It’s Paul Homewood whose claims are false.
One look at their graph shows this is patently not true, despite grossly misleading linear fit, intended to fool people.
What graph, you wonder? This one:
It shows each year’s minimum sea ice extent in the Arctic. The amount of ice changes with the seasons: more in winter/spring, less in summer/fall, and each year it reaches its minimum some time in September; the graph shows how much ice remained even at the year’s lowest point.
Here’s my version, which I think is a bit easier to see:
I included a best-fit straight line (by least squares regression, shown as the red line), as did the U.K. Met Office.
Paul Homewood says he knows what’s really going on:
It is very easy to show that Arctic sea ice has stabilised. As their graph itself shows, there have only been three years since 2007 with lower ice extent than that year, and eleven have had higher extents.
Also the average of the last ten years is higher than 2007’s extent.
It’s obvious that this is how Paul Homewood sees the graph:
He’s drawing a rather bold conclusion from a very short time span and a very small number of data values. He himself says:
In itself, this is too short a period to make any meaningful judgements. But that is no excuse for the Met Office to publish such a manifest falsehood.
I find it ironic that he says outright “this is too short a period to make any meaningful judgements,” just after saying that “It is very easy to show that Arctic sea ice has stabilised.”
The linear fit shown by the U.K. Met Office (and in my 1st graph too) is estimated by least squares regression, which not only gives us an estimate of the rate (i.e., how fast is the variable rising or falling), it also estimates the uncertainty in our estimate.
I computed linear fits for both time spans discussed by Paul Homewood: the pre-2007 values and the post-2007 values (shown as red lines in the graph of how Homewood sees things). This gives me an estimate of the rate, and of its uncertainty, for each time span separately, which enables me to compute the likely range in which their rates fall. I can then compare their estimates to see how different they are, and I’ll do so with this graph showing the ranges for each interval:
For the first interval (pre-2007), the annual minimum extent of arctic sea ice was definitely declining, at a rate estiated at 55,000 km^2/year, but not less than 35,000 km^2/year or more than 75,000 km^2/year. For the later interval (post-2007), we can’t tell from the available data whether it’s rising or falling, be we can put limits on its rate. It’s not increasing faster than 47,000 km^2/yr, nor decreasing faster than 69,000 km^2/year.
Think about that. Using only the data since 2007, sea ice might be increasing or it might be decreasing faster than it was before. The rate estimate is too uncertain to tell. But Paul Homewood believes he can tell — by just “one look at their graph.”
Unlike Paul Homewood, I’m far more interested in what’s really happening to arctic sea ice than in heaping scorn at the U.K. Met Office.
First and foremost, the yearly minimum is only one day out of the year. We have sea ice extent data throughout the year, and what happens during the rest of the year counts. Instead of using the annual minimum, let’s use the annual average. To avoid losing the most recent data, I’ll compute the yearly average for October throught the following September rather than the usual (but arbitrary) January through December. I’ll also omit October 1978 through September 1979 because that year is incomplete. I get this:
I wonder what Paul Homewood will conclude from “one look at the graph.”
The annual averages show much less fluctuation than the annual minima, so we can estimate things like rates of change with greater precision. I find that there is statistical evidence that the rate changed over time. One model of such changes uses three straight-line segments with their changes chosen to best-fit the data, like this:
This fit also enables us to estimate the rates, and their uncertainties (and therefore likely ranges), for each time span. Here they are:
From this analysis, I estimate the rate of change of Arctic sea ice extent as follows: from 1979 to 2002 it decreased at a rate of 31,000 (+/- 9,000) km^2/year; from 2002 to 2007 it decrease super-fast, at a rate of 143,000 (+/- 36,000) km^2/year; since 2007 it has declined at a rate of 42,000 (+/- 15,000) km^2/year.
Note that during the final time span, post-2007, the rate is negative with statistical significance. That wasn’t true for the annual minima, mainly because the degree of fluctuation of the yearly minimum is so much bigger than the degree of fluctuation for the anual average, that our rate estimate from annual minimum is far more uncertain. So much more so, we can’t even tell with confidence whether it’s increasing or decreasing.
But with the annual averages, the answer is clear. Using only data since 2007, we can still show the statistically significant decline in Arctic sea ice.
Just like the U.K. Met Office said.
As for “Arctic sea ice has stabilised,” no — it hasn’t.
I think I know Paul Homewood’s method of analyzing climate data: one look at a graph, to see what you want to see.
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