Tipping Points: Melting Ice, Rising Oceans

Global warming IS a time bomb.

There may still be time to defuse it, but that requires policy-makers to take the actions that are needed, not the ineffectual actions they are discussing.

Despite the publicity that global warming has received, there is a large gap between what is understood by the relevant scientific community, and what is known by the people who need to know, the public and policymakers. Global warming is small compared to day-to-day weather fluctuations, so it is hard for people to recognize that we have a crisis – but we do.

The climate system has great inertia, caused, e.g., by the 4-kilometer-deep ocean and the thick ice sheets on Antarctica and Greenland, which have only partly responded to the human-made changes of atmospheric composition. That inertia is not our friend. It is a Trojan horse. By the time the public notices that change is underway the momentum of the climate system may be sufficient to guarantee much larger changes. The climate system can pass tipping points, such that large change continues out of our control.

The bad news is that we have already passed into a dangerous range of atmospheric carbon dioxide.

The good news is that if we act smart and promptly it is still feasible to achieve a safe level of atmospheric gases, and the actions needed to achieve that would have multiple benefits in addition to climate stability.

There are several climate tipping points of special concern.

Tipping points are “non-linear” phenomena, which means that they can reach a point at which rapid catastrophic change occurs. It is inherently difficult to determine the time at which non-linear collapse will occur, even in cases where such rapid change is certain.

The mechanism that seems to be most important for disintegration of the great ice sheets that cover Antarctica and Greenland begins with ocean warming. Ocean warming leads to melting of ice shelves, which are tongues of ice that stretch out into the ocean. The ice shelves buttress the ice sheets, so when ice shelves disappear, the more mobile parts of the ice sheet, the ice streams, can surge into the ocean. Thus, removal of the ice shelves is somewhat akin to taking the cork out of a bottle — it allows the material behind to flow rapidly.

We know from Earth’s history that once ice sheet disintegration is well underway, sea level can rise by several meters per century.

Let’s look more at processes contributing to ice sheet disintegration.

We have accurate satellite measurements of the area on Greenland with summer melting on the snow and ice surface. The area with melting fluctuates from year to year, depending on year-to-year weather fluctuations, but there is a long-term increase of the melt area.

The area with summer melting is shown by the red area, with maps for 1992 and 2007, the years with the least and most melt area. The trend has an increase of 50 percent in melt area during the past three decades.

The meltwater runs to a low spot on the ice surface, where it burrows a hole in the ice sheet that carries the water all the way to base of the ice sheet.

There, the meltwater lubricates the base of the ice sheet, accelerating the discharge of giant icebergs to the ocean.

This is one of the processes causing ice loss from Greenland and Antarctica to increase.

Another process, probably even more effective than surface melt, is melting of ice shelves by warmer ocean water.

Ice shelves are tongues of ice extending from the large ice sheets into the ocean. They buttress the large ice sheet, helping to keep it in place. As a warming ocean melts the ice shelves, icebergs begin to stream more rapidly into the ocean. Until recently, some scientists argued that global warming may cause ice sheets to grow, because a warmer atmosphere holds more water vapor and thus produces greater winter snowfall. Of course, common sense suggests that the ice sheets will become smaller as the planet becomes warmer, but accurate measurements are needed to prove what is happening.