Figure 1. Daily Arctic sea ice extent for September 12, 2008, was 4.52 million square kilometers (1.74 million square miles). The orange line shows the 1979 to 2000 average extent for that day. The black cross indicates the geographic North Pole. Sea Ice Index data. About the data.
—Credit: National Snow and Ice Data Center
High-resolution image

Overview of conditions

On September 12, 2008 sea ice extent dropped to 4.52 million square kilometers (1.74 million square miles). This appears to have been the lowest point of the year, as sea has now begun its annual cycle of growth in response to autumn cooling.

The 2008 minimum is the second-lowest recorded since 1979, and is 2.24 million square kilometers (0.86 million square miles) below the 1979 to 2000 average minimum.

Update 8:00 am MT October 2:

Final analysis indicates that the 2008 Arctic sea ice minimum occurred on September 14, 2008. For more information see the NSIDC press release: Arctic Sea Ice Down to Second-Lowest Extent; Likely Record-Low Volume.

Figure 3. Daily Arctic sea ice extent for September 12, 2008, the date of this year’s minimum (white) is overlaid on September 16, 2007, last year’s minimum extent (dark gray). Light gray shading indicates the region where ice occurred in both 2007 and 2008. Sea Ice Index data. About the data.
—Credit: National Snow and Ice Data Center
High-resolution image

Overlay of 2007 and 2008 at September minimum

The spatial pattern of the 2008 minimum extent was different than that of 2007. This year did not have the substantial ice loss in the central Arctic, north of the Chukchi and East Siberian Seas. However, 2008 showed greater loss in the Beaufort, Laptev, and Greenland Seas.

Unlike last year, this year saw the opening of the Northern Sea Route, the passage through the Arctic Ocean along the coast of Siberia. However, while the shallow Amundsen’s Northwest Passage opened in both years, the deeper Parry’s Channel of the Northwest Passage did not quite open in 2008.

A word of caution on calling the minimum

Determining with certainty when the minimum has occurred is difficult until the melt season has decisively ended. For example, in 2005, the time series began to level out in early September, prompting speculation that we had reached the minimum. However, the sea ice contracted later in the season, again reducing sea ice extent and causing a further drop in the absolute minimum.

We mention this now because the natural variability of the climate system has frequently been known to trick human efforts at forecasting the future. It is still possible that ice extent could fall again, slightly, because of either further melting or a contraction in the area of the pack due to the motion of the ice. However, we have now seen five days of gains in extent. Because of the variability of sea ice at this time of year, the National Snow and Ice Data Center determines the minimum using a five-day running mean value.

Ongoing analysis continues

We will continue to post analysis of sea ice conditions throughout the year, with frequency determined by sea ice conditions. Near-real-time images at upper right will continue to be updated every day.

In addition, NSIDC will issue a formal press release at the beginning of October with full analysis of the possible causes behind this year’s low ice conditions, particularly interesting aspects of the melt season, the set-up going into the important winter growth season ahead, and graphics comparing this year to the long-term record. At that time, we will also know what the monthly average September sea ice extent was in 2008—the measure scientists most often rely on for accurate analysis and comparison over the long-term.

For previous analysis, please see the drop-down menu under Archives in the right navigation at the top of this page.

Overview of conditions

Arctic sea ice extent on September 3 was 4.85 million square kilometers (1.87 million square miles), a decline of 2.47 million square kilometers (950,000 square miles) since the beginning of August.

Extent is now within 370,000 square kilometers (140,000 square miles) of last year’s value on the same date and is 2.08 million square kilometers (800,000 square miles) below the 1979 to 2000 average.

Regional ice loss contributes to decline

What part of the Arctic contributed most strongly to the rapid August decline? Through spring and early summer, ice losses were largest in the Beaufort Sea. In August, the pattern of ice loss changed, with the greatest ice losses shifting to the Chukchi and East Siberian Seas.

The shift in location of maximum ice losses was fueled by a shift in atmospheric circulation. A pattern of high pressure set up over the Chukchi Sea, bringing warm southerly air into the region and pushing ice away from shore. August air temperatures in the Chukchi Sea (at 925 millibars pressure, roughly 750 meters [2,500 feet] in altitude) were 5 to 7 degrees Celsius (9 to 13 degrees Fahrenheit) warmer than normal. Ice loss in the Chukchi and East Siberian Seas averaged 14,000 square kilometers (5,400 square miles) per day faster than in 2007.

Sea ice also experienced an unusual retreat north of Ellesmere Island during August. Partial collapse of ice shelves in the region attended this retreat. Visit the Trent University press release at: http://www.trentu.ca/newsevents/newsreleases_080903iceshelf.php.

Warm ocean temperatures

Mike Steele and Wendy Ermold from the University of Washington’s Applied Physics Laboratory Polar Science Center have been closely monitoring sea surface temperatures in the Arctic.

Positive sea surface temperature anomalies for August 2008 correspond with areas of ice retreat. When the ice melts, it exposes open water that absorbs solar energy; the warm ocean waters then favor further sea ice melt. An interesting phenomenon, in this regard, is that sea ice this August has been drifting into the Beaufort Sea only to melt when it encounters these warm ocean waters.

As autumn comes to the Arctic, the ocean will begin to lose its heat back to the atmosphere. This means that regions of high sea surface temperatures seen in August will be manifested as above-average air temperature in corresponding regions as autumn unfolds.

To view both August 2008 and 2007 sea surface temperature anomalies, click on Figure 4.

Figure 5. Monthly August ice extent for 1979 to 2008 shows 2008 as the second-lowest August on record.
—Credit: National Snow and Ice Data Center High-resolution image

 

August 2008 average extent compared to past Augusts

Arctic sea ice extent averaged over the month of August was 6.03 million square kilometers (2.33 million square miles). This is 1.64 million square kilometers (633,000 square miles) below the 1979 to 2000 August average,

However, August 2008 was still 670,000 square kilometers (260,000 square miles) above August 2007, despite the record-breaking rate of decline over the past month. Why would this be? The best explanation for this is that this summer did not experience the “perfect storm” of atmospheric conditions seen throughout the summer of 2007.

Even though August ice extent was above that of August 2007, the downward trend for August ice loss has now gone from -8.4% per decade to -8.7% per decade.

 

Figure 1. Daily Arctic sea ice extent for August 26, 2008, fell below the 2005 minimum, which was 5.32 million square kilometers (2.05 million square miles). The orange line shows the 1979 to 2000 average extent for that day. The black cross indicates the geographic North Pole. Sea Ice Index data. About the data.
—Credit: National Snow and Ice Data Center
High-resolution image

Overview of conditions

With several weeks left in the melt season, sea ice extent dipped below the 2005 minimum to stand as the second-lowest in the satellite record. The 2005 minimum, at 5.32 million square kilometers (2.05 million square miles), held the record-low minimum until last year.

Recent ice retreat primarily reflects melt in the Chukchi Sea off the Alaskan coast and the East Siberian Seas off the coast of eastern Russia.

Update 9:15 am MT August 27:

Arctic sea ice extent on August 26 was 5.26 million square kilometers (2.03 million square miles), a decline of 2.06million square kilometers (795,000 square miles) since the beginning of the month. Extent is now within 430,000 square kilometers (166,000 square miles) of last year’s value on the same date and is 1.97 million square kilometers (760,000 square miles) below the 1979 to 2000 average.

For previous analysis, please see the drop-down menu under Archives in the right navigation at the top of this page.

Figure 1. Daily Arctic sea ice extent for August 24, 2008, was 5.47 million square kilometers (2.11 million square miles). The orange line shows the 1979 to 2000 average extent for that day. The black cross indicates the geographic North Pole. Sea Ice Index data. About the data.

—Credit: National Snow and Ice Data Center
High-resolution image

Overview of conditions

Arctic sea ice extent on August 24 was 5.47 million square kilometers (2.11 million square miles), a decline of 1.85 million square kilometers (714,000 square miles) since the beginning of the month. Extent is now within 580,000 square kilometers (220,000 square miles) of last year’s value on the same date and is 1.84 million square kilometers (710,000 square miles) below the 1979 to 2000 average.

Animation provides a closer look at the melt

NSIDC has released new animations for Google Earth showing daily sea ice concentrations and extent in the Arctic. Click on the still image in Figure 3 to view a Quick Time animation of daily sea ice concentration over the past 90 days. Note the recent strong losses of ice north of Siberia.

Users who wish to manipulate the animation and access the daily updated animation may now download them from the NSIDC Virtual Globes page. Daily sea ice extent updates are automatically loaded into Google Earth so users always have the most recent files; 30-, 60-, and 90-day versions are available. To download the Google Earth files, learn more about Google Earth, or to find out how to read the time-slider tutorial, see http://nsidc.org/data/virtual_globes/.

Figure 4. Ice thickness measurements for summer 2008 indicate melt at the ice surface (red) versus the underside (yellow). This image shows changes in ice thickness at buoy locations (white circles), overlaid on the NSIDC sea ice concentration field for August 20. The numbers above each bar plot indicate total ice thickness at the beginning of the melt season compared to August 20.
—Credit: From National Snow and Ice Data Center courtesy D. Perovich, CRREL
High-resolution image

Changing ice thickness

As discussed in a number of previous postings (July 17, April 7), sea ice thickness is a key measure of the health of the sea ice. While the NASA ICESat sensor can give an overall picture of ice thickness over the Arctic Ocean, the most accurate way to measure ice thickness is by taking point measurements on the ground. Don Perovich, Jackie Richter-Menge, Bruce Elder, and Chris Polashenski at the United States Army Cold Regions Research and Engineering Laboratory track the evolution of sea ice thickness year round using autonomous buoys. The buoys are deployed as part of the North Pole Environmental Observatory, the Beaufort Gyre Observatory, and the DAMOCLES project. The buoy data have indicated increased amounts of melt on the underside of the ice cover in recent years; bottom melt last year was particularly extreme.

The pattern for 2008 has been more mixed. The ice at some buoy locations has thinned by more than a meter through the melt season because of strong melt both on the surface and the underside of the ice. Other locations show strong thinning caused by surface melt, while only modest thinning is apparent in others. Differences in surface melt from location to location reflect factors such as air temperature, the ice albedo, and cloud conditions. The wide range in bottom melt points to variations in the amount of ocean heat absorbed. In recent days, the buoys have indicated sub-freezing temperatures with surface melt coming to an end; however, bottom melt will continue for at least two to three more weeks and the ice extent decline, while slowing, will also continue.

For previous analysis, please see the drop-down menu under Archives in the right navigation at the top of this page.

Figure 3. Sea-level pressure for August 8, 2008, shows a weather pattern favoring ice melt. Areas of high pressure are shown in yellow and red; areas of low pressure are shown in blue and purple.
—Credit: From National Snow and Ice Data Center courtesy Climate Diagnostic Center
High-resolution image

Storms trigger increased melt

A series of storms north of Alaska and Siberia in late July and early August have helped break up the thin ice and have brought warm southerly winds into the region.

Subsequently, a pattern has developed with high pressure over the Beaufort Sea and low pressure over the Laptev and East Siberian Seas (Figure 3). In accord with Buys Ballot’s Law, this pattern has brought southerly winds to the region, enhancing melt, breaking up ice, and pushing the ice edge northward.

Figure 4. Passive-microwave satellite data shows ice concentration on August 10, 2008, over the Northwest Passage region. The yellow line indicates Amundsen’s historic route through the passage. NASA AMSR-E data.
—Credit: From National Snow and Ice Data Center courtesy University of Bremen
High-resolution image

Figure 1. Daily Arctic sea ice extent for July 31, 2008 was 7.71 million square kilometers (3.98 million square miles). The orange line shows the 1979-2000 average extent for that day. The black cross indicates the geographic North Pole. Sea Ice Index data. About the data

—Credit: National Snow and Ice Data Center
High-resolution image

Overview of conditions

Arctic sea ice extent on July 31 stood at 7.71 million square kilometers (2.98 million square miles). While extent was below the 1979 to 2000 average of 8.88 million square kilometers (3.43 million square miles), it was 0.89 million square kilometers (0.34 million square miles) above the value for July 31, 2007. As is normal for this time of year, melt is occurring throughout the Arctic, even at the North Pole.

Figure 2. Daily sea ice extent; the blue line indicates 2008; the gray line indicates extent from 1979 to 2000; the dotted green line shows extent for 2007. Sea Ice Index data.
—Credit: National Snow and Ice Data Center
High-resolution Image

Conditions in context

Sea ice extent continues to decline, but we have not yet seen last July’s period of accelerated decline. Part of the explanation is that temperatures were cooler in the last two weeks of July, especially north of Alaska.

Because we are past the summer solstice, the amount of potential solar energy reaching the surface is waning. The rate of decline should soon start to slow, reducing the likelihood of breaking last year’s record sea ice minimum.

Figure 3. Using average long-term decline rates is one way to project sea ice extent at the end of the 2008 season. The bottom dashed line shows decline rate one standard deviation faster than normal, the middle dashed line shows decline at average rates, and the top dashed line shows decline rate one standard deviation slower.
—Credit: National Snow and Ice Data Center
High-resolution image

Slower decline than 2007

To estimate the range of possibilities, we have used average long-term daily decline rates to project ice extent during the rest of the season (dashed blue lines). The bottom dashed line shows decline rate one standard deviation faster than normal, the middle dashed line shows decline at average rates, and the top dashed line shows decline rate one standard deviation slower.

If the Arctic experiences a normal decline rate, the minimum extent will be between the second-lowest extent, which occurred in 2005, and the third-lowest extent, which occurred in 2002. Even at a rate one standard deviation faster than normal, the extent will not fall below last year’s minimum—so it appears unlikely that we will set a new record low.

Figure 4. Passive-microwave satellite data shows ice concentration on July 31, 2008. Widespread areas of low concentration ice exist, shown in yellows. NASA AMSR-E data.

—Credit:From National Snow and Ice Data Center courtesy University of Bremen
High-resolution image

Figure 5.Visible-band satellite imagery confirms the low-concentration ice cover seen in Figure 4. This view places NASA MODIS Aqua data in a perspective generated in Google Earth, simulating a view from far above Earth.

—Credit: From National Snow and Ice Data Center courtesy NASA
High-resolution image

Visible imagery confirms weak ice cover

Visible-band imagery from the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) sensor shows a more detailed picture of the ice than AMSR-E. Looking east into the Northwest Passage on July 28, the image confirms the low ice concentrations revealed in the AMSR-E data.

So, will we break last year’s record low minimum extent? Will the North Pole become ice-free? Probably not this year. However, the ice is in a vulnerable state and there are six weeks of melting left, so a lot can still happen.

And perhaps the most important point as we continue to watch this season’s evolving ice cover is that, whether or not Arctic sea ice sets a new record low, this year continues the pattern of well-below-average ice extent seen in recent years.

For previous analysis, please see the drop-down menu under Archives in the right navigation at the top of this page.

Figure 1. Daily Arctic sea ice extent for July 16, 2008 was 8.91 million square kilometers (3.44 million square miles). The orange line shows the 1979-2000 average extent for that day.About the data.
—Credit: National Snow and Ice Data Center
See High Resolution Image

Overview of conditions

Arctic sea ice extent on July 16 stood at 8.91 million square kilometers (3.44 square miles). While extent was below the 1979 to 2000 average of 9.91 square kilometers (3.83 million square miles), it was 1.05 million square kilometers (0.41 million square miles) above the value for July 16, 2007 (see Figures 1 and 2).

Figure 2. Daily sea ice extent; the blue line indicates 2008; the gray line indicates extent from 1979 to 2000; the dotted green line shows extent for 2007.
—Credit: National Snow and Ice Data Center
See High Resolution Image

Conditions in context

The current pattern of sea ice retreat is noticeably different than last summer, with some areas showing less ice and others showing more. For example, in mid-July 2007, a large area of the southern Beaufort Sea north of Alaska still had ice; this year, it is already ice-free (see Figure 1). in 2007, large areas along the Siberian coast had melted out by mid-July; as of July 16, 2008, the Siberian sector remained largely ice-covered. Although the Siberian area still shows ice, satellite data reveals that the ice is low concentration and thus prone to melting.

Colleagues at the University of Bremen, Germany post daily satellite images from the NASA Advanced Microwave Sounding Radiometer (AMSR) at http://www.iup.uni-bremen.de:8084/amsr/amsre.html. The images not only show the low ice concentrations in the Siberian sector discussed above; they also show that a large part of the northern Beaufort Sea is covered by ice with low concentrations of 50 to 75 percent, and is likely to melt out soon. An unusual area of low ice concentration is also developing near 85 degrees North latitude.

Figure 3:  Above is a map of sea-level pressure, averaged for the the three-week period from June 23 through July 13, 2008. Areas of high pressure are shown in yellow and red; areas of low pressure are shown in blue and purple.Thin lines indicate isobars, which connect areas of equal pressure. Winds between high- and low-pressure areas, indicated by “H” and “L” respectively, are from the south.
—Credit: From National Snow and Ice Data Center courtesy Climate Diagnostic Center
See High Resolution Image

Winds from the south

The spatial pattern of sea ice concentration, discussed above, is influenced by atmospheric circulation patterns. Figure 3 shows the pattern of sea-level pressure averaged over the three-week period from June 23 through July 13, 2008.

Winds tend to blow parallel to the isobar lines with a strength proportional to the pressure gradient; the closer the spacing between the isobars, the stronger the wind. The direction of the wind is determined from Buys Ballot’s Law, which states that if you are standing with your back to the wind in the Northern Hemisphere, low pressure will be on your left, and high pressure will be on your right.

So, from Figure 3, we can see that the tightly spaced isobars between the high-pressure cell in the Canadian Arctic Islands and lower pressure in the East Siberian Sea points to strong and persistent winds from the south. This wind pattern helps transport sea ice poleward, away from the Alaska coast, and favors the development of open water in the area.

Figure 4.  This image is a map of air-temperature anomalies at the 925 millibar level (about 3,000 feet above the surface), averaged for the three-week period from June 23 through July 13, 2008.  Yellow and red indicate areas with above-average temperature; blue and purple indicate areas with below-average temperature.

—Credit: From National Snow and Ice Data Center courtesy Climate Diagnostic Center
See High Resolution Image

 

Warm winds contribute to melt

In addition to pushing sea ice poleward, winds from the south tend to be warmer. Figure 4 shows a map of air-temperature anomalies averaged for the three-week period from June 23 through 13, 2008. While temperatures over most of the Arctic Ocean have been above normal, they have been particularly warm north of Alaska. There, persistent winds from the south are causing strong melt and reduced ice concentrations.

How is this different from what we saw in the record-breaking year 2007? In early July 2007, an atmospheric pattern developed that featured high pressure over the Beaufort Sea. This pattern promoted especially strong sea ice loss. The pattern that has dominated the summer of 2008, so far, seems less favorable for ice loss. However, the melt season has a long way to go. Furthermore, as discussed above, large areas of the pack ice with fairly low concentrations are likely to melt out soon.

Figure 5. On the left are maps of ice thickness for late winter of the last three years; note the widespread coverage of fairly thin ice (purples and blues) in February-March 2008. The graphs on the right show the statistical distribution of ice thickness; note the thinner multiyear ice in 2008. Data from ICESat.

—Credit: From the National Snow and Ice Data Center courtesy Ronald Kwok, NASA Jet Propulsion Laboratory.
High Resolution Image not available

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.

For previous analysis, please see the drop-down menu under Archives in the right navigation at the top of this page.

Figure 1. Arctic sea ice extent for June 2008 was 11.44 million square kilometers (4.42 million square miles). The magenta line shows the median ice extent for June from 1979 to 2000. About the data.

—Credit: National Snow and Ice Data CenterSee High Resolution Image

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 (see Figure 3).

While the monthly average was slightly less than June 2007, Figure 2 indicates that 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 percent. So if the concentration is 15 percent 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.

Figure 3. Average June ice extent for 1979 through 2008
—Credit: National Snow and Ice Data CenterSee High Resolution Image

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.

Figure 4. The colors in the above image indicate date of onset of melt over the Arctic Ocean. Light gray indicates areas that have not yet begun to melt this year, or areas for which data is not available. Data from the SSM/I sensor; algorithm used to process the data came from Thorsten Markus at Goddard Space Flight Center.

—Credit: Natonal Snow and Ice Data CenterSee High Resolution Image

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 (see Figure 4). 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.

Figure 5. This image shows the percent anomaly of ocean absorption of solar heat from January 1 to September 21, 2007, compared to the 1979 to 2005 average. Dark red and orange indicate areas with especially low albedo. Data from SSM/I sensor.
Figure 4 was updated on July 3, 2008, with data through July 1. A previous version, posted on July 2, used data from June 10, 2008.

—Credit: From the National Snow and Ice Data Center courtesy Don Perovich, U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory See High Resolution Image

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 (see Figure 5). 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.

A community sea ice outlook

The Study of Environmental Arctic Change program has released a Sea Ice Outlook for 2008. Their May report has predictions from a number of different scientific groups (including NSIDC) of how much sea ice will be left in the Arctic at the end of the melt season. The predictions range widely above and below last year’s record minimum of 4.13 million square kilometres (1.59 million square miles).

References

Perovich, D. K., J. A. Richter-Menge, K. F. Jones, and B. Light (2008), Sunlight, water, and ice: Extreme Arctic sea ice melt during the summer of 2007, Geophys. Res. Lett., 35, L11501, doi:10.1029/2008GL034007.

Stroeve, J.C., T. Markus and W.N. Meier (2006), Recent changes in the Arctic melt season, Annals of Glaciology, 44, 367-374

For previous analysis, please see the drop-down menu under Archives in the right navigation at the top of this page.

Overview of conditions

Arctic sea ice extent for May stood at 13.18 million square kilometers (5.09 million square miles), which is 0.28 million square kilometers (0.11 million square miles) greater than May 2007, but is still 0.42 million square kilometers (0.16 million square miles) less than the 1979 to 2000 average for the month.

Conditions in context

Although ice extent is slightly greater than this time last year, the average decline rate through the month of May was 8,000 square kilometers per day (3,000 square miles per day) faster than last May. Ice extent as the month closed approached last May’s value.

Average Arctic Ocean surface air temperatures in May were generally higher than normal. While anomalies were modest  (+1 to 3 degrees Celsius, +2 to 5 degrees Fahrenheit) over most of the region, temperatures over the Baffin Bay region were as much as 6 degrees C (11 degrees F) above normal. The atmospheric circulation in May was highly variable. The first half of the month saw strong winds blowing from east to west over the southern Beaufort Sea. This wind pattern probably contributed to polynya formation near Banks Island and along the northwestern coast of Alaska.

Multi-year ice continues to be low

The relative lack of thick, resilient multi-year ice in the Arctic discussed in earlier postings finds further support in the latest analysis from the United States National Ice Center (NIC). NIC uses a variety of satellite imagery, expert analysis, and other information to provide information on the amount and quality of sea ice for ships operating in the Arctic. NIC scientist Todd Arbetter suggests that much of the first-year ice is likely to melt by the end of summer, saying that despite the total ice extent appearing normal, the relative amount of multi-year ice going into this summer is very low when compared to climatological averages. NIC has found that the relative fraction of multi-year ice in the central Arctic has plummeted since the mid-1990s, creating an Arctic prone to increased melt in summer. Arbetter said, “This may be a primary reason for record summertime minimums in recent years.”

However, the unusual location of some of this year’s first-year ice may help more of it survive than otherwise might be expected. This year, much of the first-year ice is farther north than normal, and those northern areas receive weaker solar radiation. So, northern first-year ice may be less vulnerable to melt than first-year ice in typical locations.

Overview of conditions

For the month of April, Arctic sea ice extent stood at 14.49 million square kilometers (5.59 million square miles), which is 0.61 million square kilometers (0.24 million square miles) greater than April 2007, but is still 0.51 million square kilometers (0.20 million square miles) less than the 1979 to 2000 average for April.

Conditions in context

Although there is more ice than this time last year, the average decline rate through the month of April was 6,000 square kilometers per day (2,300 square miles per day) faster than last April.

Estimating September extent based on past conditions

As discussed in our April analysis, the ice cover this spring shows an unusually large proportion of young, thin first-year ice; about 30% of first-year ice typically survives the summer melt season, while 75% of the older ice survives. For a simple estimate of the likelihood of breaking last year’s September record, we can apply survival rates from past years to this year’s April ice cover.  This gives us 25 different estimates, one for each year that we have reliable ice-age data (see Figure 4).  To avoid beating the September 2007 record low, more than 50% of this year’s first-year ice would have to survive; this has only happened once in the last 25 years, in 1996. If we apply the survival rates averaged over all years to current conditions, the end-of-summer extent would be 3.59 million square kilometers (1.39 million square miles). With survival rates similar to those in 2007, the minimum for the 2008 season would be only 2.22 million square kilometers (0.86 million square miles). By comparison the record low extent, set last September, was 4.28 million square kilometers (1.65 million square miles).

Forecasting September extent with climate predictors

Sheldon Drobot at the Center for Astrodynamics Research at the University of Colorado at Boulder and colleagues have developed a sophisticated forecasting technique. The forecast considers sea ice extent, ice age, summer and winter temperatures, cloudiness, the phase of the Atlantic Oscillation, and climate trends as predictors (see the papers cited below for details; visit the Arctic Oscillation Index).  As reported last month, the Arctic Oscillation was in its positive phase through the winter season, associated with a wind pattern helping to flush thick ice out of the Arctic, leaving thinner ice.  This is one of the factors helping to set the stage for pronounced ice losses this summer. Drobot predicts a 59% chance of a new record minimum this year; read the press release. Todd Arbetter of the U.S. National Ice Center tells us that his group is working to implement a version of Drobot’s analysis scheme for operational forecasting.

Ronald Lindsay of the University of Washington’s Applied Physics Laboratory and collaborators recently published results from their own ice prediction system, based on a retrospective analysis of the modeled state of the ice and ocean system (see the paper cited below for details).  The model is successful in explaining around 75% of the year-to-year variations for the past few decades; for 2008, the model implies a very low, but not extreme, sea ice minimum. Lindsay cautions that sea ice conditions are now changing so rapidly that predictions based on relationships developed from the past 50 years of data may no longer apply.