Arctic Sea Ice decline in the 21st Century

Guest Commentary by Cecilia Bitz, University of Washington

Last month a paper I co-authored received considerable media attention. Headlines read “Experts warn North Pole will be ‘ice free’ by 2040”, “The Big Melt: Loss of Sea Ice Snowballs“, and “Arctic Clear for Summer Sailing by 2040: Models Predict Rapid Decline of Sea Ice”. The story also reached NPR, BBC, CBC, the Discovery channel, and Fox News, among others. Dr. Marika Holland, the first author of the paper, was inundated with media attention. About a dozen journalists contacted me too. I was impressed by the questions they posed — questions that probably reflect what the public most wants to know. However, after giving lengthy interviews, I would read the resulting article and see my explanations boiled down to a few lines. In this essay, I’d like to explain the science in the paper and give my answers to the most often asked questions.

In our paper (with co-author Bruno Tremblay), we examined the September Arctic sea ice cover in the 20th and 21st centuries in climate models, and found occasional decades of very rapid retreat. The most extreme case was a decrease from 6 to 2 million square kilometers in a decade (see Fig 1). This is about 4 times faster than the decline that has been observed in the past decade.

Figure 1: (a) Northern Hemisphere sea ice extent in September from one integration of the Community Climate System Model version 3 (CCSM3) with observations from the satellite era shown in red. The light blue line is a 5-yr running mean. The three lower panels show the September ice concentration (ice floes are separated by open water) in three select decades.

It is common practice to run climate models multiple times with slight variations to the initial conditions. Because the system is chaotic, the natural variability in each run is random and uncorrelated from one run to the next. When an ensemble of runs is averaged, the natural variability is reduced in the ensemble mean, and it is easier to detect a significant trend.

An ensemble of runs offers an opportunity to evaluate rare events too, such as extreme sea ice decay. We were in search of evidence for “tipping points“, which several authors have speculated might exist in sea ice. RealClimate places sea ice in the category of systems with “known unknowns” with regard to tipping points. This means we know there are thresholds involving sea ice (e.g., it can cease to exist), but we don’t know when, or if, the climate will arrive at one.

Only one of seven ensemble members had an event as extreme as quoted above, and it resulted in near ice-free conditions for September by 2040 (see Fig 1d). (The sea ice grows back at least for some portion of winter for the duration of the 21st century.) However, every ensemble member had an event 5 years or longer at some time in the 21st century when the sea ice retreat was about 3 times faster than the observed retreat since 2001 (see Fig 2). These ensemble members took about 5–10 years longer to become nearly ice-free in September than the most extreme case.

As illustrated in Fig 1, the sea ice retreat accelerates during the 21st century as the ice decays and more sunlight is absorbed by the ocean (the positive ice-albedo feedback). Increasing ocean heat transport under the sea ice adds to the melt back. The retreat appears abrupt when natural variability in the ocean heat transport into the Arctic Ocean is anomalously high. We did not find clear evidence of a threshold, which can be difficult to identify given the variability and complexity of the climate system. Therefore we can neither verify or rule-out the existence of a tipping point. Regardless, the rapid declines seen in our runs are a serious concern.

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