The extraordinary decline in Arctic sea ice during 2012 is finally over. Sea ice extent bottomed out on September 16, announced scientists at the National Snow and Ice Data Center (NSIDC) on Wednesday. The sea ice extent fell to 3.41 million square kilometers, breaking the previous all-time low set in 2007 by 18% --despite the fact that this year's weather was cloudier and cooler than in 2007. Nearly half (49%) of the icecap was gone during this year's minimum, compared to the average minimum for the years 1979-2000. This is an area approximately 43% of the size of the Contiguous United States. And, for the fifth consecutive year -- and fifth time in recorded history -- ice-free navigation was possible in the Arctic along the coast of Canada (the Northwest Passage) and along the coast of Russia (the Northeast Passage or Northern Sea Route). "We are now in uncharted territory," said NSIDC Director Mark Serreze. "While we've long known that as the planet warms up, changes would be seen first and be most pronounced in the Arctic, few of us were prepared for how rapidly the changes would actually occur. While lots of people talk about opening of the Northwest Passage through the Canadian Arctic islands and the Northern Sea Route along the Russian coast, twenty years from now from now in August you might be able to take a ship right across the Arctic Ocean."
Figure 1. Arctic sea ice reached its minimum on September 16, 2012, and was at its lowest extent since satellite records began in 1979. Image credit: National Snow and Ice Data Center (NSIDC).
When was the last time the Arctic was this ice-free?
We can be confident that the Arctic did not see the kind of melting observed in 2012 going back over a century, as we have detailed ice edge records from ships (Walsh & Chapman, 2001). It is very unlikely the Northwest Passage was open between 1497 and 1900, since this spanned a cold period in the northern latitudes known as "The Little Ice Age." Ships periodically attempted the Passage and were foiled during this period. Research by Kinnard et al. (2011) shows that the Arctic ice melt in the past few decades is unprecedented for at least the past 1,450 years. We may have to go back to at least 4000 B.C. to find the last time so little summer ice was present in the Arctic. Funder and Kjaer (2007) found extensive systems of wave generated beach ridges along the North Greenland coast, which suggested the Arctic Ocean was ice-free in the summer for over 1,000 years between 6,000-8,500 years ago, when Earth's orbital variations brought more sunlight to the Arctic in summer than at present. Prior to that, the next likely time was during the last inter-glacial period, 120,000 years ago. Arctic temperatures then were 2-3 °C higher than present-day temperatures, and sea levels were 4-6 meters higher.
Figure 2. Year-averaged and 3-month averaged Northern Hemisphere sea ice extent from Chapman and Walsh (2001), as updated by the University of Illinois Cryosphere Today. I've updated their graph to include 2011 plus the first 9 months of 2012.
Figure 3. Late summer Arctic sea ice extent over the past 1,450 years reconstructed from proxy data by Kinnard et al.'s 2011 paper "Reconstructed changes in Arctic sea ice over the past 1,450 years." The solid pink line is a smoothed 40-year average, and the light pink areas shows a 95% confidence interval. Note that the modern observational data in this figure extend through 2008, though the extent is not as low as the current annual data due to the 40-year smoothing. More commentary on this graph is available at skepticalscience.com.
When will the Arctic be ice-free in summer?
So, when will Santa's Workshop need to be retrofitted with pontoons to avoid sinking to the bottom of the Arctic Ocean in summer? It's hard to say, since there is a large amount of natural variability in Arctic weather patterns. Day et al. (2012) found that 5-31% of the changes in Arctic sea ice could be due to natural causes. However, the sea ice at the summer minimum has been declining at a rate of 12% per decade, far in excess of the worst-case scenario predicted in the 2007 IPCC report. Forecasts of an ice-free Arctic range from 20-30 years from now to much sooner. Just this week, Dr. Peter Wadhams of Cambridge University predicted that the Arctic will be ice-free in summer within four years. A study by Stroeve et al. (2012), using the updated models being run for the 2014 IPCC report, found that "a seasonally ice-free Arctic Ocean within the next few decades is a distinct possibility." Of the 21 models considered, 2022 was the earliest date that complete Arctic sea ice occurred in September.
Video 1. A powerful storm wreaked havoc on the Arctic sea ice cover in August 2012. This visualization shows the strength and direction of the winds and their impact on the ice: the red vectors represent the fastest winds, while blue vectors stand for slower winds. According to NSIDC, the storm sped up the loss of the thin ice that appears to have been already on the verge of melting completely. Video credit: NASA.
But Antarctic sea ice is growing!
It's a sure thing that when Arctic sea ice hits new record lows, global warming contrarians will attempt to draw attention away from the Arctic by talking about sea ice around Antarctica. A case in point is an article that appeared in Forbes on Wednesday by James Taylor [a noted minion of denial think tanks]. Mr. Taylor wrote, "Antarctic sea ice set another record this past week, with the most amount of ice ever recorded on day 256 of the calendar year (September 12 of this leap year)...Amusingly, page after page of Google News results for Antarctic sea ice record show links to news articles breathlessly spreading fear and warning of calamity because Arctic sea ice recently set a 33-year low. Sea ice around one pole is shrinking while sea ice around another pole is growing. This sure sounds like a global warming crisis to me."
This analysis is highly misleading, as it ignores the fact that Antarctica has actually been warming in recent years. In fact, the oceans surrounding Antarctica have warmed faster than the global trend, and there has been accelerated melting of ocean-terminating Antarctic glaciers in recent years as a result of warmer waters eating away the glaciers. There is great concern among scientists about the stability of two glaciers in West Antarctica (the Pine Island and Thwaites Glaciers) due the increase in ocean temperatures. These glaciers may suffer rapid retreats that will contribute significantly to global sea level rise.
Despite the warming going on in Antarctica, there has been a modest long-term increase in Antarctic sea ice in recent decades. So, how can more sea ice form on warmer ocean waters? As explained in an excellent article at skepticalscience.com, the reasons are complex. One reason is that the Southern Ocean consists of a layer of cold water near the surface and a layer of warmer water below. Water from the warmer layer rises up to the surface, melting sea ice. However, as air temperatures warm, the amount of rain and snowfall also increases. This freshens the surface waters, leading to a surface layer less dense than the saltier, warmer water below. The layers become more stratified and mix less. Less heat is transported upwards from the deeper, warmer layer. Hence less sea ice is melted (Zhang, 2007). As the planet continues to warm, climate models predict that the growth in Antarctic sea ice will reverse, as the waters become too warm to support so much sea ice.
Figure 4. Surface air temperature over the ice-covered areas of the Southern Ocean surrounding Antarctica (top) and sea ice extent, observed by satellite (bottom). Image credit: (Zhang, 2007).
Commentary: Earth's attic is on fire
To me, seeing the record Arctic sea ice loss of 2012 is like discovering a growing fire burning in Earth's attic. It is an emergency that requires immediate urgent attention. If you remove an area of sea ice 43% the size of the contiguous U.S. from the ocean, it is guaranteed to have a significant impact on weather and climate. The extra heat and moisture added to the atmosphere as a result of all that open water over the pole may already be altering jet stream patterns in fall and winter, bringing an increase in extreme weather events. This year's record sea ice loss also contributed to an unprecedented melting event in Greenland. Continued sea ice loss will further increase melting from Greenland, contributing to sea level rise and storm surge damages. Global warming doubters tell us to pay attention to Earth's basement -- the Antarctic -- pointing out (incorrectly) that there is no fire burning there. But shouldn't we be paying attention to the steadily growing fire in our attic? The house all of humanity lives on is on fire. The fire is certain to spread, since we've ignored it for too long. It is capable of becoming a raging fire that will burn down our house, crippling civilization, unless we take swift and urgent action to combat it.
References
Funder, S. and K. H. Kjaer, 2007. A sea-ice free Arctic Ocean?, Geophys. Res. Abstr., 9 (2007) 07815.
Kinnard et al., 2011. Reconstructed changes in Arctic sea ice over the past 1,450 years, Nature.
Walsh, J. E and W. L.Chapman, 2001. Twentieth-century sea ice variations from observational data, Annals of Glaciology, 33(1) (January 2001) 444-448.
Related info
Half of the polar ice cap is missing: Arctic sea ice hits a new record low. September 6, 2012, blog post
Wunderground's Sea Ice page.
http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2237
Posts
No comments:
Post a Comment