There are several research centers that monitor Arctic ice extent, especially NSIDC (USA), DMI (Denmark), JAXA (Japan), and NANSEN (Norway). All start with the same data from passive microwave sensors on NASA satellites. Slight differences arise from different algorithms used to process the inputs into ice extent estimates.
Operational ice charts are an alternative measure of Arctic sea ice extent. These are prepared daily by Canada, Russia and US maritime authorities to assist ships navigating in Arctic waters. For example, the US National Ice Center (NIC) provides an index called MASIE (Multisensor Analyzed Sea Ice Extent). NIC charts are based upon not only passive microwave numbers, but also satellite imagery and reports from planes and ships operating in the regions. Operational ice charts are the most detailed pre-satellite records, with the Russian archives being the oldest.
What’s the Issue?
On another blog, a climate person put it to me this way:
“Based on what I’ve read if I had no other source, or were heading out in a boat, I would most certainly use the operational indices such as NIC. For climate change trends, I’d go with NSIDC, JAXA … Do you know of any climate scientists that prefer NIC?”
Measuring anything in the Arctic is problematic due to the conditions. And any technology has limitations and uncertainties. Thus it is useful to have more than one estimate of ice extent. Comparisons of the two types of data show the passive microwave results underestimate ice extent, especially during the late summer minimum. The difficulty is mistaking surface melt water for open water, failing to discern the ice underneath.
“Passive microwave sensors from the U.S. Defense Meteorological Satellite Program have long provided a key source of information on Arctic-wide sea ice conditions, but suffer from some known deficiencies, notably a tendency to underestimate ice concentrations in summer. With the recent release of digital and quality controlled ice charts extending back to 1972 from the U.S. National Ice Center (NIC), there is now an alternative record of late twentieth century Northern Hemisphere sea ice conditions to compare with the valuable, but imperfect, passive microwave sea ice record.”
“This analysis has been based on ice chart data rather than the more commonly analyzed passive microwave derived ice concentrations. Differences between the NIC ice chart sea ice record and the passive microwave sea ice record are highly significant despite the fact that the NIC charts are semi-dependent on the passive microwave data, and it is worth noting these differences. We compare the ice chart data to ice concentrations from the NASA Team algorithm which, along with the Bootstrap algorithm [Comiso, 1995], has proved to be perhaps the most popular used for generating ice concentrations [Cavalieri et al.,1997]. We find a baseline difference in integrated ice concentration coverage north of 45N of 3.85% ± 0.73% during November to May (ice chart concentrations are larger). In summer, the difference between the two sources of data rises to a maximum of 23% peaking in early August, equivalent to ice coverage the size of Greenland.“ ( My Bold)
The differences are even greater for Canadian regions.
“More than 1380 regional Canadian weekly sea-ice charts for four Canadian regions and 839 hemispheric U.S. weekly sea-ice charts from 1979 to 1996 are compared with passive microwave sea-ice concentration estimates using the National Aeronautics and Space Administration (NASA) Team algorithm. Compared with the Canadian regional ice charts, the NASA Team algorithm underestimates the total ice-covered area by 20.4% to 33.5% during ice melt in the summer and by 7.6% to 43.5% during ice growth in the late fall.”
From: The Use of Operational Ice Charts for Evaluating Passive Microwave Ice Concentration Data, Agnew and Howell
Or, if you don’t like what the US or Canada puts in their ice charts, you can get a third, independent perspective from Russia. The AARI has been studying and mapping the polar regions for a very long time, currently one of them chairs the ETSI and they maintain a global sea ice database (the other one is at NSIDC). Their ice charts can be accessed here:
A warning note: The Russians are not alarmed by what they see in the Arctic.
“In winter, the newly formed ice actively grows up to a 1.2 meter thick layer, while the coastal ice grows up to 2.0 meters. Consequently, the Arctic sea ice layer does not change significantly. Moreover, according to Genrikh Alekseev, in the summer, ice melts in various seas unequally. This year, the seas through which the Northern Shipping Route passes are covered with an unusually thicker ice layer. The Barents Sea is covered by a thin ice layer, but the amount of ice in the Kara, Laptev, East-Siberian and Chukotskiy seas exceeds the level of 2007. The conditions in the Arctic in the warm summer can be considered abnormal, but the Northern Shipping Route has not been completely freed from ice yet. This means icebreakers will be needed in the future, says the scientist.”
The extreme melting of ice in the summer 2012 is most likely the last gesture that the warming is ending. In fact, ice is a product of climate, and when comparing the graphs of the air temperature and melting ice, one can see that they coincide, Genrikh Alekseev said.
Operational ice charts are more variable due to human error in their production.
Climatologists prefer passive microwave indices of ice extent because they are consistently wrong.
One wonders about their preference if satellites were overestimating ice. For myself I am glad for two mostly independent measures, one done by people who only want to get it right that day.
Note: As of March 2015, NSIDC has been showing MASIE off-line since mid November 2014. They say NIC results will be back in April.
Update on April 2: NSIDC now says MASIE will be back after April.