An age in ice
An expedition above the Arctic Circle reveals climate records, locked in ice.
By Holly McKelvey. Photography from NEEM.
In the crystalline sharpness of a frozen world, an international team of scientists drilled down kilometers beneath their feet to draw out long, slender cylinders of ice: Ice that would answer urgent questions about our changing climate.
The team hailed from 14 different countries, as part of the North Greenland Eemian Ice Drilling Project, or NEEM. From 2007 to 2011, their camp was perched on the northern edge of the world, far above the Arctic Circle, as they drilled out a 2.5 kilometer ice core— reaching the bottom of the ice sheet, and spanning the last 150 thousand years.
The core that the NEEM project collected would answer some of the biggest questions we face in a world of unprecedented climatic change. The biggest unknown? What Earth will look like 4° or 5° Celsius warmer – or more. To find an answer, scientists looked back, into a past as warm as our future.
In this case, the past being investigated happened between 130 and 115 thousand years ago, and is recorded in a section of the ice core. In that fifteen thousand year window, dubbed the “Eemian Period”, sea levels rose by 4 to 8 meters, global temperatures increased by as much as 8° Celsius, and ice caps shrank – but didn't vanish entirely.
It was a brief respite of relative warmth— called an interglacial— in an ice age that still continues today. In fact, we are currently in another of these warm interglacial periods, called the Holocene. And this time, we have no idea how warm it will get. Estimates suggest 2°-4° Celsius per century, but scientists are uncertain. Records from the warmer Eemian Period might help us better predict what changes lie ahead.
What lies in the ice?
As the climate has fluctuated between warmer and colder ice age periods, Earth’s ice sheets have advanced and retreated. And as they've melted and regrown, the ice has stored a frozen record: annual layers of snow compacted over time. NEEM and other research teams have studied the chemicals, gas, and particles frozen into each of these annual layers to reveal the climatic history of an ice age.
The layers themselves tell a story: like tree rings, they store a record of changes in temperature and rain or snowfall from year to year. The thickness of each layer can tell scientists how much snow fell and stuck, a proxy for how much it snowed in winter and how warm the summers were.
The layers of ice that make up the warmer period NEEM studied are about 1 cm thick: thick winter ice capped with summer meltwater. Thick snow tells scientists it was a cold winter, and meltwater points to warm summer rains. 130 thousand years ago, the ancient summers were long and warm, and much of the winter snowfall melted away.
The ice layers are important, but even more information is stored in the water molecules themselves. The oxygen atoms stored inside each H2O molecule are an elegant tool to interpret climatic patterns. It all comes down to what kind of oxygen is in there.
There are two garden-variety types of oxygen: 16O (the lighter one) and 18O (the heavier one). In theory, they behave the same; after all, they’re both oxygen. But in practice, they act quite differently. Since 16O is lighter, it tends to be more mobile. A water molecule containing 16O therefore evaporates more easily. The heavier 16O water molecules, meanwhile, are slower to evaporate. They aren’t carried as far inland by clouds, which tend to rain out the heavier water first.
Imagine: water evaporates off the surface of the ocean, moves inland in the form of clouds, and falls as rain or snow. During a warm period like the Eemian, most of this water flows back to the sea; but during colder glacial periods, it will freeze, becoming locked up on land in snow and ice. Since water with 16O is more mobile, it is mainly this lighter water that gets locked up. This means that the ice formed during colder periods tends to contain a lot of this lighter oxygen.
Today, climate scientists know that glacial ice layers with more 16O formed during colder periods. And they know that during these colder periods, huge amounts of water from the ocean must have been frozen at the poles, meaning that sea levels were much lower. Equally, less 16O in the ice indicates that sea levels were higher.
It's not just water molecules. The air trapped in ice also tells a story.
Air is trapped slowly, as tiny bubbles. As new snow is buried and compacted into ice, the air between the flakes and clumps of snow is trapped by the new snow layers above. Each bubble is a tiny pocket of ancient air, a photograph of the atmosphere at the moment it was trapped. And the bubbles in each ice layer are slightly different: they contain different concentrations of gases, recording changes in the air above the ice. Today, researchers can compare the bubbles in different ice layers to understand how the atmosphere has changed over millenia.
The ice, therefore, tells a story far more rich in detail than one might expect at first glance: ice cores contain complicated chemical stories, where portraits of climatic changes are described in the trapped air bubbles and type of water. The patterns of sea level change, atmospheric composition, temperature change and more are locked up in these layers. And the North Greenland Eemian Ice Drilling project is one of many actors translating this data from ice to words.
In 2013, NEEM researchers published results from their ice core analyses in the international scientific journal Nature. They found that the Eemian was about 8° Celsius warmer than today; that’s far warmer than scientists had previously thought. The ice sheet contracted in that heat, but didn't melt entirely; in fact, it only shrank by about 25%, meaning that it only partially contributed to the Eemian’s dramatic sea level rise of 4 to 8 meters. The fact that the Greenland ice sheet was more resilient than expected in the face of climate change is not necessarily good news, however; climate scientists who analyzed the NEEM ice core are concerned that the majority of Eemian sea level rise was due to melting from Antarctica.
Understanding the climatic history of the Greenland ice sheet, therefore, can help scientists understand how the Earth reacted to a warm period in the past. And that information is crucial to understanding how our planet might change during the next decades and centuries.
All photos courtesy of NEEM ice core drilling project, www.neem.ku.dk. Cover image by Martin Skrydstrup.
The North Greenland Eemian Ice Drilling - NEEM - is an international ice core research project aimed at retrieving an ice core from North-West Greenland (camp position 77.45°N 51.06°W) reaching back through the previous interglacial, the Eemian. The project logistics is managed by the Centre for Ice and Climate, Denmark, and the air support is carried out by US ski equipped Hercules managed through the US Office of Polar Programs, National Science Foundation.
Holly is a graduate student in Environmental Management at the University of Kiel, Germany, where she cycles a lot, drinks tea, and enjoys the brief lapses of sunshine. She can be reached at firstname.lastname@example.org