By Graham Cogley
Pine Island Glacier is a giant, an outlet glacier draining about 160,000 km2 of the West Antarctic Ice Sheet. It is the focus of intense current concern because the area near its grounding line, where it feeds a floating ice shelf, has exhibited rapidly increasing rates of thinning and concurrent retreat of the grounding line. With its neighbours along the coast of the Amundsen Sea, it is now contributing something like 0.15 to 0.30 mm per year to a total rate of sea-level rise of about 2.5 to 3.2 mm/yr.
It is natural to be rattled by these observations. There is no immediately obvious reason why the rate of ice loss should not continue to increase. Indeed, the recent observations might presage even faster acceleration, perhaps involving the discharge of a substantial fraction of the 1500 mm of sea-level equivalent still stored in Pine Island Glacier and its neighbours. And we have a serious enough problem even if Pine Island Glacier simply maintains its present rate of loss.
Knowing what they know and what they don’t know, “alarmist” is therefore not a label about which glaciologists need to be embarrassed. But they also know that alarmist projections have a way of turning out to be exaggerated.
Consider the energy-balance models, that describe how the climate responds to changes in radiative forcing. The two first such models, published independently by Mikhail Budyko and William Sellers in 1969, projected that the Earth’s surface temperature would drop to tens of degrees below freezing if the output of the Sun were to decrease by only two percent. That made people sit up, and yielded a flurry of publications showing that there are plenty of ways in which the climate system moderates the severity of the negative feedback which was the basis for the original findings.
Even though they are based on measurement rather than on modelling, might our concerns about the recent behaviour of outlet glaciers in Antarctica and Greenland be similarly exaggerated? In a recent modelling study, Ian Joughin and co-authors suggest that the answer is “Probably, but not necessarily”.
The model is not quite state-of-the-art, in that it does not solve the full Stokes equation but a simpler form of the dynamical system that is appropriate for ice shelves and ice streams. The authors were obliged to handle the grounding line, where the grounded ice stream feeds into the floating ice shelf, somewhat roughly. Nevertheless the calculations allow for careful treatment of the rapid sliding at the base of the ice stream, and the implied very large rates of basal melting. And the model does a good job of reproducing the documented behaviour of Pine Island Glacier up to 2009.
Most of the ice in the Pine Island Glacier catchment is flowing very slowly indeed, at a few metres per year at most. But as it converges on the outlet of the catchment it accelerates spectacularly, and is moving at thousands of metres per year by the time it starts to float at the grounding line. Most of the speed is the result of basal sliding, so the ice stream is not unlike a rigid plug, punching its way through the much slower ice on its flanks. This peculiar setup is the core of the problem.
Joughin and his co-authors simulated responses of the glacier to a variety of scenarios that might or might not represent the next hundred years. Even the more extreme scenarios, featuring basal melting at four times the present rate, did not lead to flotation of the entire 200-kilometre length of the ice stream, as one earlier study had suggested. Nor did the model come anywhere close to an even simpler extrapolation of current behaviour, based on kinematics rather than dynamics.
Don’t breathe out yet, however. The results considered by the authors to be the most probable have Pine Island Glacier continuing to lose mass at rates comparable to the recent rates. It doesn’t continue to accelerate, but it doesn’t slow down either. The grounding line doesn’t continue to migrate inland, but the inland thinning implied by the fast flow does continue.
It would be wrong to write off this heroic but tentative modelling effort, which is an important step towards the goal of understanding Pine Island Glacier. Models like this one, and like the energy-balance models that followed up on Budyko and Sellers, are part of the learning process. They suggest that doomsday isn’t going to happen just yet. But, in short, doomsday scenarios are educational.