What might cause the Arctic sea ice to have a tipping point?
Are we about to reach, or have we already reached a tipping point?
This series is a follow-on to my 3-part series Arctic Ice 2010.
The current series began with part #1: Background And Recent History. In part #2 and part #3 I wrote about various ways in which ice could melt, and how the melt rate might be accelerated. In Arctic Tipping Points - #4: The Broken Bridges Of Nares I reported the current state of the Nares Polynya as compared with previous years. Arctic Tipping Points - #5: Where Warm Water Meets Ice discussed the basic roles of meltwater and warm water currents.
I now propose to show that there are positive feedback mechanisms which are capable of causing a composite positive feedback effect in which Arctic sea ice once reduced beyond a limit will disperse very rapidly and will fail to recover.
Please note that I prefer the term 'dispersal' to describe ice loss from Arctic regions. Ice which is fragmented and dispersed melts more readily. Even when ice melts in situ the meltwater may readily re-freeze unless dispersed, as e.g. by mixing with sea water.
The Coriolis effect
North of the equator a mass propelled by a force seems to veer to the right of its path. A mass will continue in its state of rest or motion unless acted on by a force, so it appears that some sort of force is acting so as to deflect the mass from its path. In reality there is no force. While the mass is moving across the planet, the planet is rotating. To an observer rotating with the planet the path of the mass appears curved.
The Coriolis effect is a major component of the interdependent dynamic systems of Arctic ice consolidation and dispersal. To a first approximation, when the wind pushes ice to the South along a coast, the Coriolis effect provides a negative feedback to ice dispersal. In the major areas of ice dispersal the Coriolis effect causes ice to be forced into coastlines. This can be readily seen in Baffin Bay and Fram Strait. The ice tends to compact, with more mobile slabs of ice over-riding the more stationary ones. This thicker and more compact ice is better able to resist surface melting and the underflow of warm currents than scattered ice.
Whenever there is an offshore wind on such coasts, the ice is driven away from the coast. The open inshore lead might well be expected to contribute to ice loss, but the Coriolis effect makes a far greater contribution. The wind tends to separate fragments at the offshore edge of the pack ice and to drive them into more open waters. The Coriolis effect deflects this fragmented ice towards the warmer and more open waters to the South.
All it takes is a change of wind direction and the Coriolis effect switches from being a negative feedback to being a positive feedback in the ice dispersal mechanism. It is like throwing a switch. However, this particular mechanism cannot alone produce a tipping point.
Ice is blown offshore in Baffin Bay, left.
The Nares Strait, right, is exporting ice.
The North West Passage, top, begins to open.
April 30 2010 - MODIS/aqua 2km resolution.
Thermal mass
Open water absorbs heat. In the Arctic that heated water is not easily dispersed, due to the Arctic's being almost entirely surrounded by land. In addition, normal flows and occasional pulses of warm water from the Atlantic and Pacific contribute to the store of heat. As the Arctic Summer ends, this water releases its store of heat into the atmosphere. Temperatures are maintained higher than the freezing point of sea water.
Until sufficient heat is lost to allow new ice to form the old ice will continue to melt, albeit ever more slowly. The more that ice is lost in Summer, the more open water there is to absorb and retain heat. This is a positive feedback mechanism tending to shorten the Winter freeze, hence giving the next Summer melt season less ice to melt. Ultimately, this one feedback working alone could lead to Arctic Summers free of sea ice.
The Arctic helps to cool the planet, most especially the Northern hemisphere. As the planet warms, the Arctic warms faster. It is by no means certain - but I consider it highly probable - that a point could be reached where the excess warming due to loss of Summer sea ice is sufficient together with the effects of the Gulf stream to keep the Arctic sea-ice free all year round.
A tipping point
As the amount of open water increases and ice reduces there is an acceleration in ice loss. Different computer models1 give different predictions as to the final outcome. If the season does not provide enough time for all of the ice to melt then perhaps recovery is possible. Even if all of the ice melts, some models suggest that a cooler climate would allow recovery.
Given that an ever-increasing amount of CO2 in the atmosphere does not suggest any likelihood that the planet may suddenly enter a cooling phase, I consider it unlikely that sea ice, once vanished from the Arctic, would return soon by any known climatic means.
Considering the mechanisms which I have so far discussed, an ice-free Summer sets the scene for all future summers to be ice-free. Ice forming in winter will, by definition, be young ice. Young ice driven into the pack heaps up and consolidates. Arctic sea ice thickens mainly by consolidation. Snow adds little to the ice compared with compaction. After an ice-free summer there is no pack ice into which the young ice may be driven. It may be driven into a floating mass close to the pole, or it may be driven into coasts.
The Arctic ice is now in a lose-lose situation. If the ice remains thin it is vulnerable to surface melting. If it compacts it thereby reduces in area, thus increasing the rate of melting through the albedo feedback effect. Barring a 'perfect winter' in which the entire Arctic ocean freezes over, it appears that recovery is unlikely. Factor in probable changes in the distribution of Arctic thermohaline currents and recovery may well be impossible.
Tipping point 2007
I believe that we already reached a tipping point back in 2007. Throughout that year various climatic anomalies were observed in the Arctic. Quite apart from the amount of ice lost, the manner of its loss - through multiple mechanisms acting together - was most unusual.
A recent study2 showed that about 10% of ice loss was through the Nares Strait.
Sea ice flux through the Nares Strait is most active during the fall and early winter, ceases in mid- to late-winter after the formation of ice arches along the strait, and re-commences after breakup in summer. In 2007, ice arches failed to form. This resulted in the highest outflow of Arctic sea ice in the 13-year record between 1997 and 2009.
The Lincoln Sea area at the northern end of the Nares Strait was less fragmented at any time during 2007 than it is now, in April 2010.
April 30 2010 - MODIS/terra image of Nares Strait - Lincoln Sea area at 500 meter resolution.
Some of the early ice loss in Baffin Bay 2007 was due to a combination of wind and Coriolis effect which, rather than compacting ice against the west coast of Baffin Bay, drove it away from shore and dispersed it to the south.
The North West Passage, for many centuries found to be generally un-navigable, opened in August 2007. The loss of multi-year ice in the various passages between the islands appears to be irreversible. It is important to realise that prior to 1944, navigations of the passage had required overwintering: as the ice closed in on the ship it could proceed no further until - if luck prevailed - the next melt season freed the ship.
Roald Amundsen was the first person to sail the entire Passage from east to west. It took him three years to complete the journey. The first vessel to complete the voyage in both directions was the RCMP vessel St. Roch, pronounced 'rock'.
R.C.M.P. marine patrol vessel St. Roch in winter quarters at Herschel Island in the Beaufort Sea.
St. Roch is rightly famous as the first ship to completely circumnavigate North America; the second sailing vessel to complete a voyage through the Northwest Passage; the first ship to complete the Northwest Passage in the direction west to east, 1940-1942; first to complete the voyage in one season, 1944.
The first commercial ship sailed through the North West Passage November 28 20083.
The North West Passage has now been safely navigated by a number of leisure craft.
The ice in 2007 was still shorebound enough to block the North East Passage. In September 2009, the North East Passage was open to shipping for the first time ever without ice breakers4.
Concluding remarks:
Once the Arctic is ice free, whether seasonally or annually, the local climate will be substantially warmer for longer. This will undoubtedly affect the permafrost, coastal erosion rates and the calving rates of Arctic glaciers. It might well bring forward a Greenland ice sheet tipping point.
Continued on next post.
Footnotes:
[1] - Discussions of Arctic tipping points:
http://www.pnas.org/content/105/6/1786.full
http://www.pnas.org/content/106/1/28.full
http://www.pnas.org/content/106/49/20590.full#sec-5
[2] - Large sea ice outflow into the Nares Strait in 2007, Kwok et al
http://www.agu.org/pubs/crossref/2010/2009GL041872.shtml
[3] - http://www.cbc.ca/canada/north/story/2008/11/28/nwest-vessel.html
[4] - http://www.examiner.com/x-4648-Atlanta-Weather-Examiner~y2009m9d6-Arctic...
Further reading:
Melting Sea Ice Major Cause Of Arctic Warming - Study
http://www.vancouvermaritimemuseum.com/page216.htm
http://northwestpassagefilm.com/arctic/
No comments:
Post a Comment