Last week, Edinburgh and Day (2016) used historical monitoring records to conclude that “the [Antarctic sea ice] levels in the early 1900s were in fact similar to today“. Apparently this was a surprising finding for those who assume anthropogenic greenhouse gases largely determine net changes in polar sea ice.
Perhaps it may also be surprising for those who only focus on the 1979-to-present satellite era to learn that Arctic sea ice has also remained essentially unchanged since the 1930s and 1940s too, and is overall still quite high relative to recent centennial- and millennial-scale historical periods. Even for the last few decades, the trends are not unusual.
For example, the IPCC referenced NOAA satellite data that extended back to 1972, not 1979, in the first UN report (1990). It showed that there had been a slight increasing trend in sea ice for 1972-1990 due to the low extent recorded during the early 1970s, and the very high extent in the late 1970s, when the current satellite datasets begin. Now, the IPCC (and NOAA, NSIDC) discard the 1972-1978 data from the sea ice record, instead using 1979 as the starting point, or the year with the highest sea ice extent since the early 20th century. This way, the decline in sea ice extent to the present can be steepened considerably in modern graphics.
Between 1990 and 2006, Arctic sea ice declined rapidly. Since 2006, however, the sea ice decline has undergone a pause, as shown in NSIDC data (using WoodForTrees.org interactive graphs):
Graph generated using WoodForTrees.org
Including the 1972-1978 trend with the 1979-2016 anomaly data (with added trend line) looks like this:
University of Illinois graph:
For the early 20th century, there was a dramatic decline in Arctic sea ice between the 1920s and 1940s that was concomitant with the as-warm-as-present Arctic surface temperatures (top graph). After this abrupt warming trend ended, the Arctic cooled for several decades and a subsequent increase in sea ice occurred through the late 1970s. Hoffert and Flanney (1985) furnish a graph with recorded sea ice trends for 1920-1975.
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