Cruise Overview
The Cruise
On January 30th I will join a team of scientists in Christchurch, New Zealand, to embark on a cargo plane bound for the U.S. McMurdo station in the southwest Ross Sea, Antarctica. There we will board the U.S. research icebreaker Nathaniel B. Palmer, which will be home for the following eight weeks as we make our way towards our final destination of Puntas Arenas, Chile. We will collect oceanographic samples in the coastal seas off west Antarctica to improve our understanding of how the ocean interacts with the West Antarctic Ice Sheet (WAIS). The Chief Scientist for NBP0702 is Stan Jacobs of Lamont-Doherty Earth Observatory (LDEO), Columbia University, New York, who kindly invited me to participate for seagoing experience as I begin my PhD studies in polar oceanography. This endeavor has been made possible by the support and funds from the Office of Polar Programs of the US National Science Foundation (NSF).
What is the WAIS?
Describing Antarctica in terms of east and west, from a bird's eye view, may seem to put one on a path of going in circles; yet, the continent is generally referred to by an east and west side that is determined by the prime meridian. The "West Antarctic" is the region west of the prime meridian to the Transantarctic Mountains, while the "East Antarctic" is the remainder of the continent. Hence, the West Antarctic Ice Sheet extends over the region that includes the Ross Sea, Antarctic Peninsula, and the Weddell Sea. As you will read later, the WAIS is an important element of our cruise objectives.
Before explaining the importance of the WAIS, however, the term "ice sheet" deserves further explanation. An ice sheet describes a large area of ice that has built up from snowfall over millennia. They seem solid and permanent, but in fact ice sheets flow slowly downhill by way of glaciers or ice streams, moving as fast as 100s of meters to a few kilometers in one year. Upon reaching the ocean, these ice streams detach from the ground and begin to float-continuing to extend out to sea as floating ice shelves, the ends of which break off and form icebergs.
Why are we studying the WAIS?
How does this information relate to our cruise? The importance of the research on NBP0702 is explained in three words: sea level rise. If the WAIS were to collapse completely, global sea level would rise by 5-6 m, or about 16 to 20 ft. This rise is enough to flood large swaths of coastal land including major population centers like London and Florida. But is a collapse really possible? And what does "collapse" even mean when we're talking about a large, thick chunk of ice (up to 4000 m thick!) that mostly sits on land? How realistic is a total collapse of the WAIS and are there more likely scenarios that we should be aware of, such as rapid collapse of only a small portion of the ice sheet? These are some of the questions that we are helping to answer by better understanding the ocean's interaction with ice shelves which, to reiterate, are the floating edges of the ice sheet.
While there are many processes that affect how the Antarctic ice sheet reacts to changing climate conditions, as oceanographers we are mostly interested in the ocean's ability to modify the thickness, extent, and stability of ice shelves. The modern approach to understanding this problem is through a combination of long-term measurements from space, field measurements like ours, and computer modeling. On our cruise we will attempt to collect data from the Amundsen Sea, a region where the grounded ice sheet is known to be thinning and the melt rates on the ice shelves are large. But research in Antarctica is a tricky business: In some years, the sea ice in this region is too thick for icebreakers to cut through and research plans need adjustment. If this happens during our cruise, we will shift our sampling focus further north along the Antarctic Peninsula to the ice shelves that feed into the Bellingshausen Sea. The data that we will collect will help modelers determine if their models can predict present-day changes in the ice shelves, so that we can improve our trust in their ability to predict future variability under likely climate-change scenarios.
Oceans, Ice Shelves, and Sea Level Rise
Now that the purpose of the cruise is explained, we need address one remaining question: How can the ocean influence sea level rise when it interacts with a part of the ice sheet that is already floating?
Try this experiment. Put some ice blocks in a glass of water and carefully note the water level. When the ice blocks melt, check it again. The water level is the same. So, melting the ice shelves does nothing to sea level, right? But when the ice was first added to the water, the water level did rise. In the ocean, this difference is similar to the melting of already-floating ice shelves versus the addition of non-floating ice from land-based ice sheets.
The importance of ice shelves is that they act as a buttress to the ice streams and glaciers flowing downhill and thereby dam up the flow of ice moving off the continent and into the ocean. If an ice shelf is removed then ice can flow more quickly from the land-based ice sheet into the ocean and cause sea level to rise. The Larsen B ice shelf collapse in 2002 provided a great example of this process. Satellite measurements of the motion and thickness of the grounded ice behind the shelf before and after the shelf collapse showed the ice accelerating and losing thickness. The potential contribution of ice near Larsen B to sea level is small, but if other west Antarctic ice shelves are removed then the effect could be catastrophic.
For oceanographers thinking about why an ice shelf might collapse, we start with the hypothesis that there is a minimum ice shelf thickness for which a shelf is stable. This hypothesis is based on the simple exercise of looking at the thicknesses of shelves just prior to their collapse. So, what sets shelf thickness? It is mostly set by the thickness of the ice reaching the coast, and the dynamical properties of the ice as it transitions from rubbing against bedrock or a sticky bed of "till" (a mixture of sediment and water) to flowing smoothly over the ocean; however, ocean contact with the ice shelf may melt the ice shelf if the ocean temperature is warmer than the freezing point of water at the given salinity and depth that the ocean interacts with the ice shelf. So, we'd like to know the temperature of the water flowing into the ocean cavity under the ice shelf, how quickly the water sweeps through the cavity, and the properties of the water that leaves the cavity after melting the ice. If we can then create models that accurately represent these data, we will be confident that the models can be used to predict future changes.
Thus, this one small cruise for me aims to help answer a large question for humanity.
Note: satellite measurements of the Antarctic region are crucial for maintaining the long term measurements that will help us fully understand these dynamics. Sadly, the refocus on exploring outer space combined with budget restrictions means that NASA's satellite coverage of the polar regions is at risk. Find out more in this 01/16/2007 New York Times article ("Scientists warn of diminished earth studies from space", by A.C. Revkin).