Sea ice extent is the total area of ocean covered by at least 15% sea ice, measured daily via satellite passive microwave sensors (e.g., NSIDC Sea Ice Index). It differs from sea ice area (which accounts for actual ice concentration) or volume/thickness. Extent is a key climate indicator because it reflects seasonal freeze/melt cycles and long-term changes driven by warming.

Seasonal cycle: Arctic sea ice grows in winter (maximum typically in March) and melts in summer (minimum in September).

Units: Million square kilometers (km²).

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From Science Matters

By Ron Clutz

The Arctic ice extents are now fully reported for 2025, ending the year below average despite a higher rate of growth through December.

Note MASIE 2025 started 1M km2 (or 1 Wadham) below the 19 year average, but cut the deficit to 428k km2, or a gap of 3%. SII v.4 tracked lower than MASIE during December, drawing closer the last week. The chart below shows the distribution of ice extent across the Arctic regions at yearend 2025.

Region	2025365	Day 365 Average	2025-Ave.	2024365	2025-2024
(0) Northern_Hemisphere	12611676	13039302	-427626	12435177	176499
(1) Beaufort_Sea	1071070	1070458	612	1071001	69
(2) Chukchi_Sea	966006	964771	1235	965989	17
(3) East_Siberian_Sea	1087137	1087133	4	1087137	0
(4) Laptev_Sea	897845	897841	4	897845	0
(5) Kara_Sea	867623	887208	-19585	876527	-8904
(6) Barents_Sea	254882	423978	-169096	345715	-90832
(7) Greenland_Sea	668550	595658	72891	574537	94013
(8) Baffin_Bay_Gulf_of_St._Lawrence	768306	982649	-214343	982716	-214409
(9) Canadian_Archipelago	854931	853618	1313	854878	53
(10) Hudson_Bay	1256284	1215695	40589	922416	333868
(11) Central_Arctic	3174354	3206560	-32206	3207164.49	-32811
(12) Bering_Sea	350519	403002	-52483	325489.93	25029
(13) Baltic_Sea	14031	31873	-17843	15271.77	-1241
(14) Sea_of_Okhotsk	366812	393742	-26929	302941	63871

The major deficits are in Barents Sea and Baffin Bay (Atlantic basins), along with smaller losses in Bering and Okhotsk (Pacific basins).

Some years ago reading a thread on global warming at WUWT, I was struck by one person’s comment: “I’m an actuary with limited knowledge of climate metrics, but it seems to me if you want to understand temperature changes, you should analyze the changes, not the temperatures.” That rang bells for me, and I applied that insight in a series of Temperature Trend Analysis studies of surface station temperature records. Those posts are available under this heading. Climate Compilation Part I Temperatures

This post seeks to understand Arctic Sea Ice fluctuations using a similar approach: Focusing on the rates of extent changes rather than the usual study of the ice extents themselves. Fortunately, Sea Ice Index (SII) from NOAA provides a suitable dataset for this project. As many know, SII relies on satellite passive microwave sensors to produce charts of Arctic Ice extents with complete coverages going back to 1989.  Version 3 was more closely aligned than Version 4 with MASIE, the modern form of Naval ice charting in support of Arctic navigation. The SII User Guide is here.

There are statistical analyses available, and the one of interest (table below) is called Sea Ice Index Rates of Change (here). As indicated by the title, this spreadsheet consists not of monthly extents, but changes of extents from the previous month. Specifically, a monthly value is calculated by subtracting the average of the last five days of the previous month from this month’s average of final five days. So the value presents the amount of ice gained or lost during the present month.

These monthly rates of change have been compiled into a baseline for the period 1980 to 2010, which shows the fluctuations of Arctic ice extents over the course of a calendar year. Below is a graph of those averages of monthly changes up to and including this year. Those familiar with Arctic Ice studies will not be surprised at the sine wave form. December end is a relatively neutral point in the cycle, midway between the September Minimum and March Maximum.

The graph makes evident the six spring/summer months of melting and the six autumn/winter months of freezing.  Note that June-August produce the bulk of losses, while October-December show the bulk of gains. Also the peak and valley months of March and September show very little change in extent from beginning to end.

The table of monthly data reveals the variability of ice extents over the last 4 decades, with gains in blue cells and losses in red cells.

The values in January show changes from the end of the previous December, and by summing twelve consecutive months we can calculate an annual rate of change for the years 1980 to 2025.

As many know, there has been a decline of Arctic ice extent over these 40 years, averaging 70k km2 per year. But year over year, the changes shift constantly between gains and losses, ranging up to +/- 500k km2, (2024 being exceptional). Since 1989 the average yearend gain/loss is nearly zero, -0.049k km2 to be exact.

Moreover, it seems random as to which months are determinative for a given year. For example, much ado was printed about 2023 losing more ice than usual June through September. But then the final 3 months of 2023 more than made up for those summer losses, resulting in a sizeable gain for the year.

As it happens in this dataset, October has the highest rate of adding ice. The table below shows the variety of monthly rates in the record as anomalies from the 1980-2010 baseline. In this exhibit a red cell is a negative anomaly (less than baseline for that month) and blue is positive (higher than baseline).

Note that the  +/ –  rate anomalies are distributed all across the grid, sequences of different months in different years, with gains and losses offsetting one another.  As noted earlier,  in 2023 the outlier negative months were June through September where unusual amounts of ice were lost.  Then unusally strong gains in October to December resulted in a large annual gain, compared to the baseline. The bottom line presents the average anomalies for each month over the period 1979-2025.  Note the rates of gains and losses mostly offset, and the average of all months in the bottom right cell is virtually zero.

A final observation: The graph below shows the Yearend Arctic Ice Extents for the last 35 years.

Year-end Arctic ice extents (last 5 days of December) show three distinct regimes: 1989-1998, 1998-2010, 2010-2025. The average year-end extent 1989-2010 is 13.4M km2. In the last decade, 2011 was 13.0M km2, and six years later, 2017 was 12.3M km2. 2021 rose back to 13.0  2024 slipped back to 12.2M, and 2025 is back up to 12.4M. So for all the the fluctuations, the net is virually zero, or a loss of one tenth of a Wadham (0.1M) from 2010. Talk of an Arctic ice death spiral is fanciful.

These data show a noisy, highly variable natural phenomenon. Clearly, unpredictable factors are in play, principally water structure and circulation, atmospheric circulation regimes, and also incursions and storms. And in the longer view, today’s extents are not unusual.

It uses red bars for year-to-year values (reconstructed pre-1979, satellite-observed post-1979).

A blue trend line is overlaid.

The annotation points to a low point around the 1930s–1940s and accuses “alarmists” of starting trends from ~1979 (satellite era) while “ignoring” earlier lows.

There was a significant dip in sea ice during the Early Twentieth Century Warming (ETCW, ~1920s–1940s), with extent roughly comparable to (or only slightly higher than) recent lows. This suggests natural variability played a big role, and modern decline isn’t unprecedented.

The method relies on a statistical relationship between summer Arctic surface air temperatures (from weather stations) and satellite-era sea ice extent. They essentially used temperature data as a proxy to “back-calculate” ice extent before direct observations.

This reconstruction is not mainstream.