Gone for good: Arctic Ocean ice free all year by the 2040s?

By Gareth Renowden 18/10/2010

A few days ago I used a combination of Arctic sea ice volume data from the University of Washington’s PIOMAS model and NSIDC sea ice extent numbers to project that the Arctic Ocean would be effectively ice-free in late summer within ten years. The key to that exercise was the rate at which the volume of sea ice has been declining — 350 km3 per year over the last 30 years for the full dataset, 410 km3 per year over the last 20, and 740 km3 over the last decade at summer minimum. The rate of volume decline has obviously been increasing. Using those numbers to project ice extent in the future is one thing, but they also tell us something interesting about the overall Arctic heat budget — and we can use that to make a rough guess about when the Arctic will become ice-free year round. The answer is surprising…

The Arctic heat budget is the balance between the heat being shipped to the top of the planet by the summer sun, the atmosphere and ocean currents, and the heat lost over a sunless winter. The sea ice volume trends tell us that the polar heat budget over the last three decades has been dominated by excess incoming heat — enough to melt hundreds of cubic kilometres of ice every year. We don’t need to worry about the details at this point — we don’t need to look at all the items in the budget, positive and negative, but like a good CEO we can focus on the bottom line, and that’s what the volume decline represents.

Another aspect of the volume data tells us something else about the budget: the amount of heat lost over winter. If we take the full 30 year PIOMAS data, the average maximum ice volume is 28,600 km3, and the average minimum is 13,400 km3. In other words, in what used to be an average winter the Arctic lost enough heat to grow 15,200 km3 of new ice. Let’s be generous, and assume that in current circumstances (starting from a low summer minimum) winter ice growth is potentially around 20,000 km3.

Once the perennial ice has gone, the Arctic heat budget will not simply stop showing a surplus. Instead of melting ice, energy will go into heating the ocean and atmosphere. More heat will have to be lost each autumn before ice can start to reform, and the arrival of the first ice of winter will be delayed. This effect can already be seen in the data. Take a look at the area anomalies at Cryosphere Today — they are already greatest after the late summer minimum, reflecting the delayed formation of new ice (more on that here). The greater the delay, the smaller the volume of ice that will regrow over what’s left of winter. At some point in the future enough heat will have accumulated in the ocean to prevent winter ice formation, and the Arctic will have completed its transition to an ice-free state.

…it may be already too late to do anything to prevent the Arctic sea ice disappearing completely

How soon could that happen? The ice volume data provides a simple way to arrive at an estimate: divide the normal winter heat loss by the annual heat surplus. At 350 km3 of ice lost per year (the 30 year volume trend), it will take 57 years. If the Arctic is seasonally ice-free by 2020, then it will be ice-free year round by 2080. However the recent volume loss has been running at 740 km3 per year, and if that rate continues it might only take 27 years for the winter ice to disappear. If the summer ice is gone by 2016, as might be possible at that rate of decline, then the Arctic could be ice-free in winter as early as 2043. This is not only much faster than most researchers have been willing to countenance, but also falls within the 30 year climate commitment — the inevitable warming caused by current greenhouse gas levels, as the climate system comes back into thermal equilibrium. If my sums are right (please, prove me wrong!) then it may be already too late to do anything to prevent the Arctic sea ice disappearing completely.

This speculation is clearly a considerable over-simplification — not a back-of-the-envelope calculation so much as a topologically infeasible back-of-the-back-of-an-envelope scribble — but the basic logic seems sound to me. The mechanisms that are creating the Arctic’s energy surplus may not operate the same way in future — negative feedbacks may come into play (increased cloudiness, freshening of the surface layers of the ocean), but so will positive feedbacks such as the “albedo flip” (replacing ice with dark ocean will increase heat absorption from solar radiation). The balance between those factors will determine how long it will actually take to move to a permanently open ocean, but there are signs we are heading that way at some speed.

We have a good analogue in the geological record for what a warm Arctic might be like — the Paleocene Eocene Thermal Maximum 58.5 million years ago. A sudden warming episode, probably associated with the release of a large amount of methane from oceanic methane hydrate deposits, saw Arctic sea surface temperatures spike up to 24ºC, and there’s evidence of sub-tropical vegetation (specifically palms) growing on neighbouring land masses. Not good news for the Greenland ice sheet, or the huge amounts of methane hydrates trapped under an already melting permafrost cap on the seabed off Siberia…

When I wrote Hot Topic (over NZ summer 2006/7), I suggested that the Arctic might be ice free in summer during my lifetime and thought I was going out on a limb by saying that. The record retreat of the sea ice in 2007 was a worrying confirmation of my gut feeling. Now it seems possible that if I can hang on until I’m 89, I might see the Arctic lose all its sea ice. The complete and utter transformation of a planet in my lifetime. And still we do too little, too late…

[The Shins]