Several years ago, I travelled to the Gold Coast in Queensland and stayed in a motel just behind Main Beach, and at high tide I had to stand with my nose almost plastered against the window of my room to see the sand on the beach, it had retreated so far. Indeed, we were able to get down to the beach in only a few places because so much sand has been washed away that at the back of the beach, just below our motel, was a 3 metre high sand cliff with dangling grass and shrub roots. Shoot forward to a few weeks ago and I was in Sydney, staying at Narrabeen. The beach was only accessible nearby at the local surf lifesaving club, because they had dumped sand in front of it to form a ramp to allow club members and the public to get down the beach. In most places the back of the beach was a sand or boulder cliff, with collapsed lawns and dangling ends of fences hanging over the precipice. On the beach below where I was staying, half way between the cliff and the water’s edge was a hemisphere of concrete with a fencepost sticking out of it. The ocean always wins.
More than half of Earth’s freshwater resources are held by the Antarctic Ice Sheet, and it represents by far the largest potential source for global sea-level rise under future warming conditions. The stability of the ice sheet determines the fate of our coastal cities1 and our beaches. It would be comforting to think that if we stopped producing greenhouse gases some time soon, then the Antarctic ice sheet will return to stability and all will be well. This would firstly require the relationship between global temperature rise and the melting of the ice sheet to be linear (i.e. 2 degrees rise in temperature would lead to twice as much melting as a 1 degree temperature rise would). In addition, it would require the climate to have little or no inertia. Unfortunately, feedbacks between the ice sheet, the atmosphere and the ocean are nonlinear1, and the climate has a great deal of inertia2. A new paper in one of the world’s premier scientific journals, Nature, has shown that the Antarctic Ice Sheet has several temperature thresholds beyond which ice loss is irreversible1.
In their paper in Nature, the authors have found that for global warming levels around 2 degrees Celsius above pre-industrial levels (we are already at 1 degree)3, West Antarctica is committed to long-term partial collapse owing to the marine ice-sheet instability. The ice sheet’s temperature sensitivity is such that we are committed to 1.3 metres of sea-level rise per degree of warming up to 2 degrees above pre-industrial levels (as I say above, we are already at 1 degree), almost doubling to 2.4 metres per degree of warming between 2 and 6 degrees and increasing to about 10 metres per degree of warming between 6 and 9 degrees. If we get to 6 degrees of warming, sea level rise will be the least of our worries. Each of these temperature thresholds gives rise to what is called hysteresis* behaviour; that is, the currently observed ice-sheet configuration is not regained even if temperatures are brought down to to present-day levels. In particular, for the West Antarctic Ice Sheet to regrow to its modern extent, we would need to decrease global temperatures to at least one degree Celsius lower than pre-industrial levels (i.e. 2 degrees below current levels). Their results show that if the Paris Agreement, to keep global temperature rise to 2 degrees or less, is not met, Antarctica’s long-term sea-level contribution will dramatically increase and exceed that of all other sources (e.g. Greenland). So, a 2 degree rise in global temperature will see a 2.6 metre rise in sea level, and many coastal towns and cities will suffer and many beaches will disappear completely.
*Hysteresis: the phenomenon in which the value of a physical property lags behind changes in the effect causing it.