Have satellites overestimated Antarctic sea ice growth?
It’s puzzling why Antarctic sea ice seems to be growing while earth’s other icy expanses are shrinking as temperatures rise.
Now new research questions whether there has been much of a rise in Antarctic sea ice after all. The paper suggests the small but significant growth scientists thought had occurred since 1979 could be little more than a “spurious artifact” of how satellite data is interpreted.
But other polar scientists tell us the implications of the new findings” are very limited indeed” and they’re confident Antarctic sea ice is still growing.
Bucking the trend
Scientists know ice is being lost from both the Greenland and Antarctic ice sheets. They also know the amount of sea ice in the Arctic is rapidly decreasing.
But satellite data suggest the amount of sea ice around Antarctica has been growing since 1979. A report from the Intergovernmental Panel on Climate Change (IPCC) last year put the size of the increase at 1.5 per cent on average per decade.
For comparison, that’s about a third of the rate of sea ice retreat in the Arctic. A new paper just published in journal The Cryosphere explains the puzzle this poses for scientists:
“[T]here has been substantial interest in the trend in Antarctic sea ice extent â?¦ primarily because of the observed asymmetry between increasing ice extent in the Antarctic and rapidly diminishing ice extent in the Arctic, and the inability of current climate models to capture this.”
The new paper raises an interesting point. It notes that the growth in Antarctic sea ice in the latest IPCC report is much bigger than suggested in the previous report in 2007. The authors say:
“[The 2007 report] reported the trend in Antarctic sea ice extent to be small and statistically indistinguishable from zero”.
The authors explain how scientists assumed the increase in Antarctic sea ice between the two reports was down to having more data, and set about trying to figure out why it might be happening.
Some scientists say it could be down to water melting beneath the ice shelves and refreezing on the surface. Others suggest local wind patterns are involved.
But the new paper say there may be another explanation – a change in the way measurements are made, rather than in the sea ice itself.
Satellite errors
Scientists use a number of techniques to translate satellite measurements into estimates of sea ice cover. It’s not straightforward – they have to account for interference from clouds, for example. Professor Andy Shepherd, polar observation expert at the university of Leeds tells us:
“Although we have several decades of satellite observations with which to chart changes in sea ice extent, the changes have been small in comparison to natural variability, and it has been difficult to establish the underlying (decadal) trend with confidence.”
And here’s Professor John Turner from the British Antarctic Survey to explain why it’s quite such a challenge (sorry about the poor quality, we’re working on our video skills).
In 2007, NASA updated its main sea ice dataset to include the latest scientific understanding. At the time, the update was “generally viewed within the community as having a negligible impact” on the size of the trend in Antarctic sea ice, the authors explain in the new paper.
But when the authors analysed the whole satellite record again, they found the NASA update had the effect of pushing sea ice values down a bit before 1991 and up a bit afterwards. Looking at the whole record, that makes the growth in sea ice appear quite a lot bigger than before, they explain.
This could mean one of two things, the paper concludes. Either the old dataset is wrong, or the newer one is. The authors say they can’t be sure yet but if it’s the latter, that raises the possibility that Antarctic sea ice growth has been overestimated.
Limited significance
Not so fast, say scientists at the British Antarctic Survey. Turner says the new study doesn’t question the current scientific understanding of what’s happening to Antarctic sea ice. He tell us:
“What they found in this latest study is that there was an error in the transition between two satellites in 1991 â?¦ It’s good to know this because we want to make sure we get the best record of sea ice extent”.
But the implications of the error are “very limited indeed”, Turner adds. Other independent sea ice datasets give very similar results to NASA’s updated one, which means scientists have confidence in what it says about Antarctic sea ice trends, he adds:
“The error they found only really affects estimates of sea ice extent up to about 2008. We can have high faith that the estimates that were used for the latest IPCC 5th assessment report are correct”.
The prospect of errors during satellite processing adds confusion to an already challenging field, warns Shepherd:
“The lesson is to take great care when combining measurements from different satellite sensors, and when trying to detect increasingly subtle changes of the climate system.”
Why Antarctic sea ice seems to be bucking the global trend for ice loss is an interesting question. But it looks like that puzzle isn’t going away on the back of this new research. Instead, the paper suggests that if it hadn’t been for a satellite processing error, scientists would have known about the growth in Antarctic sea ice a lot sooner.