Picking cherries in the snow

Here is figure SPM3(a) from the IPCC AR5 SPM, captioned “Northern Hemisphere March-April average snow cover extent”.

Here is the corresponding paragraph, in which the IPCC excels in misleading cherrypicking:

There is very high confidence that the extent of Northern Hemisphere snow cover has decreased since the mid-20th century (see Figure SPM.3). Northern Hemisphere snow cover extent decreased 1.6 [0.8 to 2.4] % per decade for March and April, and 11.7 [8.8 to 14.6] % per decade for June, over the 1967–2012 period. During this period, snow cover extent in the Northern Hemisphere did not show a statistically significant increase in any month.

There is so much misleading spin here that it takes some time to unpack it all.
The first thing to note is that the graph does not show much of a decrease, certainly no decrease in the last couple of decades – another indicator of the pause in warming.
The first cherry-pick is that it refers only to the Northern Hemisphere; but this is reasonable since there is so much less land area in the Southern Hemisphere.  Now look at the months selected. Why are ‘March and April’ chosen, and why are they lumped together? (The same thing was done in the AR4 SPM.) And why mention June – a month not generally associated with snow?

Graphs of snow cover for different months can be found at the Rutgers site. (click on ‘Monthly Anomalies’ on the left).

Here is the graph for March. You can see that there has been a decrease, a rather abrupt one in the late 1980s, but since then there has been no decrease. The numbers are all over the place, so to talk of a linear trend per decade is misleading.

What about the winter months, that the SPM doesn’t mention? Well they show either no change, or an increase, for example in this graph for December.  The IPCC wording carefully avoids admitting this by saying there is no ‘statistically significant’ increase in any month.

I wonder if the decrease for March or April, considered separately, would count as statistically significant?

If you look at the ‘Seasonal extent’ graphs at the Rutgers site, you can see that although spring snow cover shows a decline, both the fall and winter graphs show a slight increase. So the IPCC statement claiming very high confidence in decreasing snow cover is not true.

In response to the similar but slightly differently worded claim in the draft SPM, one reviewer wrote “Misleading claim. Rutgers GSL data shows winter snow cover has not decreased.”

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2 thoughts on “Picking cherries in the snow

  1. I seem to recall an Independent Article from march 2000 when it was said:
    According to Dr David Viner, a senior research scientist at the climatic research unit (CRU) of the University of East Anglia,within a few years winter snowfall will become “a very rare and exciting event”. “Children just aren’t going to know what snow is,” he said.

    I rightly took this as a prediction of a catastrophic collapse of the snow cover before my 2year child was an adult. He will be an adult next year and there is absolutely no sign of a catastrophic collapse.

    I was lied to.

  2. I believe they use June because they go by calendar seasons: End of September through end of December is fall, end of December through end of March is winter and end of March through end of June is spring. It was the only idea I could come with when accused of “cherry-picking” for not using data through the end of June. Actually, I, too thought June was not spring.

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