Glaciologists have long feared certain Antarctic Glaciers have “tipping points” beyond which their decline will be almost impossible to reverse. Proof in specific cases has been harder, but now one team say they have confirmed the Pine Island Glacier, one of the greatest threats to global sea level, fits the bill and we are getting frighteningly close. 

Tipping points represent perhaps the greatest source of uncertainty in climate research. Until one is reached, a system may shift in one direction as the global temperature warms, but can be expected to reverse if we ever cool down. In some cases, however, once a threshold is crossed, even a substantial fall in temperatures will not restore the previous situation.

For certain major pieces of the climate puzzle, we know there are tipping points, although how close we are to crossing them is usually less certain. For others, it’s unclear if these thresholds exist at all. Until now, Pine Island Glacier, West Antarctica, has been one of the later, but Northumbria University’s Dr Sebastian Rosier claims to have settled that question. In fact, Rosier argues in The Cyrosphere, this mighty river of ice has three tipping points, but it is the last one that matters. 

Whether a glacier has a tipping point depends on the shape of the terrain on which it sits. That’s a challenge to determine since, by definition, the underlying rock is buried beneath vast quantities of ice. Even once the contours are known, scientists need to model what will happen as ice retreats, exposing new material to seawater, which causes fresh melting and a vicious circle.

"Many different computer simulations around the world are attempting to quantify how a changing climate could affect the West Antarctic Ice Sheet but identifying whether a period of retreat in these models is a tipping point is challenging,” Rosier said in a statement. "However, it is a crucial question and the methodology we use in this new study makes it much easier to identify potential future tipping points." 

Once crossed, the first two tipping points, the paper reports, will cause modest areas of the glacier to melt, even if temperatures stop rising. The third, however; “Leads to a retreat of the entire glacier that could initiate a collapse of the West Antarctic Ice Sheet.” 

The authors predict this will occur at ocean temperatures around 1.2 ºC (2.2 ºF) warmer than initial conditions. 

Along with its neighbor the Thwaites Glacier, Pine Island is one of Antarctica's fastest-moving glaciers. The pair’s rapid melt accounts for 10 percent of global sea-level rise at the moment – and possibly much more in the future. These glaciers and the area they protect not as large as East Antarctica but are considered much more vulnerable. Since they are easily sufficient to overwhelm many of the world’s great cities and most populated river deltas, they could make what happens to East Antarctica further into the future almost irrelevant.