This is a question that’s been asked for decades. There is always some danger in picking who first asked it, but glaciologist John Mercer in 1968 is a strong candidate. Mass changes in the ice sheet translate into changes in sea level, and a lot of people live close enough to sea level to be displaced if the ice sheet were to be lost, while many more enjoy the beaches and ports that would be affected.

Many things affect sea level. For example, more snow on an ice sheet — as we expect on the Antarctic ice sheet and central parts of Greenland in a warming world — tends to lower sea level by taking water that evaporated from the ocean and storing it on top of an ice sheet. But more melt on an ice sheet — as we expect in parts of Greenland with warming — takes water from the ice sheet and puts it back in the ocean, raising sea level. Melting of mountain glaciers has a similar effect as melting the ice sheet, as does pumping of water out of the ground to irrigate crops or for other uses, because most of that water ends up in the ocean rather than back in the ground. Warming the ocean causes expansion of the water and raises sea level. All of these are interesting and important influences, with notable uncertainties, but we don’t think that those uncertainties are huge.

Another way to raise sea level is for ice sheets to spread more rapidly under their own weight, taking ice from above sea level and delivering it to the ocean to make icebergs. This influence on sea level is complicated, and is where various uncertainties arise. Many factors control how rapidly ice flows, and thus how rapidly ice sheets can transfer land ice to the ocean to raise sea level.

Today, around most of Antarctica and parts of Greenland, the ice reaching the ocean does not immediately break off to make icebergs; instead it remains attached while spreading over the ocean, forming an ice shelf. The ice shelves almost all exist in bays or fjords, and thus have friction with their sides; the undersides of many ice shelves also hit local high spots in the seafloor, generating additional friction. Furthermore, the undersides of the ice shelves, where they are in contact with the ocean, are at their melting point. Warming ocean water tends to thin the shelves — a warming of 1 degree Celsius increases melting by about 10 meters per year — reducing the friction, and thus allowing faster flow of the ice that feeds the shelves, raising sea level.

Various features of the geologic record, modern observations, and investigations with models point to the importance of “threshold” behavior. Increasing the ocean temperature increases the ice’s speed, with the potential that at some high-enough temperature, the speed will jump rather abruptly and irreversibly. Such behavior is especially interesting and important, but also difficult to predict. You can undoubtedly think of many questions: What happens if the water temperature stays the same, but the rate of ocean circulation changes? What about warming adding meltwater into crevasses that could wedge them open and remove the friction that way? If an ice shelf is thinned, by how much does the flow speed increase? What are the thresholds?

A large and vigorous community of scientists in the field, remote-sensing experts, and modelers is working to measure, understand, project, and test the projections. And, we’re doing so with some urgency — we want to get answers in time to provide useful guidance to people making decisions about energy and the environment.