Judith Curry
Pursuant to Part I, i ask the following questions:
- Whence an ‘ice free’ Arctic?
- Does an ‘ice free’ Arctic matter?
Whence an ‘ice free’ Arctic?
‘Ice free’ is put in quotes, because ‘ice free’ as commonly used doesn’t mean free of ice, as in zero ice. The usual definition of ‘ice free’ Arctic is ice extent below 1 M sq km (current minimum extent is around 3.5 M sq km). This definition is used because it is very difficult to melt the thick ice around the Canadian Archipelago. And the issue of ‘ice free’ in the 21st century is pretty much a non issue if your require this thick ice to disappear.
The sea ice minimums in 2007 and 2012 have triggered numerous predictions of an ice free Arctic. Following the 2007 minimum, Jay Zwalley of NASA predicted ice free by 2012.
There have been numerous recent articles based on interviews with sea ice and climate scientists, that include predictions of when we can expect an ice free Arctic:
The most alarming prediction was from Paul Beckwith, who made a prediction on Aug 10 that there would be zero ice in the Arctic Ocean by the end of September.
Peter Wadhams is predicting ice-free by 2015, as are several others. Many experts are predicting ice free within a decade or by 2020. Andy Revkin at Dotearth poses an interesting question: do you agree with the statement that there is a 50-50 chance of ice free in the next two decades? The respondents seem to think that is about right, cautioning that natural variability is a wild card.
What do the climate models have to say? Wang and Overland’s analysis of CMIP5 simulations find ice free by the 2030’s for the culled selection of ensemble members.
For the next two decades, natural variability will trump any direct effects from AGW by a long shot. This statement from Levina and Lenton makes sense:
However, all early warning indicators show destabilization of the summer-autumn sea-ice since 2007. This suggests the new low ice cover state may be a transient feature and further abrupt changes in summer-autumn Arctic sea-ice cover could lie ahead; either reversion to the normal state or a yet larger ice loss.
The issue is whether the ice is now sufficiently thin that it would be difficult to reverse the decline. Growing and diminishing the sea ice pack are not symmetric processes: ice export that contributes to diminishing the sea ice pack does not have a reverse counterpart; at best you stop the export and stop the decrease.
So the question then becomes what processes could contribute to a recovery of the Arctic sea ice on the time scale of two decades?
Recovery (?)
Here are some processes that would contribute to a recovery:
- Reduction of the sea ice export through the Fram Strait
- Reduction of warm water inflow from the Atlantic and Pacific
- Fewer clouds in winter and/or more clouds in summer
- Less snow fall on ice in autumn and more in spring
- No rainfall on snow covered ice before mid June
- Fewer storms in summer causing ice breakup and more storms in autumn/early winter causing ice ridging/rafting
These processes depend on both random weather patterns and the teleconnection climate regimes. Can I predict how this might go over the next two decades? Heck no, other than that I suspect that the cool phase of the PDO will persist and at some point probably within two decades we will switch to the cool phase of the AMO.
And then there are the known unknowns: what solar radiation will do (looks like cooling), volcanoes are always a wild card, and then there are the less known unknowns such as cosmic ray effects, magnetic field effects, etc. And in terms of climate shifts, there may be something happening on much longer time scales (e.g. AMOC) that could influence the next climate regime shift. Focusing on CO2 as the dominant influence on the time scale of two decades seems very misguided to me.
Does ‘ice free’ matter?
I will leave the issue of ecological and socioeconomic and political implications for another post. Here, lets discuss the implication for the global and regional climate.
The first issue to debunk is that an ‘ice free’ Arctic is some sort of ‘tipping point.’ A number of recent studies find that in models, the loss of summer sea ice cover is highly reversible.
The impact of September sea ice loss on the ice albedo feedback mechanism is interesting. The minimum sea ice occurs during a period when the sun is at low elevation, so the direct ice albedo effect isn’t all that large. Less sea ice in autumn means more snowfall on the continents, which can have a larger impact on on albedo.
The impacts of the freeze-thaw over the annual cycle influences ocean circulations. But sea ice would continue to freeze and thaw on an annual cycle.
Clouds would change, atmospheric circulation patterns would change. The net effect on climate outside the Arctic Ocean would be what? More snow during winter on the continents is the most obvious expected change. But we really don’t know.
There would likely be regional triggers that could feedback onto larger scale regime shifts. Would any of these patterns or extreme events fall outside the envelope of what we have seen over the past century? Hard to know.
Would melting sea ice trigger some sort of clathrate methane release into the atmosphere? Well in terms of thawing permafrost, it seems like more snow fall on the continents would inhibit permafrost thawing. Same for the stability of the Greenland ice cap.
These are all qualitative speculations, but I am not seeing a big rationale for climate catastrophe if the see ice melts? I would be interested in other speculations on this. Like I said, I will discuss ecosystem, socioeconomic, national security impacts in a future post.