|Above: The Russian research station Vostok sits on the polar plateau in East Antarctica. This photo was taken during the summer of 2000-01. The coldest temperature ever recorded on earth was –89.2°C (–128.6°F) at Vostok in 1983. Image credit: Josh Landis, National Science Foundation.|
Paleoclimate researcher Pete Akers (Institut des Géosciences de l’Enivironnement or IGE in Grenoble, France) is a participant in the 2019-20 East Antarctic International Ice Sheet Traverse (Project EAIIST). Pete is writing about the project in a special series for Category 6. See Pete’s initial post from November 8.
Aside from some rocky coastlines and exposed mountain peaks, Antarctica is an almost incomprehensibly vast expanse of snow and ice. The snow that falls on the continent never melts (except for small regions near the coast), but instead gets buried by more snow year after year until the increasing weight from above compresses it into ice. If enough snow accumulates, the tremendous weight and pressure will cause the ice to flow downhill through gravity, creating a glacier.
A mountain glacier—the type you are probably more familiar with—is constrained by the surrounding landscape; it flows from the higher elevation near a mountain peak down valleys to lower elevations. In contrast, Antarctica’s glaciers are in the form of an ice sheet, where the ice completely covers all bedrock and the highest elevations are actually parts of the ice sheet itself. These high points are known as domes, and ice will flow from a dome toward the coast, where it will eventually break off into the ocean as an iceberg. Since all the ice in Antarctica is slowly moving, this means that the science stations and other structures on the surface of the ice sheet are also slowly moving. In fact, scientists at the Amundsen-Scott South Pole Station actually have to move the stake marking the true position of the South Pole about 30 feet (9 meters) every year to compensate for the flow of the ice!
Antarctica is actually capped by two main ice sheets: the smaller, lower, and warmer West Antarctic Ice Sheet and the massive and frigid East Antarctic Ice Sheet. An ice sheet has existed in Antarctica in some form for the past 45 million years, and the East Antarctic Ice Sheet was relatively stable (though likely smaller) throughout the warm Pliocene Epoch from 3 to 5 million years ago. Much of the news you read today about rapid glacial melting and collapsing ice shelves is from West Antarctica, including the Antarctic Peninsula, which are warming at a very rapid pace. The base of the West Antarctic Ice Sheet actually sits well below sea level, and scientists worry that this makes the ice sheet especially at risk of irreversible collapse if it loses a certain threshold of ice. Unfortunately, we don’t know what that threshold is, and a lot of research is going into learning how stable that whole system currently is.
Happy birthday A68! The world's biggest iceberg broke away from the Larsen C ice Shelf in July 2017. Initially, it didn't move very far, very fast. But it's been quite nimble of late. This #Sentinel1 movie shows the progress since Jan 2018. https://t.co/3DPkILvSKR @adrian_luckman pic.twitter.com/v4rSG5ymna— Jonathan Amos (@BBCAmos) July 11, 2019
In contrast, East Antarctica holds over 10 times more ice than West Antarctica and is not warming as quickly. Although a “cooling” of East Antarctica has been commonly referenced as evidence against global warming, this cooling appears to have been restricted to the 1980s and 1990s, and warming is now observed across the entire continent.
|Figure 1. Long-term changes in yearly surface temperature in and around Antarctica between 1981 and 2007, based on thermal infrared (heat) observations made by a series of NOAA satellite sensors. Image credit: NASA Earth Observatory image by Robert Simmon, based on data from Joey Comiso, GSFC.|
The East Antarctic Ice Sheet is over three miles deep at its thickest point, and a complete melting of all the ice would raise sea levels over 200 ft (60 m) Luckily, even the most severe projections of global warming from climate models do not show a complete (or even close to complete) melting of Antarctica. However, each 1% loss of ice from the continent will raise the oceans by two to three feet. As a result, even small uncertainties in projected future ice losses can mean a world of difference for millions of people and could affect trillions of dollars in infrastructure on the coasts.
Snow accumulation and sea level rise: a complex picture
My specific role in EAIIST focuses on improving our interpreation of ice core records in order to better understand how snow accumulation rates have changed in East Antarctica. This is important because snow falling on top of an ice sheet counteracts the loss of ice when icebergs break off (or calve) at the coast and then melt in the ocean. Conditions are too cold for direct melting of the ice sheet surface to be a factor in Antarctica.
In a stable ice sheet, the amount of ice lost at the coasts is compensated by the amount of new snow that falls inland on the ice sheet. If more ice breaks away than new snow falls, the ice sheet will shrink and sea level will rise. Likewise if more snow falls than ice is lost to calving, the ice sheet will grow, more of Earth’s water will be stored as ice on land, and sea level will fall. This is known as the glacial mass balance.
|Figure 2. Simplified diagram of the mass balance processes of the East Antarctic Ice Sheet. Image credit: Pete Akers.|
Currently, we know that ice loss is greater than snow gain in Greenland and West Antarctica, which is leading to modern sea level rise (along with thermal expansion of the oceans due simply to heating). However, studies of East Antarctica are less clear. The region is so vast and cold that some studies suggest the increased water capacity of warmer air temperatures is producing enough additional snow to match or exceed any increased ice loss. Changes in atmospheric circulation in response to regional warming may also be bringing more snow onto the continent. If this proves true and continues through the near future, East Antarctica would actually serve to slow the rate of sea level rise and give us valuable extra time for costly climate adaptations in coastal communities.
However, surface weather observations in East Antarctica are very sparse, with only three stations manned year-round on the entire ice sheet (America’s Amundsen-Scott Station, Russia’s Vostok Station, and Europe’s Concordia Station), and the longest records barely span a half century. And as you can imagine, something as simple as under- or overestimating snow accumulation by even 1 cm/yr becomes a big problem when you have to extrapolate the mass of snow that is falling over an entire continent! So even if our observations of East Antarctica currently gaining mass are true, it is difficult to know if this is a trend that will continue or is simply an odd short-term blip. And if East Antarctica shifts instead to losing more ice than it gains, it has the potential for massive effects on human civilization from the increased sea level rise.
To better understand how Antarctica responds to global climate changes, we can look at ice cores. There are a few “sweet spots” at the ice sheet’s domes and ridges where the net flow of the local ice is almost zero, since the ice flows radially in all directions from these high points. Snow that falls on the surface of these spots will get buried and turned to ice layers that get increasingly “smooshed” and thinned, but never fully transported away. When snow falls, the chemistry of the snow itself as well as other trapped compounds and air will reflect the current climate and environment. These chemical signals are preserved when the snow transforms into ice, and by performing many chemical analyses on each individual ice layer, we can reconstruct an amazing amount of climate information.
Thursday AWI shallow ice core test drilling at S7 2019 reached 40.3m depth and was finished. In the picture Johannes is operating the shallow drill at the S7 test site. pic.twitter.com/ZQ9Up0mU1g— EastGRIP Camp (@egripcamp) May 17, 2019
At many of these spots, scientists have drilled and recovered cores of ice many miles long that extend back as far as 800,000 years, and several new missions are dedicated to recovering ice cores that extend back even farther. At these long timescales, we observe several cycles between glacial periods (colder times when multiple large continental ice sheets covered much of the Earth) and interglacials (warmer times like the present, when ice sheets are restricted to Greenland and Antarctica). These extreme changes offer some of our best comparative examples when studying how Antarctica reacts to rapid and massive global changes in climate.
Scientists test and study each ice core in a -10 degree freezer pic.twitter.com/4EzOi82R6W— Business Insider (@businessinsider) June 23, 2019
Deducing the rate of snowfall from nitrogen
I specialize in using the chemistry of nitrate (NO3-) to reconstruct past snow accumulation rates. Nitrate is created naturally in the atmosphere from nitrogen oxides (produced by lightning, wildfires, and also manmade exhausts), and it falls in trace amounts in Antarctic snow. However, if ultraviolet radiation from sunlight strikes the nitrate, it can turn it back into nitrogen oxide in a process called photolysis (literally, light-breaking), and the nitrogen oxide escapes back into the atmosphere. Eventually, as more snow falls, the nitrate will be buried below levels where light can reach, and the photolysis will stop.
However, there are two types of nitrate: a common one where the nitrogen atom has the typical seven protons and seven neutrons (14N) and a rarer one with seven protons but eight neutrons (15N). These two different nitrogen types are called isotopes, and the ratio between the amounts of nitrate with 15N and nitrate with 14N can tell us lots of information about the environment. In my particular Antarctic case, the process of photolysis is more likely to happen to nitrate with 14N. As long as the nitrate is exposed to sunlight and can undergo photolysis, the ratio between 15N and 14N in the nitrate left in the snow becomes more and more skewed toward 15N. Once the nitrate is buried, the isotopic ratio is locked in and preserved indefinitely. And by now you might have figured out that if we measure the 15N/14N ratio for snow or ice from a given year, we can estimate how long it took to get buried, aka the rate of snow accumulation!
|Figure 3. Overview of nitrogen isotope chemistry in Antarctic snow. In step one, nitrate falls onto the snow surface. In step 2, ultraviolet light cause some nitrate to react and leave the snow. This process, called photolysis, favors nitrate whose nitrogen is a 14N, skewing the remaining nitrate’s ratio between 15N and 14N toward 15N. In step 3, two scenarios are shown: one with little snow and one with more snow. In step 4, we see how some of the nitrate in the little snow scenario is still in the layers that light can penetrate, leading to more nitrate loss and even greater isotopic ratio skewing. Image credit: Pete Akers.|
By translating 15N/14N ratios into snow accumulation rates, you could take samples from any East Antarctic ice core and reconstruct snowfall changes for periods ranging from thousands to hundreds of thousands of years. But like any good translation, you need a reference. For the past decade, researchers at IGE have been taking snow samples along Antarctic traverses ranging from high snowfall areas near the coast all the way to the dry domes where the ice cores were drilled. These samples have given us the ability to translate 15N/14N ratios into snow accumulations for past times when snowfall at the domes was similar to today or greater. However, we need to expand our reference to include times when the climate was drier. Our journey to the arid Megadunes will finally provide us with samples from a site generally similar in environment to the domes, but significantly drier. And by taking multiple samples as we travel to and from the Megadunes site, we will capture a full gradient of accumulation rates.
Once I'm back at IGE in Grenoble, I will process our ice samples to extract the isotopic information in them, working with nitrate amounts of only a few micrograms. I will cover this in more detail in a future post, but it is quite an innovative technique that actually uses specially cultured bacteria to digest the nitrate and produce a gas we can run through our equipment. In my next post, I will be in Antarctica at the coastal Dumont d’Urville station, and I hope to provide some updates and insight into life at the station and our logistical preparations (shaky internet service willing!).
Pete Akers has created this and other articles in this series in his personal capacity. The views expressed are his own and do not necessarily represent the views of IGE, CNRS, IPEV, or EAIIST.