Ocean overturning and ice-sheet tipping points

5 problems

• Is the AMOC already weakening toward collapse today?

  Contested

  Some studies claim statistical warning signs that the overturning circulation is losing stability, while direct measurements since 2004 are too short to show a clear trend. The question is whether a collapse is near.

• At what warming or freshwater input does the AMOC collapse?

  Contested

  The Atlantic meridional overturning circulation (AMOC, the ocean conveyor carrying warm surface water north) can in theory switch to a weak state if the North Atlantic gets too fresh. Nobody knows how much warming or meltwater would cause that switch.

• What controls melting at the base of floating ice shelves?

  Open

  Warm ocean water melts Antarctic ice shelves from below, which weakens the resistance they give to the ice flowing behind them. How fast this melting happens depends on thin turbulent water layers right under the ice that models cannot yet capture well.

• Does marine ice-cliff instability operate on real ice sheets?

  Contested

  If an ice shelf breaks off, it may leave a tall ice cliff at the ocean edge that collapses under its own weight, exposing an even taller cliff behind it. Whether this runaway collapse really happens decides how fast Antarctica could add to sea level.

• Is West Antarctic retreat already irreversible?

  Contested

  Much of the West Antarctic ice sheet rests on bedrock that deepens inland, so once the ice front starts retreating it may keep going on its own. The question is whether that process has already begun for glaciers like Thwaites.