Climate Brief: Vignettes from the front lines - the Arctic sea ice maximum

Introduction: A quick overview of the last 30 years

If you look back at my story record from say 2010 through 2012, you’ll find some diary entries from 2010 through 2012 titled “News From the Arctic: ...”. For awhile I wrote them weekly, but I slowed over time and then stopped for good in May 2012. For this week’s Climate Brief, I thought it would be interesting to look at Arctic sea ice as we approach the Arctic sea ice maximum, which typically occurs any time from the last few days of February to as late as the 2^nd of April (in 2010).

A note on how sea ice extent is measured

Satellite observations are used to determine sea ice extent. If a water “pixel” (~3x3 km) has a 15% or higher sea ice coverage, it is considered to be covered with ice. The 15% threshold is used to reduce the large variability found at low percentage coverage. that result from day-to-day weather variations.

I used the banner graphic to display the time series of the last several months’ Arctic sea ice extent (from 1 December 2021 through 9 March 2022, in light blue) from the National Snow and Ice Data Center (NSIDC), located at the University of Colorado — Boulder. Other data are plotted through April 30:

• The minimum sea ice extent year (2011-12) is in dark green dash for comparison.
• The heavy gray line is the median value (i.e. in the exact middle) for Arctic sea ice extent from 1981-2010
• The dark shading represents the “interquartile range (from 25% to 75%)
• The wider, lighter shading is the interdecile range (10% to 90%).

There has been no year since 2001 with maximum sea ice extent as high as the 1981-2010 median, with every year since 2006 in the bottom 10% of the 1981-2010 range.

Next, here’s a plot of the 1981-2010 median Arctic sea ice extent (heavy gray line) with the 1979-1990 (gold), 1991-2000 (light green), 2001-2010 (black), and 2011-2020 (blue) averages.

You can easily see the rapid decrease in warm season Arctic sea ice minima, especially over the latest two decades. In 1979-90, mean sea ice minimum averaged about 7,000,000 km²; while the following three decades averaged about 6.5 million, 5.5 million, and 4.4 million km², respectively. The final 2011-2020 decade includes the record low minimum sea ice coverage of about 3.4 million km², which occurred in August 2012, when an abnormally strong cyclone entered the Arctic Ocean basin and caused massive disruption of the sea ice in an area where it was already unusually thin.

The Road to This Year’s Arctic Sea Ice Maximum

The 2021-22 sea ice freeze season started from the highest sea ice minimum since 2009. The summer season, though 1-2^oC warmer than normal over much of the Arctic Ocean basin, was also characterized by lower pressure and more cloud than normal, which limited sunlight available for absorption by exposed water. Because of wind anomalies consistent with the lower than normal pressure, circulating sea ice spread out more than in a typical summer. This led to larger areal coverage of 15% or more sea ice (but not more overall sea ice) than in recent years.

I’ve annotated a graphic from NSIDC showing annual or season to date Arctic sea ice extents for 2021 and 2022. For comparison, I’ve included the ±2 standard deviation from the 1981 to 2010 median, and the decadal averages shown before (same color scheme).

The 2021-22 Arctic sea ice season began with a minimum higher than any since 2014, but on a date only two days later than the average of 14 September. October found faster growing sea ice than in recent years because of favorable conditions on the Siberian side of the Arctic Ocean. The faster than normal increase in sea ice area continued in November, and by the end of the month was just above the lowest 10% values over the satellite observation period for the first time since 2017. This was in spite of essentially no sea ice in the whole of Hudson Bay.

In December, colder than normal weather in central Canada froze over the entire Hudson Bay, bringing it to near-average ice cover. And while well-below normal, Arctic sea ice extent was the highest since 2014. By the end of January, sea ice extent was ranked higher than it had been since 2009. February saw a slowing of sea ice growth leaving area coverage at the lowest 10% of the recorded values for any given day. Finally, as of yesterday, 9 March, sea ice extent is actually about 100,000 km²lower than it was at the end of February which, if trends continue, would mark one of the earliest maxima in the satellite record.

Can We Tell What the Melt Season Will Bring?

To do any predicting for the melt season, we need more than just sea ice coverage. We need to know the sea ice volume: the area times the ice thickness. The latter is often difficult to “get at” from observations, though innovations in satellite data processing, as in many other Earth observational methods, allow for decent estimates of sea ice thickness when combined with sea ice volume models, discussed next.

Sea ice volume models

Sea ice volume can be obtained from satellite estimates of sea ice thickness, Naval submarines, moorings from ongoing field campaigns out on the ice, etc.). These are not continuous given current capabilities, so a sea ice volume is used to make the data continuous and consistent over time. The Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS) is just such a numerical model, developed by the Polar Science Center in association with the University of Washington. with components for sea ice and ocean, and a data assimilation system to constrain the ice volume to best fit the analysed weather conditions, like wind, surface air temperature, and cloudiness (for incoming solar and outgoing terrestrial (long wave) radiation). The necessary sea ice concentration information from NSIDC comes from satellite data and sea surface temperature (SST) data from the NOAA Ocean Prediction Center are used for ice-free areas.  The wind, surface air temperature, and cloud cover (to compute solar and long wave radiation) are specified from the joint National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. The pan-Arctic ocean model is provided input data from a global ocean model at its open boundaries located at 45^oN.

History of Sea Ice Volume

Three graphics tell the story about sea ice volume during the satellite data record starting in 1979 …decline. Satellite data on sea ice drift can be used to calculate the age of sea ice. First year ice was born in the most current freeze-up season, with any ice that survives a melt season denoted as multiyear ice, whose age is the number of melt seasons it has survived, The age of sea ice can be used as a proxy for sea ice thickness. The contrast between 1984 and 2021 is striking! In 1985, more than half of the sea ice was more than four years old, while in 2021, almost all of it has vanished, with most of the ice from just the previous freeze-up.

How does this translate to volume? Here we’ll use the PIOMAS model results described previously. April is used for the volume maxima and September for the volume minimum, with data plotted from 1979 — 2021, The right edge of the graphic which shows 2021 is, unfortunately, cut off.

Again, the trends are clear, with decreases of 2.6±1 km³/decade for volume maxima and 3.2±1 km³/decade for volume maxima. In the last decade (2012-21) the pattern seems to have flattened out somewhat, especially for the sea ice volume minimum. Note the record low volume for maximum ice was recorded in April 2017.

Forecasts for September 2021 Sea Ice Minimum

Yes, there is a forecast contest for best prediction of Arctic sea ice minimum extent run by an organization called the Arctic Sea Ice Prediction Network, called the Sea Ice Outlook. In essence it's an experiment to learn the economic value of these sea ice outlooks and to investigate the relative importance of each component that determines the evolution of sea ice during the melt season. Currently the experiment is in the last year of its second phase; the first being run from 2014-17.

There were 160 forecasts made for the 2021 sea ice minimum. Each participant, usually a full organization affiliated with either an educational or private sector institution, sent in forecasts in June, July, August, and early September for the September 2021 minimum, using different methods. The results are shown below, with individual forecasts shown as horizontal bars, the median of all forecasts as a vertical dashed line, the 2020 observed minimum as a vertical blue line, and the verification for 2021 as a heavy black line.

Dependencies on weather

The forecasts provided by the contest participants woefully low balled the 2021 verification. As you may recall, when I talked about the 2021 summer season, note was made of the high amount of cloudiness (thus low solar radiation in, and low infrared radiation out) and the wind-driven dispersion of the ice so that a wider area met the criterion of 15% ice cover than might be expected otherwise. So one could say that these forecasters were heavily influence by last summer’s minimum (what one could call using “persistence” in your forecast), rather than looking at what was actually persisting … the months-long weather pattern unfavorable for melting more ice.

Conclusions

While we cannot predict how much or how little sea ice will exist at a given maximum or minimum for any year, it’s unequivocal that the Earth is losing its sea ice in the Arctic. Interestingly, the Antarctic melt season (remember that southern hemisphere seasons are reversed from ours in the north) concluded with a record minimum sea ice extent in late February 2021.

Arctic/Antarctic sea ice and snow act as a sort of air conditioner that tempers the global mean temperature. Unfortunately, it also is highly sensitive to, and is a major drive of, global warming. As snow and ice melt, more solar radiation is absorbed rather than reflected. Ocean water goes from reflecting ~65% of incoming solar radiation when covered by ice and snow, to only about 8%, and land snow/ice from about 70-80% to 15-20%. This is why polar regions are showing a larger temperature response to greenhouse gas emissions than anywhere else in the world.

The result has been loss of permafrost, arctic habitats, native culture and livelihood, and so on. One of the most depressing emblems of arctic warming and the loss of sea ice is the polar bear. I’m sure you’re all familiar with that picture that makes you want to bawl your eyes out, so I won’t torture myself (or DKos readers) with that image. Let’s suffice it to say that things aren’t looking too good.

Comments are welcome, please, including corrections of anything I may have misstated or omitted.