The following brief communication was published on the The Cryosphere Discussions website two days ago: Does it matter exactly when the Arctic will become ice-free?
It's a good question, albeit a rhetorical one. The authors argue that a "robust definition of ice-free may reduce confusion in the community and amongst the public", and start by asking what the exact definition of ice-free is.
We consider four plausible definitions of the date of an ‘ice-free Arctic’. We apply the commonly-used threshold, for which northern hemispheric sea ice extent (defined as the total area of ocean with a sea ice fraction greater than 15%) is less than 1 million km2. The threshold of 1 million km2, rather than zero, is used because ice can be expected to remain for some time along the northern coast of Greenland, whilst for navigational purposes the central Arctic is ice-free. The ‘first ice-free year’ is then defined as:
A. The first year that at least one day is ‘ice free’.
B. The first year when the September mean is ’ice free’.
C. The first time the final year of a 5 year running mean of September monthly mean extents is ice-free.
D. The final year of 5 consecutive September monthly means which are ’ice-free’.
The question is an interesting one to ponder, and not just from a scientific perspective. Still, discussing the exact definition of ice-free may itself become a smoke screen that shrouds a more important issue, and thus cause even more confusion. Fortunately, the authors seem to be aware of this when they state in their paper's final paragraph:
Many of the impacts of decreasing ice cover will be felt irrespective of the precise date when the Arctic is declared seasonally ice-free.
I've said it before, and I'll say it again: An ice-free Arctic isn't some starting shot after which the consequences of Arctic sea ice loss spring into action. They already did so a while ago, but we're just not seeing it clearly as the signal hasn't crossed the bounds of natural variability for long enough yet.
Of course, scientists wouldn't be scientists if they wouldn't try to make things clearer. And thus they investigate things like the release of methane from permafrost and clathrates, changes in atmospheric patterns and melting rates of Greenland's ice sheet and glaciers, as these are all logically and mutually linked to Arctic sea ice loss.
Coincidentally, a paper was just published on the Journal of Climate website, by Liu, J., Z. Chen, J. Francis, M. Song, T. Mote, and Y. Hu.
Its title and abstract:
Has Arctic sea-ice loss contributed to increased surface melting of the Greenland ice sheet?
In recent decades, the Greenland ice sheet has experienced increased surface melt. However, the underlying cause of this increased surface melting and how it relates to cryospheric changes across the Arctic remain unclear. Here we show that an important contributing factor is the decreasing Arctic sea ice. Reduced summer sea ice favors stronger and more frequent occurrences of blocking-high pressure events over Greenland. Blocking highs enhance the transport of warm, moist air over Greenland, which increases downwelling infrared radiation, contributes to increased extreme heat events, and accounts for the majority of the observed warming trends. These findings are supported by analyses of observations and reanalysis data, as well as by independent atmospheric model simulations using a state-of-the-art atmospheric model that is forced by varying only the sea ice conditions. Reduced sea ice conditions in the model favor more extensive Greenland surface melting. We find that a positive feedback between the variability in the extent of summer Arctic sea ice and melt area of the summer Greenland ice sheet, which affects the Greenland ice sheet mass balance. This linkage may improve the projections of changes in the global sea level and thermohaline circulation.
Things aren't fine until the Arctic is ice-free (whatever your definition). The consequences of Arctic sea ice loss will most probably become worse when the Arctic is ice-free, but we may be able to prevent it from getting even worse after that, and eventually reverse the process. Together.