Ann Arbor (Informed Comment) – Jun-Young Park et al. write in Nature Communications that the collapse of the Arctic and Antarctic ice sheets may come sooner than climate scientists had expected if humanity misses the Paris Climate Treaty goal of keeping extra global heating to 2.7° F. (1.5° C ) above the pre-industrial average.
Humanity will certainly miss the 1.5 C. goal.
The average temperature of the earth’s surface has already shot up 1.4° F. (0.8° C.) since 1880. The average temperature of the earth in the 20th century was 53.6°F (12.0°C), but that was already warmer than the average in the nineteenth century. The eighteenth century, before the industrial revolution, was positively cold in comparison. In contrast, the first 22 years of the twenty-first century have been unprecedentedly hot. Here’s a visualization from NOAA showing all this:
As with the William J. Ripple article I discussed last week, the Park et al. article tries to build more positive feedback loops into models of ice sheet collapse.
For instance, when ice at the poles melts, it flows as fresh water into the salty seas, tending to occupy the layers of the ocean more toward the surface. As Charles Harvey at ClimateWire put it, “Because fresh water is less dense than salt water, large influxes of meltwater could fail to mix in with the rest of the ocean and instead form a layer that rests on the surface of the water. This cold sheet of liquid traps heat beneath it and causes deeper layers to warm up.”
The Park et al. team tried to build this sort of effect into their model.
They found that if we did keep additional heating to 1.5C above the pre-industrial average, then the ice sheets would be more likely to remain at least somewhat intact.
If we go to 2° C. (3.6F) above the preindustrial average, then the ice sheets are more likely to collapse. In fact, the tipping point may be 1.8° C (3.24° F). They admit that their calculations are conservative, but they see as much as 2.3 feet of sea level rise by 2100, not counting that the hotter ocean will expand and that will additionally contribute to sea level rise.
But beyond 130 years, if we heat the average surface temperature of the earth by 2C, eventually all the ice at the poles would come tumbling down into the seas.
The last time the whole earth was tropical and there was no polar ice cap, there was about a third less land area than there is now.
Even just a few feet of sea level rise will be a big problem for cities such as New York, San Francisco and New Orleans. Lower Manhattan is also lower in elevation and may not be there.
They warn that there are lots of other changes that might accelerate ice sheet melting, such as narrow ocean currents, that are not in their model. Also, scientists still do not understand processes like the “calving” of the ice sheets very well.
But their findings are alarming enough, even though they expect the worst to be felt over many decades.