If you read The Guardian’s news coverage of the extent of Arctic Sea Ice, you might be forgiven for thinking that something special had happened.
They state that “2017 is the third year in a row the Arctic’s winter ice has set a new low.“. And they quote the director of the US National Snow and Ice Data Centre (NSIDC) as saying
“I have been looking at Arctic weather patterns for 35 years and have never seen anything close to what we’ve experienced these past two winters,”.
But the truth is simpler and can be seen and understood by a child.
The extent of Arctic Sea Ice is declining year on year.
It has been happening for a couple of decades and we have no reason to think it will stop.
The graph at the head of the page shows the extent of Arctic Sea Ice in millions of square kilometres. This has been assessed by satellites* every day since 20th October 1978 and the data can be downloaded from here.
As the graph shows, each year the sea ice grows in the northern hemisphere winter by an astonishing 10 million square kilometres. And shrinks by a corresponding amount in the summer.
The graph shows that on average:
- The maximum extent of the sea ice in winter has been falling by about 44,000 square kilometres every year.
- The minimum extent of the sea ice in summer has been falling about twice as fast – by about 84,000 square kilometres every year.
So since 1979,
- the extent of the winter sea ice maximum has fallen by about 1.6 million square kilometres and,
- the extent of the summer sea ice minimum has fallen by about 3.2 million square kilometres .
To put that into context, the 3.2 million square kilometres is about 12 times the land area of the UK – or roughly the land area of India.
The two graphs below show the decline in winter maxima and summer minima in more detail.
And what is clear is that the decline in Arctic Sea Ice this year is pretty much exactly what we would have anticipated.
What happens next?
Well, we are now talking about ‘the future’ so the answer has to be ‘nobody knows’.
But the trends look to be well-established, and in our best understanding, the ultimate cause of the decline – the warming of our planet’s surface – will not abate for many decades.
So eventually we will see the Sea Ice Extent fall to zero in the summer. Drawing a straight line through the data, one obtains an estimate of about 2065.
However many ‘so-called experts’ think that an ice-free summer will come much sooner. They argue that sea ice extent is a 2-dimensional measure of a 3-dimensional quantity – the volume of sea ice.
They argue that accompanying the decline in sea ice area, there has been a thinning of the sea ice.
Satellite measurements of sea ice thickness are relative new, and don’t yet show any clear trend. But despite that, scientists have been combining the sparse data that do exist with the data on sea ice area to produce an estimate for Sea Ice Volume . Their estimates are shown below.
Now we can see the true drama of the situation. While the sea ice minimum area has declined by approximately 30%, the sea ice minimum volume has declined by approximately 70%.
For this data, a linear decline no longer captures the trend of the data. Fitting a quadratic trend and extrapolating, the estimate of the date at which summer sea ice volume reaches zero moves forward from 2065 to 2021.
As I mentioned, we are discussing ‘the future’ so no-one knows what is really going happen: 2021 is probably too early, but 2065 is probably too late
But as the trend continues, the likelihood is that the sea ice will become more fragile, and eventually it will become thin enough that even mild storms will break it up.
In our lifetimes** we will reach a condition where the sea ice in the northern hemisphere entirely melts every summer. The North Pole will have become the North Pool.
*Reader: I had thought the measurement was made from analysing visual images, but in fact it is made using microwaves. The emission of microwaves from water and ice have different characteristic polarisations and the contrast allows the fraction of sea-ice to be estimated. Details here. Sorry for the initial mistake, and thank you to Victor Venema for spotting it.
**Reader: I hope your life is long and healthy.