Where does that data come from?

Arctic Sea Ice End of August 2013

Graph from the US National Snow and Ice Data Centre showing the extent of Arctic Sea Ice at the end of August 2013.

Good news! As the graph above shows, it looks like Arctic Sea Ice has not collapsed this year as it did in 2012. The graph below shows projections based on previous seasons and it seems very unlikely that sea ice extent will fall much further in the last month of the ‘melt season’. Excellent.

Graph from the US National Snow and Ice Data Centre showing the extent of Arctic Sea Ice at the end of August 2013.

Graph from the US National Snow and Ice Data Centre showing the predicted extent of Arctic Sea Ice from now to the end of the melt season. The red line at the bottom shows the minimum value reached last year.

This excellent page at the US National Snow and Ice Data Centre shows some of the details behind these summary graphs and discusses in detail the difference between weather patterns this year and last.

But although this data is relatively simple to understand, it is not at all obvious what exactly is being measured! And just where does this data come from anyway!

Many regions of the Arctic have partially-melted ice in the summer, and the quantity plotted above (sea ice extent) is somewhat-arbitrarily defined as being the area in which sea ice coverage exceeds 15%. So, for example, a decline in coverage in one particular area from (say) 30% to (say) 20% wouldn’t show up on these graphs. The map below shows the data from which the ‘Sea Ice Extent’ graphs are derived. It is pretty clear that the data present a complex picture.

Colour-coded map showing the sea ice coverage.

Colour-coded map showing Arctic sea ice coverage on 23 August 2013. The purple areas are essentially 100% covered, but large areas are only 50%-75% covered, even quite close to the North Pole itself.

This data is derived from the Advanced Microwave Sounding Radiometer 2 (AMSR-2) on board the “Shizuku” (GCOM-W1) satellite launched by the Japanese Space Agency in 2012 to replace an instrument which stopped functioning. The instrument is extraordinary. It measures the extremely weak emissions from the Earth in the microwave part of the electromagnetic spectrum.

As it orbits 700 km above the Earth, a 2 metre diameter mirror captures the microwaves and focusses them onto 6 different detectors sensitive to different microwave frequencies. The mirror rotates every 1.5 seconds allowing the sensors to take data from a swathe of the Earth more than 1000 km across.

The reason I am mentioning all this is to make this simple point: even when one sees relatively simple-to-understand graphs such as those at the start of the page, they are often derived in quite a complex way. Now not everyone needs to know all the details of how every graph we see is derived. But it’s a question which is worth asking every time we see any graph – “Where does that data come from?”

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