Daily changes in the graphs, maps and numbers assessing sea ice are entertaining to follow. The day-to-day discrepancies between output from different research teams get close scrutiny by those of us on the outside, trying to learn more about what’s happening to the ice. Seen from the step-back view of monthly averages, however, daily disagreements blur into substantial agreement. Figure 4 illustrates by plotting time series of September means for five main indices produced by the National Snow and Ice Data Center (NSIDC: website, data), the University of Bremen’s Institute of Environmental Physics (Uni Bremen: website, technical paper), IARC-JAXA Information Systems (IJIS: website, data) and the Polar Research Group at the University of Illinois (Cryosphere Today: website, data). So far as I know, Figure 4 is the first graphic to show these five September series together. Disagreements among the indexes appear minor; all five vary together around declining area or extent trends that steepen over the period of observation.
In some respects the agreement is not surprising; the indexes all are estimating the same physical reality, either the area (nominally, the area with 100% ice concentration) or extent (area with at least 15% concentration) of Northern Hemisphere sea ice. Moreover, there are some commonalities among their sources. Both Uni Bremen and IJIS, for example, base their estimates on observations from the Advanced Microwave Scanning Radiometer (AMSR-E) instrument designed by the National Space Development Agency of Japan (JAXA) and orbiting on NASA’s Aqua EOS satellite. The two teams start with different AMSR-E data products, however, which provide different spatial resolution (6.25 km Bremen, 12.5 km IJIS). They apply different algorithms (Bremen, IJIS) and land/ocean masks to process the data, and differ also in what they view as the time span for each “day” reported. The third extent index discussed here, from NSIDC, is based instead on passive-microwave observations that have a 25 km spatial resolution. NSIDC applies 5-day smoothing in its published graphs; IJIS employs 2-day smoothing, while Bremen shows 1-day values.
Notable differences in level and movement sometimes appear when the day-to-day values are compared, as widely (and often, informatively) done on this blog. Discrepancies are much reduced, though, when we step back to view monthly means. The separately-derived NSIDC and Uni Bremen extent values for September 2011, for example, differ by just 30,000 km2 — less than 1%.
NSIDC extent, 4.61 million km2
Uni Bremen extent, 4.58 million km2
IJIS extent, 4.71 million km2
NSIDC area, 3.20 million km2
Cryosphere Today area, 3.19 million km2
The falling September minimum has been most dramatic, but Arctic sea ice extent and area have declined significantly in every month of the year. This is illustrated in Figure 5, a cycle plot of the longest series, Uni Bremen extent (an earlier post showed similar cycle plots of NSIDC area and extent). Each squiggly line in Figure 5 depicts 39 or 40 years of data for that month. The squiggle at lower right in Figure 5 (September) is simply a miniature version of the Uni Bremen line in Figure 4. The 2011 values, emphasized in this plot, have been among the lowest recorded for each month.
In terms of the more volatile daily numbers, 2011 set new minimum records for Uni Bremen extent (Figure 6) and Cryosat Today area (Figure 7). In both cases, the 2011 values passed (by thin margins) the previous one-day records set in 2007. NSIDC called the 2011 daily minimum “a close second” to that to 2007, and IJIS ranked it second as well.
The exceptionally low ice area and extent that occurred in September 2007 resulted partly from winds that favored compaction and blew ice out through Fram Strait into the Atlantic, where it melted. Similar winds have no doubt blown many times before, but they had exceptional effects in 2007 because they were pushing around thinner, patchier and less extensive ice that had been reduced through decades of melting by warmer air above and warmer water below (Polyakov, Kwok & Walsh 2011). In 2011 ice area and extent reached levels comparable to 2007, but this time largely by melting in place without much help from the wind. This result, together with reports of thin ice encountered by icebreakers near the North Pole in 2011, detailed analysis of multiyear ice reductions, and other lines of evidence, suggests that the classic indexes of sea ice area or extent in Figures 4–7 might actually be understating how much ice has been lost.
Which brings up the next topic, past and possible future trends in Arctic sea ice volume.