After the dramatic Arctic sea melt of the summer of 2007, many people become quite alarmed about what this meant for the future of the arctic. The notion was that as the white reflective ice gave way to dark absorptive ocean waters, more of the suns energy would be absorbed which would further speed up the melting ice. It was even speculated that this could lead to a sudden tipping point after which the ice would decline rapidly and the air would warm at an even more alarming rate.
This was made even more worrisome because of the vast amounts of methane (a very potent greenhouse gas) stored in the permafrost could be released as the arctic warms.
Thankfully a recent paper indicates that any such tipping points are not nearly as close as many feared.
In light of the rapid recent retreat of Arctic sea ice, a number of studies have discussed the possibility of a critical threshold (or ‘‘tipping point’’) beyond which the ice–albedo feedback causes the ice cover to melt away in an irreversible process. The focus has typically been centered on the annual minimum (September) ice cover, which is often seen as particularly susceptible to destabilization by the ice–albedo feedback. Here, we examine the central physical processes associated with the transition from ice-covered to ice-free Arctic Ocean conditions. We show that although the ice–albedo feedback promotes the existence of multiple ice-cover states, the stabilizing thermodynamic effects of sea ice mitigate this when the Arctic Ocean is ice covered during a sufficiently large fraction of the year. These results suggest that critical threshold behavior is unlikely during the approach from current perennial sea-ice conditions to seasonally ice-free conditions. In a further warmed climate, however, we find that a critical threshold associated with the sudden loss of the remaining wintertime-only sea ice cover may be likely.
Or in other words even if we lose the summer sea ice (a near certainly in the coming decades), we still wont hit the tipping point. That being said this paper does confirm the tipping point does exist and given further warming we could risk the sudden loss of the remaining wintertime-only sea ice.
The charts we see show much more decline in summer sea ice than in winter sea ice. But it happens that the latitudes of winter-only sea ice are mostly covered in land. If you plot, instead of ice area, the latitudinal extent of sea ice, you actually get very similar declines for all seasons.
Secondly, the 2007 anomaly that everybody got all excited about wasn’t as spectacular as it looks. Five or six comparable anomalies have occurred on the latitude metric, just in other seasons. It’s simply the fact that the 2007 anomaly occurred in the season of the sea ice minimum that it jumps out visually. It’s a display of quantitative information question. Plots of annual ice minimum looked really scary in in late ’07, but other ways of looking at the data much less so.
None of this is to say the ice isn’t in decline, nor that we won’t have ice free summers soon. It is to say that the scare of 2007 was a bit overblown and that though there may well be a tipping point in the Arctic, it’s fairly far off at present, at least in Eisenman’s opinion.
And Michael’s second point bears repeating. Many people (myself included) blew the 2007 decline out of proportion. It was a dramatic decline, but it stood out only because it occurred in the season of sea ice minimum. It was also just a single data point, not a long term trend. These aren’t excuses, but a reminder that we all need to remain skeptical, and not jump to conclusions that suit our preconceived notions of reality.
And I apologize for failing to do that.
This fall, Dr. David Barber and his colleagues were cruising the western Arctic Ocean in the icebreaker/research ship CCGSAmundsen to study multi-year sea ice, the kind that has formed the permanent ice cap in the Arctic for hundreds of thousands of years. They were guided by satellite observations that suggested that solid ice was present throughout this part of the Arctic. What they saw instead was something Dr. Barber says he’s never observed before – broken, slushy, decayed ice with a thin veneer of harder ice over it, which their ship pushed through as if it wasn’t there. This new kind of ice had fooled the satellites, and suggests that the permanent Arctic ice cover is in even more trouble than had been previously thought.
Listen to the whole interview with Dr. David Barber from Quirks and Quarks.
Robert Grumbine has the relevant details for those of you not wanting to listen to the whole interview:
Our standard method for observing sea ice from space is to use satellites to measure the microwave energy emitted from the earth’s surface. Sea water is a very bad emitter, so has a very low brightness temperature. Sea ice is a pretty good emitter, so has a much warmer brightness temperature. You go through a couple of elaborations on this, and out pops the fraction of the surface that is sea ice instead of sea water.
You can also be a little more demanding than that. Particularly in the Arctic, this makes sense. The elaboration comes from the fact that not all ice emits microwaves equally well. Salty ice is a better emitter than fresher ice. In the summer time, when ice floes do some melting (but not enough to get rid of the whole floe for the ones we’re interested in), it is the saltier parts of the floe that melt away first. This is the same thing happening when you salt the sidewalk — the salt lowers the melting point, and the salty parts melt first. When you get to this time of year, and the melting stops, what is left is a relatively fresh ice floe. It is also called ‘multiyear’ ice, since it’s now in its second winter. With a more detailed analysis of the microwaves, you can try to distinguish between the first year ice (saltier and a better emitter) from the multiyear ice (less salty, but still a better emitter than sea water) from the sea water (very poor emitter)…
Now for the spot where we were fooled.
Up until basically this year, it was a standard assumption that multiyear ice was thick ice. It made sense. Ice that was thick enough to survive the summer can’t be terribly thin (we figured). And in the next winter it has a chance to pile up even more thickness either by freezing more ice on, or by getting piled up as the circulation jammed ice floes on top of each other. This had been confirmed many times in field expeditions starting long before the satellites were flying.
What Barber found instead was that the ice breaker was cruising along at 13 knots (25 kph) through what the satellite said was multiyear ice. He was amazed. That is about the speed the breaker would go through open ocean! What he realized had happened, after going up to the bridge, was that the satellite was seeing just a thin scum of the fresh multiyear ice, that was on top of also a thin layer of first year ice. There was only just enough multiyear ice for the satellite to see that. It is no longer the case that we can assume that the sound of multiyear ice means thick ice. There’s still ice, so methods for deciding total ice coverage are ok. But we can’t use the multiyear ice signature to mean ‘thick’ any more. Probably (my opinion) this has become a progressively less accurate thing to do over the last few years, and it’s only now that the difference has become as drastic as David found.
So on the one hand we have the good news of the albedo tipping point being farther away than we first though, but on the other hand we now know that we have overestimated the amount of thick multi-year ice.
As usual the picture is remarkably complex, and that complexity is lost in the media, and the majority of the public.