Ice sheets in Antarctica: losing or gaining?

A friend recently highlighted last year’s NASA research on Antarctic ice-sheets and asked me to comment. So I’m going slightly away from the journey reprise to take a quick jaunt around some of the latest publications.

In short: some ice sheets may be thickening slightly but that does not mean we should stop worrying about seal level rise. First: any extension in sea or land ice, if it is happening, does not counteract the ice being lost elsewhere. And more worryingly, in West Antarctica we may already be at an irreversible tipping point which will put several meters on to sea level. This post seeks to summarise why, despite the NASA research, you should continue to minimise emissions and support other initiatives to contain global warming.

Last October NASA published a paper which said that many of the Antarctic glaciers were gaining ice faster than they were losing it. (The team, led by glaciologist Jay Zwally, excluded the Peninsula and the Thwaites and Pine Island regions from this conclusion.) The study made a big stir. In particular it enabled many climate change deniers to celebrate, saying that the scientists were’ wrong again’ and it showed all the evidence for global warming was incorrect.

Let’s look at the status of the study and responses. It was undertaken by highly respected scientists and was based on measuring the thickness of the ice across the continent. It suggests that the ice has gone on thickening since the last ice age, and it seems enough ice is still being generated by snow fall and compaction to more than outweigh the increasing rate of calving and glacier loss at the edges of the continent. The measurements are based on more sophisticated satellite-based altimeter studies than before, and a new iteration of the models which examine ice-cap behavior across the complex underlying topography. The study included trying to understand what happens where ice-sheets cover land which is below the surrounding sea-level. This last point is particularly complicated: the ice itself weighs down the land (known as isostatic pressure), so to model how ice will melt where it meets the sea, you need to calculate the mount of the pressure, and hence how much the land itself will rise as the ice disappears.

So this is a well-constructed study, using complex technology and models, which gives results which appear to go against other studies. That’s science: create a hypothesis, test it and assess the results. But we cannot dismiss the study as quackery.

The study followed earlier NASA research, published in February 2015, that showed that global sea ice was diminishing despite the previous southern winter showing the biggest sea-ice coverage in Antarctica in the 35 years of detailed study. (And let’s bear in mind it is only very recently we have been able to study the whole continent  at once, rather than relying on spot observations by ships, ice-berg telemetry etc. So the longitudinal data is not brilliant. Remember that weather changes all the time, while climate is a matter of trends and long term atmospheric and oceanic behavior.)

One of the particularly interesting features of this study was that sea-ice had grown in the Southern Ocean, but decreased in the Bellinghausen and Admundsen seas, around the Antarctic Peninsula which is known to have warmed significantly in the last decades. In this article, the lead scientist (Clair Parkinson, climate scientist at NASA) specifically says that the measured growth in extent of Antarctic sea ice does not cancel out losses in the Arctic.

So we have well-founded research saying Antarctica is growing ice both through increased glacial thickness across the land and apparent growth in the spread of sea ice in the winter. Surely that’s a good thing, and we can breathe a little easier? There are several reasons why not. 

The first and most obvious is that where the evidence is so different from the hypothesis and events observed elsewhere, it becomes important to understand why. In Antarctica, of course, the challenges of research are extreme, but a few clues are around. Even in 2014, NASA were wondering about this and the same team published an article which looked in detail at wind patterns. This suggests that the changes in the frequency or intensity of low pressure systems in the Amundsen Sea may be a factor, along with the impact of more melting ice from the glaciers (remember the rate of discharge – calving by glaciers – is still increasing) and even increased snowfall. As Parkinson said then ‘ there hasn’t been one explanation yet.’ Two years on that still seems to be the case: if it is the case that parts of Antarctic and the Southern Ocean are seeing increased ice, we do not know why.

All of this is well summarised in a lay article in The Washington Post in November 2015. They were able to go beyond my reach and contact other glaciologists. Their concerns relate both to the methodologies and the models used in the NASA study. As the Washington Post emphasises, ‘none of this means you should worry less about Antarctica and sea level rise.’

This is particularly true for the West Antarctic Ice Sheet, the area coming out into the Amundsen Sea, about which I wrote in the wake of the Paris talks. Research published by German climate scientists in June 2015 discussed the extreme uncertainty surrounding Antarctic melting and particularly the instability in the western sheet (which would raise sea levels by 3m if it fully melted.) The issue here is whether destabilization in the Amundsen Sea, while apparently local, could lead to the melting of the whole sheet, and whether that tipping point has in fact already been reached. This research also specifically comments on the potentially increased ice-sheet mass, saying ‘[a]lthough increased ice-sheet mass gain through snowfall may counteract imbalance, it can also lead to an increase in ice discharge resulting from the difference in surface elevation change between grounded and floating regions (34). The ice softening through higher surface temperatures tends to intensify ice flow and associated discharge.’ There might be more snow, but its impact may even accelerate calving and ice loss because of the changes of surface (and atmospheric) temperatures.

The authors go on to say, though, that their model, while closer to real-world observations than others used so far, is incomplete, especially in simulating how the ice and ocean interact. What we do know is that some glaciers in West Antarctica are retreating rapidly, and that the large Thwaites glacier (the size of Pennsylvania and about a mile thick) is also getting thinner.

Map showing ice retreat in West Antarctica

If Thwaites, which is key to the West Antarctic Ice Sheet, melted, that would probably destroy the whole of that ice sheet in a cascade which could add 3m to sea level rise. The picture (taken from the Washington Post) shows where all this is happening. The Europa trip touch the very tip of the Peninsula’s finger on the top left of the continent as oriented in this diagram. It is part of a useful article they published  last November. Again, they contacted other scientists who cautiously welcomed the modelling done by the German team, while pointing to the unknown timetables and the expectation that there will not be change so dramatic we do not have time to prepare our coastlines and cities.

The obvious conclusion from all this is that we need to know more. And indeed that seems to be the objective of the scientific community. The US National Science Ice and Data Centre, held two major workshops on the matter in April 2016 and have published the findings. They propose  a trans-national research effort, entitled how Much, How Fast, focused on the Thwaites Glacier and Amundsen Sea to understand what is going on. That paper talks about collaboration, especially with the British Antarctic Survey. BAS has a major ongoing study into the stability of the West Antarctic Sheet, known as iSTAR. Other national Antarctic  research programmes will assuredly be looking at similar questions.

Overall, don’t panic but equally, don’t stop being concerned. The deniers may wish to use the NASA research to pretend climate change is not going to hurt us. In many places, we are already seeing the impact as parts of the Solomon Islands disappear, polar bears lose their habitat and heat records are broken month after month. Climate change is real and it’s here.

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