Walking On Thin Ice
The recent spate of stories about the condition of the Antarctic ice sheets will have been consumed by the public as further evidence that man is changing the climate in bad ways.
This time the focus was on Cryosat observations of the West Antarctic ice sheet where losses are said to have doubled in the past decade or so, and a handful of important glaciers are poised to slide into the Amundsen Sea and melt raising global sea levels.
The problem with the story is that the case for these long-term changes in the Antarctic being due to mankind is a difficult one to make. In the absence of a statement saying this in a media report the public, who are so used to media reporting of mankind’s degradation of our climate, will assume it is more of the same.
In reporting climatic changes one has to be careful about timescales and errors. The Guardian superficially reported that even the East Antarctic ice sheet is now losing ice, and then omitted to say that the mass loss rate was 3 +/- 36 Gt per year which is a mass loss that is small in comparison to fluctuations in snow accumulation, and is hardly a statistically significant figure anyway! The mass loss rates from the Antarctic Peninsula are hardly catastrophic being 23 +/- 18 Gt per year.
The changes affecting the stability of the West Antarctic ice sheet are a different matter being, according to the recent Cryosat data obtained only between 2010 and 2013, 134 +/- 27 Gt per year. Even if this is compared to data obtained by other means for the period 2005-10 one has to be careful comparing them. One of the chief lessons to emerge from studying climate change data is that decadal variations are poorly understood.
This is especially important when it comes to forecasts. The timescale for the “collapse” of the West Antarctic ice sheet is measured in hundreds of years, perhaps thousands. A lot can change over that period. Add that to the fact that we do not have computer models that give us the ability to predict the behaviour of the West Antarctic ice streams and you will see that what some scientists are quoting is guesswork, which is ok except that it is reported by the media as something far more definite.
What was also not mentioned in reports is that many scientists believe that the West Antarctic changes are due to the southerly migration of the Antarctic Circumpolar Current at about a degree of latitude every 35 years.
This increases the supply of warm deep water to the Antarctic continental shelf. There is evidence that the supply of warm deep water declined in the 1990s to early 2000s, and has increased since then.
Whatever mankind’s effect on the climate nature itself brings about marked changes, as anyone who has studied the subject knows. Consider the temperature of the Holocene – mostly warmer than it is today, or the Little Ice Age now recognised as a truly global event. In recent years many have said that mankind’s influence on the climate dominates over nature as if that renders natural changes unimportant. If mankind’s effects have been exaggerated we must not forget the changes nature itself can bring. The melting of the West Antarctic ice sheet may be one of them.
Looking back the West Antarctic ice sheet has done this kind of thing before, and recovered, although mankind was not around in such numbers as he is today or was concentrated in coastal cities.
It has been said by some scientists that anthropogenic climate change is a combination of lots of things any one of which is relatively unimportant. When taken together these effects add up to an obvious overall pattern showing mankind’s fingerprint on the climate. The West Antarctic is another one of those interesting regions where evidence of that fingerprint on the changes going on there is less obvious than it once was.