Blog Archive

Wednesday, July 16, 2008

Arctic Sea ice well on its way to disappearing this summer, Part III

See below for the National Snow and Ice Data Center update.

QuikSCAT images from the Polar View project at the Technical University of Denmark (DTU), see also the many sources of ice info on these pages:



July 16, 2008 -- Click on the image to enlarge the detail.

Link to updated images:

Link to above image (click on image to enlarge it -- notice meltback on Greenland):

Link to above image (click to enlarge the detail):

From the National Snow and Ice Data Center (NSIDC):

July 2, 2008: Melt onset earlier than normal

Arctic sea ice extent for June 2008 is close to that for 2007, which went on to reach the lowest minimum since at least 1979. More notably, however, satellite data indicate that melt began significantly earlier than last year over most of the Arctic Ocean. The large area of the Arctic Ocean covered by first-year ice (described in our June analysis), coupled with the early onset of melting, may mean more rapid and more severe summer ice retreat than last year.

Overview of conditions

Arctic sea ice extent averaged for June stood at 11.44 million square kilometers (4.42 million square miles), 0.72 million square kilometers (0.25 million square miles) less than the 1979 to 2000 average for the month. This is very slightly (0.05 million square kilometers; 0.02 million square miles) lower than the average extent for June 2007, but not the lowest on record, which occurred in June 2006.

While the monthly average was slightly less than June 2007, on a daily basis, sea ice extent appears slightly higher than 2007 for most of the month. This apparent contradiction arises because of the monthly averaging calculation and because some days may have areas of missing data. To be included as an ice-covered region in the monthly average, the average concentration for that region must exceed 15%. So if the concentration is 15% for 29 days, but less than 15 percent for 1 day, it will not be included in the average ice extent for the month. Also, since ice extent decreases during June, if there is slightly more missing data in the early part of the month the monthly average could slightly underestimate the sea ice extent.

June sea ice extents in 2008 and 2007 are essentially identical, and near the lowest values for June ever recorded by satellite for the Arctic.

Conditions in context

While sea ice extent averaged for June 2008 was similar to last year, there were pronounced differences in the spatial pattern of the retreat through the month. Last year, open water quickly developed along the coasts of the Chukchi and Laptev seas. This year, an unusually large polynya has opened in the Beaufort Sea, and there is significantly less sea ice in Hudson Bay and Baffin Bay.

June 2008 compared to past Junes

June sea ice extent is very similar to last year and is now the third lowest on record. It lies very close to the linear trend line for all average June sea ice extents since 1979, which indicates that the Arctic is losing an average of 41,000 square kilometers (15,800 square miles) of ice per year in June. Last year, the rapid melt leading to the record-breaking minimum extent began in July.

Early onset of melt

Preliminary satellite data shows us that surface melt began earlier than usual over the western and central Arctic Ocean and Baffin Bay. Last year was fairly typical except for significant early melt in the Laptev and Barents seas. This year, sea ice in the Beaufort Sea began to melt on average 15 days earlier than normal, and 15 days earlier than last year. Surface melt in the Chukchi and East Siberian seas was 6 days earlier than normal, and 14 days earlier than in 2007. In the central Arctic Ocean, melt began around June 9th, which was 12 days earlier than normal and 9 days earlier than the year before. In Baffin Bay, surface melt began 14 days earlier than last year and was 16 days earlier than normal. Areas where melt occurred later, compared to last year, are confined to the margins of the ice cover. These preliminary results will be updated as more data becomes available.

Why earlier melt matters

What are the implications of this year's earlier-than-normal melt onset? As melting begins, the layer of snow on top of the ice becomes wet and then disappears, leaving bare ice and ponded water. Each of these changes reduces the reflectance of the surface—increasing absorption of solar energy, further reducing reflectance, and promoting even stronger melt. This is known as the ice-albedo feedback.

Early melt onset exposes the snow and ice to more days with low reflectance. It also increases the exposure during the critical early summer season, when solar energy is at its peak. As colleague Don Perovich of the Cold Regions Research and Engineering Laboratory notes, this combination enhances ice-albedo feedback. Perovich calculated that in 2007, some areas of the Arctic absorbed eight times as much heat because of the ice-albedo feedback, contributing heavily to last year’s record-breaking melt.

The combination of ice-albedo feedback and early melt onset in 2008 sets the stage for significant ice losses this summer. Three of the most important factors in sea ice losses are melt onset, cloud conditions throughout the melt season, and atmospheric circulation throughout the melt season. With melt onset having occurred earlier than usual, cloud and atmospheric conditions over the next two months come to the forefront. To learn more about cloud conditions and atmospheric circulation, read “More on the sea ice-atmosphere connection” in our June analysis.

Link to NSIDC webpage:


B Buckner said...

Surely you must know that the melt rates between 07 and 08 have diverged dramatically in July.

08 currently has about 900,000 km2 more ice extent than 07. With the end of the melting season nearing, and the maximum melt rate days behind us, 08 will almost certainly end up with a larger minimum ice extent than 07.

Tenney Naumer said...

Surely, you must not know the difference between area (sea surface extent in km2) and volume (thickness of the ice in km3).

B Buckner said...

these posts have been entirely about sea ice extent. I am confused as to why you bring up sea ice volume, but since you have, can you shed some light on that topic?

Tenney Naumer said...

I am not sure of the reason for your confusion since the volume of the sea ice is far more relevant that its surface area. I do not have good links to data on the volume. When I find some, I will post them.

B Buckner said...

To summarize, you have three posts related to sea ice disappearing this summer that have information only relating to ice extent. I comment that the sea ice is in fact not disappearing based on extent, and you chastise me that it is volume, not extent that is important. You then follow up with the revelation that you have no information on sea ice volume. So if it is volume and not extent that matters, what basis did you have to state that the sea ice is disappearing in the first place?

Tenney Naumer said...

This post was not about sea ice extent, per se, but about the condition of the sea ice.

You can see that the ice is melting out from the middle of the Arctic.

If you follow these things, then you know that the ice is much thinner than it was at this time last year.

Last year, the ice did not reach its minimum until the middle of September.

The intention of this blog is to post information for readers. If you wish to disagree with the information, you should take your disagreements to the source at the links provided.

Year 2008 may end up with a greater extent than 2007; however, it will be the volume that is important.

If I find data about the volume, I will certainly post it.

B Buckner said...

I found this on ice thickness, it is up to date. More info on this site, it might be of interest to you regarding another post on the topic.

Sea ice thickness update

Previous discussion (see April 7, 2008) presented evidence that much of the Arctic Ocean this winter and spring, including the area near the North Pole, was covered with fairly thin, first-year ice. This thin, young ice is vulnerable to melting completely in summer. The large areas of low-concentration ice discussed above reinforce this concern.

Figure 5 shows sea ice thickness for late winter of 2006, 2007, and 2008 derived from the NASA ICESat laser altimeter instrument and provided by Ronald Kwok at the Jet Propulsion Laboratory. Based on Kwok's analysis, the first-year ice that formed since last autumn, while spatially extensive, has a mean thickness of 1.6 meters (5.2 feet), which is close to the thickness seen in 2006 and 2007. Much of this season's first-year ice formed rather late last autumn, so we had expected to see thinner first-year ice.

So why is the first-year ice thicker than anticipated? Sparse snow cover last winter may have hastened its growth: less snow on the ice means less insulation from the frigid winter air, and faster ice growth. Much of the snowfall over the Arctic Ocean occurs in early autumn, but early last autumn much of the Arctic Ocean was still ice-free and could not collect snow. Once the ice formed, it grew quickly.

In contrast to the first-year ice, the multiyear ice in 2008 appears to be much thinner than in the past two years. One factor may be the strong basal melt, or melt at the underside of the ice, observed during summer 2007. Based on Kwok's ice motion analysis, another factor could be an unusually high export of thick ice out of the Arctic Ocean through Nares Strait, the narrow strait between the Lincoln Sea and Baffin Bay. This export augmented the multiyear ice outflow through the Fram Strait.