The Science and Politics of Mapping Sea Ice

Source: National Snow & Ice Data Center

There's an interesting story this week about mapping Arctic sea ice.  It is well known that the ice cap has been retreating in recent years, setting several records for open water in the late summer.  This graph from the National Snow and Ice Data Center illustrates the decline in sea ice over the last three decades.  The Center has also reported that the sea ice extent recorded in March 2015 was the lowest since satellite observations began.

 So that's the context for the story this week: as reported in The Globe and Mail, the federal government has released a new map of Canada, which includes the surprising news that there is apparently more Arctic ice now than in 2006. 

2006 map (source: http://geogratis.gc.ca)

Current map (source: Atlas of Canada Reference Map Series)

The basis for this change is that while the 2006 map showed only permanent sea ice at that time, the new map uses the median of September sea ice from 1981 to 2010.  (According to Yvan D�sy of Natural Resources Canada, this change makes the map consistent with how Environment Canada computes sea ice extent.)

It's a defensible way to present data, but only if we accept a crucial assumption: that there is no long-term trend in sea ice.  And that's a huge "if": one that is contradicted by the last three decades of observations.  So here we have a problem, because Statistics 101 teaches that getting an average out of a long-term trend only produces nonsense.  (An analogy would be to report a person's height as the average of her heights on each of her birthdays from age 1 to 30.)

Nonsense or not, this new map is already being drawn into political debates.  Those dismissive of climate change will see it as "proving" that sea ice melting is a myth; others will see the map as evidence that the federal government is manipulating data to be consistent with its preference for avoiding climate action.

And for those who follow climate science and politics, it's another interesting example of how the assumptions scientists make about data can have weighty political consequences.