The decline of arctic ice cover has become an icon of man-made global warming. It’s one of those factors like, temperature rise, sea level rise and ocean acidification that, according to some, when taken with the others, point to a compelling picture of what is happening to our planet.
Despite its iconic status no one knows in detail why the arctic ice is declining; warming atmosphere, warming surface or under-ice ocean, more insolation, changing currents? The usual answer is that it is multi-factorial and generally consistent with the predicted Arctic Amplification, seen in many models. But without more detailed knowledge we cannot be completely certain what we are seeing is not part of a natural cycle. If one could take a snapshot of the Earth at the majority of points in its history there would be climatic factors that are changing in either directions. In the man-made CO2 era in which we live we must not automatically look at a changing climate parameter and ascribe it to global warming just because it is moving in the expected direction. Science demands more than that.
The Arctic is warming at a faster rate than anywhere else and it seems that many diverse climate effects and feedbacks contribute to it. This is what makes predicting what will happen in the future so difficult. Since the advent of satellite data (1979) that found a decline already in progress, one could draw a straight line up to about 2000 and make a prediction. Then the decline increased up to 2007 that gave rise to claims that the Arctic ice would disappear much sooner. Since 2007 the ice is exhibiting a different behavior.
In a recent paper in Nature Climate Change two researchers make a case that the Pacific Decadal Oscillation (PDO) has an important effect on the behavior of Arctic ice. The PDO is a complex rhythm of sea surface temperatures that persists for a decade or more before changing to a different state. It effects are felt globally. The researchers conclude that winter ice loss is related to the phase of the PDO.
The PDO was relatively cool (negative phase) between 1948 – 1976 and warm (positive phase) between 1976 and 2007, and then negative until it abruptly switched to positive phase in 2013-14. This leads the researchers to conclude that in the future the rate of decline of Arctic sea ice could reduce (temporarily they emphasise) making predictions more complicated. This is a clear tendency in climate science research. The more we know the more complicated it gets as the science takes one further away from simple alarmism.
The researchers give three references to support their statement that arctic sea ice has declined “steadily” since the late 1970s “one of the most visible indications of human-induced global warming.” Those references are the latest IPCC report, a 2012 paper in Geophysical Research Letters that says it finds a “physically plausible” strong correlation between Arctic ice and atmospheric CO2 concentration. The third reference is a 2008 Geophysical Research Letters paper. A lot has happened since 2008!
This paper is far from the first to examine the influence of natural cycles on the extent of arctic ice. Curiously I note a 1999 article in Science by Richard Kerr that asked “Will the Arctic Ocean Lose All Its Ice?” Kerr notes the decline in area which is put down to the Atlantic Oscillation (AO) – a seasaw change in wind patterns. Kerr speculates, “If it’s natural fluctuations in polar climate, then the loss of arctic ice should eventually reverse. But if global warming from greenhouse gasses is at fault, the entire ice pack should eventually disappear.”
Thanks to the PDO predictions of future arctic ice will be more difficult. It will be interesting to see what happens to arctic ice especially in the context of its recent winter minimum stalling.
Some recent work on historical arctic ice extent should also be at the back of our minds when thinking about the future of arctic ice. Some recent work suggests that there was a dramatic decline in arctic ice in the 1930s and 1940s with a minimum in 1936. And how deep was that minimum? About 50% of the minima seen in 2012!