with new techniques.”

Glaciers form slowly over centuries. Gases, chemicals, and other particles in the air cling to falling snowflakes. The snow piles up on glaciers, and the airborne materials become trapped in frozen layers of ice. These layers build up year after year. Scientists can examine these layers in an ice core sample. They reveal past seasons’ snowfall amounts and climate conditions (see Natural Record Keepers).

People have recorded climate data for only 150 years. That’s why ice cores are so important to preserve, sample, and study, says Markle. Ice cores are like archives of the past. A core’s deepest layers contain its oldest ice. That can date back hundreds, thousands, or even millions of years. The longest unbroken ice core record spans 850,000 years, without missing time periods. The oldest ice samples date back about 5 million years!

After an ice core is drilled, it’s shipped in temperature-controlled boxes. It goes to a laboratory like the National Science Foundation’s Ice Core Facility (NSF-ICF) in Colorado. That’s the largest ice core archive in the world. When new samples arrive, scientists analyze their chemical composition and age. Each core is cut into 1-meter (3.3-foot) pieces. The pieces are stored inside tube-shaped cardboard containers. Labels on the containers tell the core’s original location and depth. Then the core goes into a -40°C (-40°F) freezer.

Why save all that ice? “We want to have an archive of ice cores so that after all the initial research is published, we can go back and do more analysis if needed,” says Richard Nunn. He’s the assistant curator and database manager of the NSF-ICF facility. “But, also, as technology advances, we might want to revisit a core to reanalyze it with new techniques.”

To determine temperatures in the past, climate scientists look at different isotopes of hydrogen (H) and oxygen (O) that make up water molecules (H₂O) in ice core layers. Isotopes are atoms of an element with the same number of protons but different numbers of neutrons. Heavier isotopes have more neutrons. Scientists have determined that ice samples with fewer heavy isotopes formed when temperatures were colder. Ice samples with more heavy isotopes formed when temperatures were warmer.

Scientists also measure the amount of greenhouse gases, like carbon dioxide (CO₂), in ice core samples. That way they know how much of these heat-trapping gases existed in Earth’s atmosphere at different points in history. To collect this data, scientists place a small portion of ice from the core inside a sealed chamber and either melt or crush the ice to release the gases within. The gases are then captured and analyzed. By comparing the results of these experiments with temperature data obtained from ice cores, scientists have found that higher levels of greenhouse gases always correspond with warmer temperatures throughout history.

Levels of greenhouse gases have risen and fallen periodically in the past because of natural causes such as wildfires and volcanic eruptions (see Rising Gases). But according to Markle, the current concentration of CO₂, as well as the rate of change we see in ice cores in the past 200 years, has never been seen before. The only explanation, he says, is the increase in people’s use of fossil fuels, like coal, oil, and natural gas. When we burn these, they release greenhouse gases into the atmosphere.

Water molecules (H₂O) in ice core layers hold clues about temperatures in the past. Scientists look at different isotopes of hydrogen (H) and oxygen (O) in these molecules. Isotopes are atoms of an element. They have the same number of protons but different numbers of neutrons. Heavier isotopes have more neutrons. Some ice samples contain fewer heavy isotopes. Scientists learned that these samples formed when temperatures were colder. Other ice samples contain more heavy isotopes. They formed when temperatures were warmer.

Scientists also measure heat trapping greenhouse gases like carbon dioxide (CO₂) in ice core samples. This tells them the amount of greenhouse gases in Earth’s atmosphere at different points in history. To collect this data, scientists place a small piece of the ice core inside a sealed chamber. They melt or crush the ice to release the gases inside. Then the gases are captured and analyzed. Scientists compare the results of these experiments with temperature data from ice cores. They always find higher levels of greenhouse gases and warmer temperatures together, throughout history.

Levels of greenhouse gases have risen and fallen at different times in the past. The causes were natural, like wildfires and volcanic eruptions (see Rising Gases). But Markle says that the current level of CO₂ has never been seen before. Neither has the rate of change in ice cores in the past 200 years. The only explanation is the increase in people’s use of fossil fuels, he says. These include coal, oil, and natural gas. When we burn them, they release greenhouse gases into the atmosphere.

Some of Earth’s ice masses, like those in parts of Antarctica, are likely at a lower risk of melting anytime soon, says Markle. “On the other hand, mountain glaciers are certainly retreating and experiencing a lot of melt.” That’s why a group of scientists formed the Ice Memory Foundation, to gather ice cores from at-risk locations. The group aims to collect 20 cores from 20 locations within 20 years. These samples will be stored in an ice cave that will eventually be constructed in Antarctica.

The group has collected cores from eight sites in France, Bolivia, Switzerland, Russia, Italy, and Norway. But they might not be gathering them fast enough to keep up with climate change. During a 2020 operation at the Corbassière glacier in the Swiss Alps, they discovered the glacier was melting at an alarming rate. “Meltwater will infiltrate deep into the layers of the glacier and then refreeze in the winter,” says Margit Schwikowski, a chemist with the Ice Memory project. This can erase the ancient climate records within. Schwikowski doesn’t know if there’s enough time to get the remaining 12 ice cores—but they’re determined to try.

Some of Earth’s ice masses are less likely to melt anytime soon. They include those in parts of Antarctica, says Markle. “On the other hand, mountain glaciers are certainly retreating and experiencing a lot of melt.” So a group of scientists formed the Ice Memory Foundation. They’re gathering ice cores from at-risk locations. The group aims to collect 20 cores from 20 locations within 20 years. They plan to build an ice cave in Antarctica. The samples will be stored there.

The group has already worked at sites in France, Bolivia, Switzerland, Russia, Italy, and Norway. They’ve collected eight cores. But they might not be fast enough to keep up with climate change. In 2020, they worked at the Corbassière glacier in the Swiss Alps. They discovered the glacier was melting at an alarming rate. Margit Schwikowski is a chemist with the Ice Memory project. “Meltwater will infiltrate deep into the layers of the glacier and then refreeze in the winter,” she says. This can erase the ancient climate records inside. Is there enough time to get the other 12 ice cores? Schwikowski doesn’t know. But the group is determined to try.