Olive Heffernan

A few months back, I was asked to give a talk on sea level rise as part of a routine editorial meeting, where we discuss issues of interest across various publications within the Nature family.

I was excited by the opportunity for two reasons. Firstly, while the factors contributing to sea level rise appear to be well understood, there is little consensus within the scientific community as to exactly how much of an increase we can expect this century. In fact, it’s still a topic over which research scientists routinely clash. But given that there’s agreement that sea level will rise to some extent, I was also interested to know what, if anything, is being done to prepare for it in regions that are most vulnerable. After some research, I discovered that there isn’t exactly a consensus on that either.

Having mulled the topic of sea level rise for some time, I decided it was a topic that I’d like to cover in more detail. The upshot of all this is the latest issue of Nature Reports Climate Change, which attempts to answer my two initial questions:

1) How well can we predict sea level rise?

2) Faced with uncertainty, what’s being done to prepare for the impacts?

To answer the first question (I’ll deal with the second in a subsequent post), I called on some experts who had recently aired their (quite different) opinions on the issue of how much global warming could raise sea levels by 2100.

But first, for those who haven’t followed this issue closely, a bit of background. When the Intergovernmental Panel on Climate Change released its assessment report on global warming in 2007, it was faced with a tough decision on what to say about sea level rise. In estimating how sea levels might rise in the future, the panel considered several factors such as how much the oceans will expand as they warm and how much water will be added from melting mountain glaciers and ice caps. But for one potentially important source of sea level rise – the break up of large ice sheets covering Greenland and West Antarctica – there was little information. After some consideration, the IPCC estimated a total sea level rise of 18 to 59 centimetres by the 2090s, a projection that excluded the contribution of the world’s largest ice sheets on the basis that understanding was too limited “to provide a best estimate or an upper bound”.

A number of studies published since the last IPCC assessment report have estimated much higher levels of sea level rise this century – in some cases more than 2 metres by 2100. Rather than considering the various processes that individually contribute to sea level rise, these semi-empirical approaches are based on the idea that the rate of sea level rise is proportional to the amount of global warming and they use past sea level and temperature data to quantify this effect.

Stefan Rahmstorf – who published some of the first work using this method back in 2007 – writes that “the difference between the semi-empirical estimates and the model-based estimates of the IPCC can be attributed largely to the response of continental ice to greenhouse warming”. His most recent study, with Martin Vermeer, gives a median estimate of 124 centimeters of sea level rise by 2100 or 114 centimeters by 2095.

But, write Jason Lowe and Jonathan Gregory write in a seperate commentary, the question remains as to how much the break up of the Greenland and Antarctic ice sheets will contribute to sea level this century. On this, the jury is still out, they say. Yes, satellite observations show that both ice sheets are losing mass overall and that the speed of outlet glaciers (which spit large chunks of ice into the ocean) has increased of late, but, say Lowe and Gregory, this may be due to natural variability and, as such, it may not continue.

Rahmstorf attributes the high end of his recent estimate to the fact that as temperatures rise, more ice will melt and some of that will be from ice sheets. Ultimately loss ice will depend on local climate, not on whether that ice is locked up in mountain glaciers or in ice sheets, says Rahmstorf. As he puts it “we’re not going to run out of ice to melt anytime soon”.

Lowe and Gregory, on the other hand, say that high estimates from semi-empirical methods can be attributed to large dynamical changes in ice sheets only if there was a large contribution from ice sheets to sea level in the past. They say that this wasn’t the case (because the contribution from ice sheets has only increased in recent years), and that as such, these studies lack a plausible physical mechanism to explain the projected rise in sea level.

But Rahmstorf contends that, based on recent observations, loss of continental ice could be responsible for as much as 80 percent of future sea level rise, making his estimate of 114 centimetres by 2095 feasible.

I guess the jury is still out. Both commentaries are freely accessible here.