Arctic sea ice younger, thinner as melt season begins

April 6, 2009

Arctic sea ice extent has begun its seasonal decline towards the September minimum. Ice extent through the winter was similar to that of recent years, but lower than the 1979 to 2000 average. More importantly, the melt season has begun with a substantial amount of thin first-year ice, which is vulnerable to summer melt.

Figure 1. Arctic sea ice extent for March, 2009, was 15.16 million square kilometers (5.85 million square miles). The magenta line shows the 1979 to 2000 median extent for that month. The black cross indicates the geographic North Pole. Sea Ice Index data. About the data. —Credit: National Snow and Ice Data Center
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Overview of conditions

Sea ice extent averaged over the month of March 2009 was 15.16 million square kilometers (5.85 million square miles). This was 730,000 square kilometers (282,000 square miles) above the record low of 2006, but 590,000 square kilometers (228,000 square miles) below the 1979 to 2000 average.

Figure 2. The graph above shows daily sea ice extent. The solid blue line indicates 2008 to 2009; the dashed green line shows 2006 to 2007 (the record-low summer minimum occurred in 2007); and the solid gray line indicates average extent from 1979 to 2000. Sea Ice Index data.—Credit: National Snow and Ice Data
Center
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Conditions in context

At the end of last summer’s melt season, extensive areas of open
water froze up quickly, once air temperatures cooled in
the fall. By February 28, ice extent had reached its annual maximum.
Although the maximum ice extent occurred slightly earlier than usual, ice extent remained close to the maximum level through much
of March.

Figure 3. Monthly March ice extent for 1979 to 2009 shows a decline of 2.7% per decade.—Credit: National Snow and Ice Data Center
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March 2009 compared to past Marches

Including March 2009, the past six years have all had ice extent substantially lower than normal. The linear trend indicates that for the month of March, ice extent is declining by 2.7% per decade, an average of 43,000 square kilometers (16,000 square miles) of ice per year.

Figure 4. The map of air temperature anomalies for winter 2008 to 2009 at the 925 millibar level (roughly 1,000 meters [3,000 feet] above the surface) shows warmer-than-usual conditions over much of the Arctic Ocean. Areas in orange and red correspond to strong positive (warm) anomalies. Areas in blue correspond to negative (cool) anomalies.—Credit: National Snow and Ice Data Center courtesy NOAA/ESRL Physical Sciences Laboratory
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Arctic winter warmer than average

Overall, it was a fairly warm winter in the Arctic. Air temperatures over the Arctic Ocean were an average of 1 to 2 degrees Celsius (1.8 to 3.6 degrees Fahrenheit) above normal, with notable regional variations. The Barents Sea region was over 4 degrees Celsius (7.2 degrees Fahrenheit) warmer than average this winter. This warmth probably stemmed from unusually low sea ice extent in the region throughout much of the winter, which allowed the ocean to pump heat into the atmosphere. The Bering Sea, in contrast, experienced a cool winter, with temperatures 1 to 2 degrees Celsius (1.8 to 3.6 degrees Fahrenheit) below average. The cooler conditions were consistent with the above-average sea ice extent in the Bering Sea through much of the winter.

Figure 5. These images show declining sea ice age, which indicates a thinning Arctic sea ice cover more vulnerable to melting in summer. Ice older than two years now accounts for less than 10% of the ice cover.—Credit: From the National Snow and Ice Data Center, courtesy J. Maslanik and C. Fowler, University of Colorado
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Sea ice young and thin as melt season begins

How vulnerable is the ice cover as we go into the summer melt season? To answer this question, scientists also need information about ice thickness. Indications of winter ice thickness, commonly derived from ice age estimates, reveal that the ice is thinner than average, suggesting that it is more susceptible to melting away during the coming summer.

As the melt season begins, the Arctic Ocean is covered mostly by first-year ice, which formed this winter, and second-year ice, which formed during the winter of 2007 to 2008. First-year ice in particular is thinner and more prone to melting away than thicker, older, multi-year ice. This year, ice older than two years accounted for less than 10% of the ice cover at the end of February. From 1981 through 2000, such older ice made up an average of 30% of the total sea ice cover at this time of the year.

While ice older than two years reached record lows, the fraction of second-year sea ice increased compared to last winter. Some of this second-year ice will survive the summer melt season to replenish the Arctic’s store of older ice; however, in recent years less young ice has made it through the summer. To restore the amount of older ice to pre-2000 levels, large amounts of this young ice would need to endure through summer for several years in a row.

But conditions may not always favor the survival of second-year and older ice. Each winter, winds and ocean currents move some sea ice out of the Arctic ocean. This winter, some second-year ice survived the 2008 melt season only to be pushed out of the Arctic by strong winter winds. Based on sea ice age data from Jim Maslanik and Chuck Fowler at the University of Colorado, since the end of September 2008, 390,000 square kilometers (150,000 square miles) of second-year ice and 190,000 square kilometers (73,000 square miles) of older (more than two years old) ice moved out of the Arctic. View
animation (1.1 MB).

References:

Maslanik J. A., C. Fowler, J. Stroeve, S. Drobot, J. Zwally, D. Yi, W. Emery. 2007. A younger, thinner Arctic ice cover: Increased potential for rapid, extensive sea-ice loss. Geophysical Research Letters, 34, L24501, doi:10.1029/2007GL032043.

Fowler, C., W. J. Emery, and J. Maslanik. 2004. Satellite-derived evolution of Arctic sea ice age: October 1978 to March 2003. IEEE Geosci. Remote Sensing Letters, 1(2), 71–74, doi:10.1109/LGRS.2004.824741.

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

Annual maximum ice extent confirmed

March 30, 2009

Arctic sea ice extent reached its maximum extent for the year, marking the beginning of the melt season. This year’s maximum was the fifth lowest in the satellite record. NSIDC will release a more detailed analysis of winter sea ice conditions during the second week of April.

Figure 1. Arctic sea ice extent for February 28, 2009, the date of the annual maximum, was 15.14 million square kilometers (5.85 million square miles). The orange line shows the 1979 to 2000 median 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
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Overview of conditions

On February 28, Arctic sea ice reached its maximum extent for the year, at 15.14 million square kilometers (5.85 million square miles). The maximum extent was 720,000 square kilometers (278,000 square miles) below the 1979 to 2000 average of 15.86 million square kilometers (6.12 million square miles), making it the fifth-lowest maximum extent in the satellite record. The six lowest maximum extents since 1979 have all occurred in the last six years (2004 to 2009).

Conditions in context

In the beginning of March, ice extent began to decline, and it appeared that Arctic sea ice had reached its maximum extent. However, in the second week of March the ice edge began to expand again. Ice extent grew through much of the month of March, but it did not expand to the level seen on February 28.

Such ups and downs in Arctic sea ice extent are not unusual near the annual maximum. As discussed in our March 3 post, the ice edge at this time of year consists of thin ice that is sensitive to temperature changes, and easily redistributed by storm winds.

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

Ice extent nears annual maximum

March 3, 2009

Arctic sea ice extent continued to increase through the month of February, as it approaches its annual maximum. Ice extent averaged for February 2009 is the fourth-lowest February in the satellite record. From February 18 to 22, ice extent declined slightly, primarily because of weather conditions off the coast of Alaska; ice extent then rebounded.

Figure 1. Arctic sea ice extent for February 2009 was 14.84 million square kilometers (5.73 million square miles). The magenta line shows the 1979 to 2000 average extent for February. The black cross indicates the geographic North Pole. Sea Ice Index data. About the data. —Credit: National Snow and Ice Data Center
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Overview of conditions

Arctic sea ice extent averaged for the month of February was 14.84 million square kilometers (5.73 million square miles). February extent was 800,000 square kilometers (309,000 square miles) less than the 1979 to 2000 average, and 140,000 square kilometers (54,000 square miles) less than for February 2008.

During the month of February, Arctic sea ice extent increased by 520,000 square kilometers (201,000 square miles), an average increase of 19,000 square kilometers (7,300 square miles) per day. These values are based on data from the F13 Special Sensor Microwave/Imager (SSM/I) sensor, which NSIDC is once again using because of problems with the sensor on the F15 satellite. See our February 26 post for details.

Figure 2. The graph above shows daily sea ice extent. The solid blue line indicates 2008–2009; the dashed green line shows 2006–2007 (the record-low summer minimum occurred in 2007); and the solid gray line indicates average extent from 1979 to 2000. Sea Ice Index data.—Credit: National Snow and Ice Data Center
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Conditions in context

Arctic sea ice extent continued to climb through the month of February; NSIDC scientists expect Arctic sea ice to reach its annual maximum extent sometime in March. The date of the maximum can vary by as much as 6 weeks. The average date of the maximum is March 6, based on the satellite record from 1979 to 2008.

From February 18 to 22, ice extent declined from 14.89 million square kilometers (5.75 million square miles) to 14.80 million square kilometers (5.71 million square miles), before rebounding at the end of the month. Such ups and downs are not unusual at this time of year, as ice extent nears its annual maximum.

Figure 3. Monthly February ice extent for 1979 to 2009 shows 2009 as the fourth-lowest February on record. —Credit: National Snow and Ice Data Center
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February 2009 compared to past Februaries

Monthly average ice extent for February 2009 was the fourth lowest in the satellite record. February 2005 had the lowest ice extent for the month; February 2006 was the second lowest; and February 2007 is in third place. Including 2009, the downward linear trend in February ice extent over the satellite record stands at –2.8% per decade.

Figure 4. The map of sea level pressure (in millibars) over the Arctic, averaged for February 18 to 22, 2009, shows the pressure systems that caused warm southerly winds to compact and melt the ice in the Bering Sea and Gulf of Alaska.—Credit: From National Snow and Ice Data Center courtesy NOAA/ESRL Physical Sciences Laboratory
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Short-term changes in winter ice extent

The temporary decline in ice extent from February 18 to 22 illustrates the sensitivity of Arctic sea ice extent to transient weather conditions. Conditions along southern boundary of the ice cover, such as in the Bering Sea, are typically just barely cold enough for ice to exist, and the ice there can quickly expand or retreat in response to changes in temperature and winds.

The decline in mid-February appears to have been caused by the combination of low pressure centered in the western Bering Sea and high pressure centered in the western Gulf of Alaska. This weather pattern caused warm, southerly winds between the low and high pressure cells, which pushed the ice edge to the north and promoted melt. Air temperatures in the region at the 925 millibar level (approximately 915 meters [3,000 feet] above the surface) were up to 8°C (14°F) above average.

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

Near-real-time data now available

February 26, 2009

Near-real-time sea ice data updates are again available from Arctic Sea Ice News & Analysis. We have switched to the Special Sensor Microwave/Imager (SSM/I) sensor on the Defense Meteorological Satellite Program (DMSP) F13 satellite following the sensor drift problem described in our February 18 post.

The temporary error in the near-real-time data does not change the conclusion that Arctic sea ice extent has been declining for the past three decades. This conclusion is based on peer reviewed analysis of quality-controlled data products, not near-real-time data.

Figure 1. SSM/I Arctic sea ice extent data from F13 (dashed green line) and F15 (solid blue line) were consistent until late January, when the F15 SSM/I sensor began to degrade.
—Credit: National Snow and Ice Data Center
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Impact of the F15 sensor drift

On February 18, we reported that the F15 sensor malfunction started out having a negligible impact on computed ice extent, which gradually increased as the sensor degraded further. At the end of January, the F15 sensor underestimated ice extent by 50,000 square kilometers (19,300 square miles) compared to F13. That is still within the margin of error for daily data. By mid-February, the difference had grown to 500,000 square kilometers (193,000 square miles), which is outside of expected error. However, that amount represents less than 4% of Arctic sea ice extent at this time of year. When the computed daily extent dropped sharply on February 16, the sensor failure became obvious.

NSIDC stopped displaying the problematic data, and recalculated sea ice extent using data from the DMSP F13 satellite, an older sensor in the same series of satellites. The recalculation changed the January monthly average ice extent by less than the margin of error for the sensor. As we reported in our February 3 post, growth of Arctic sea ice did indeed slow in January because of unusual atmospheric conditions. Using F13 data instead of F15, the September daily minimum that we reported on September 16, 2008, changed from 4.52 million square kilometers (1.74 million square miles) to 4.54 million square kilometers (1.75 million square miles), within the margin of error for daily data.

The F15 sensor drift does not change any of our conclusions regarding the long-term decline in Arctic sea ice extent. Such scientific conclusions, published in peer-reviewed journals, are based on quality-controlled monthly to annually averaged data. We have quality-controlled the final data through 2007; a thorough audit of the more recent data from 2008 shows that any discrepancies fall within the margin of error.

Figure 2. NSIDC acquires sea ice data from a series of Defense Meteorological Satellite Program (DMSP) satellites. Overlap between satellite sensors allows us to build a consistent long-term data record.
—Credit: Image courtesy Air Force Space Command Defense Meteorological Satellite Program
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Building long time series from satellite data

Like cars, airplanes, and other machines, satellites do not last forever. Scientists monitor satellites for problems, and new satellites must be launched to replace them. One advantage of the DMSP series of satellites is the continual overlap between one satellite and the next. This overlap allows scientists to compare and calibrate data from each satellite to ensure consistency over time. By using a series of intercalibrated sensors, we can build a consistent long-term record of sea ice extent.

Before acquiring data from F15, NSIDC obtained sea ice data from the SSM/I sensors on the DMSP F8, F11, and F13 satellites. In March 2008, we switched from the F13 satellite to the F15 satellite because the F13 SSM/I sensor had started to have regular areas of missing data. The missing data were caused by malfunctioning data recorders on the satellite, not because of any problem with the sensor itself. At the time, we were concerned that the recorder problem would become more serious. However, the F13 data recorders have not degraded further.

In comparison, the F15 sensor drift makes those data unreliable for our purposes, so we have switched back to F13. As a long-term solution, we are working on a transition to the newest sensor, which is on the F17 satellite.

Figure 3. This animated map shows how NSIDC scientists now correct for missing data in the daily time series. The daily sea ice extent map labeled “raw data” shows areas of missing data as gray wedges and speckles. The map labeled “interpolated” shows the daily image used to calculate the daily time series.
Sea Ice Index data. About the data.
—Credit: National Snow and Ice Data Center
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Correcting the daily time series

The daily extent map now shows any areas of missing data as dark gray regions, speckles, or spider web patterns. However, in the time series chart we account for the missing data by averaging the extent for that region from the day before and the day after the gap, a mathematical technique called interpolation. Interpolation is an appropriate approach because ice cover changes slowly. The animation in Figure 3 shows an Arctic sea ice extent map with regions of missing data (Raw Data: 02/08/09), and the same map, showing the filled-in data (Interpolated: 02/08/09).

Isolated areas of missing data occasionally occur in near-real-time sea ice data. We adjust for such gaps during the quality control for the final data products, which are usually released about a year after the near-real-time data. The final data products form the basis for peer-reviewed scientific analysis; this is typical for all fields that use satellite data, not just sea ice science. For more information about quality control for NSIDC sea ice data, see Do your data undergo quality control? on the Frequently Asked Questions about Arctic sea ice Web page.

Figure 4. This time series of F13 SSM/I (blue line) versus AMSR-E (red line) Arctic sea ice extent from 2006 to February, 2009 shows how SSM/I and AMSR-E data match up over the last three years. The gray line shows the 1979-2000 average.
—Credit: National Snow and Ice Data Center
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Cross-checking data

A number of other satellites can detect sea ice, and provide an independent check for consistency of the SSM/I data. The NASA Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) sensor generally serves as a good comparison for SSM/I data from the DMSP F-series satellites. Recent comparison shows that ice extent from F-13 tracks close to that from AMSR-E. AMSR-E and several other types of satellite and ground-based measurements have confirmed the long-term decline in Arctic sea ice extent over the past thirty years.

Figure 5. This time series shows the difference between F13 SSM/I (solid green line) and AMSR-E (dashed pink line) Arctic sea ice extent from December 1, 2008 to February 12, 2009. AMSR-E starts out lower than SSM/I, but ends up higher, with differences of up to 100,000 square kilometers (38,600 square miles).
Credit: National Snow and Ice Data Center
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SSM/I vs. AMSR-E

While the AMSR-E satellite can provide a good check on SSM/I data, scientists prefer not to use it to extend the long-term sea ice record. Although AMSR-E has a lower absolute error, continuing to use SSM/I provides a lower relative error, which is more important for tracking long-term changes in conditions.

Figure 5 illustrates the difference between AMSR-E and SSM/I sea ice extent. AMSR-E starts out lower than SSM/I, but ends up higher, with differences of up to 100,000 square kilometers (38,600 square miles). Experts expect this to be the case because of differences in the way the sensors detect sea ice. Thus, by comparing sea ice extent data from the same satellite sensor, we can build a more reliable picture of how extent has changed over the last three decades.

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

Satellite sensor errors cause data outage

February 18, 2009

As some of our readers have already noticed, there was a significant problem with the daily sea ice data images on February 16. The problem arose from a malfunction of the satellite sensor we use for our daily sea ice products. Upon further investigation, we discovered that starting around early January, an error known as sensor drift caused a slowly growing underestimation of Arctic sea ice extent. The underestimation reached approximately 500,000 square kilometers (193,000 square miles) by mid-February. Sensor drift, although infrequent, does occasionally occur and it is one of the things that we account for during quality control measures prior to archiving the data. See below for more details.

We have removed the most recent data and are investigating alternative data sources that will provide correct results. It is not clear when we will have data back online, but we are working to resolve the issue as quickly as possible.

Figure 1. Daily Arctic sea ice extent map for February 15, 2009, showed areas of open water which should have appeared as sea ice. Sea Ice Index data. About the data.

—Credit: National Snow and Ice Data Center
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Where does NSIDC get its data?

NSIDC gets sea ice information by applying algorithms to data from a series of Special Sensor Microwave/Imager (SSM/I) sensors on Defense Meteorological Satellite Program (DMSP) satellites. These satellites are operated by the U.S. Department of Defense. Their primary mission is support of U.S. military operations; the data weren’t originally intended for general science use.

The daily updates in Arctic Sea Ice News & Analysis rely on rapid acquisition and processing of the SSM/I data. Because the acquisition and processing are done in near-real time, we publish the daily data essentially as is. The data are then archived and later subjected to very strict quality control. We perform quality control measures in coordination with scientists at the NASA Goddard Space Flight Center, which can take up to a year. High-quality archives from SSM/I, combined with data from the earlier Scanning Multi-channel Microwave Radiometer (SMMR) data stream (1979–1987) provide a consistent record of sea ice conditions now spanning 30 years.

Figure 2. Daily total Arctic sea ice extent between 1 December 2008 and 12 February 2009 for Special Sensor Microwave/Imager SSM/I compared to the similar NASA Earth Observing System Advanced Microwave Scanning Radiometer (EOS AMSR-E) sensor.
—Credit: National Snow and Ice Data Center
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Data error sources

As discussed above, near-real-time products do not undergo the same level of quality control as the final archived products, which are used in scientific research published in peer-reviewed journals. However, the SSM/I sensors have proven themselves to be generally quite stable. Thus, it is reasonable to use the near-real-time products for displaying evolving ice conditions, with the caveat that errors may nevertheless occur. Sometimes errors are dramatic and obvious. Other errors, such as the recent sensor drift, may be subtler and not immediately apparent. We caution users of the near-real-time products that any conclusions from such data must be preliminary. We believe that the potential problems are outweighed by the scientific value of providing timely assessments of current Arctic sea ice conditions, as long as they are presented with appropriate caveats, which we try to do.

For several years, we used the SSM/I sensor on the DMSP F13 satellite. Last year, F13 started showing large amounts of missing data. The sensor was almost 13 years old, and no longer provided complete daily data to allow us to track total daily sea ice extent. As a result, we switched to the DMSP F15 sensor for our near-real-time analysis. For more information on the switch, see “Note on satellite update and intercalibration,” in our June 3, 2008 post.

On February 16, 2009, as emails came in from puzzled readers, it became clear that there was a significant problem—sea-ice-covered regions were showing up as open ocean. The problem stemmed from a failure of the sea ice algorithm caused by degradation of one of the DMSP F15 sensor channels. Upon further investigation, we found that data quality had begun to degrade over the month preceding the catastrophic failure. As a result, our processes underestimated total sea ice extent for the affected period. Based on comparisons with sea ice extent derived from the NASA Earth Observing System Advanced Microwave Scanning Radiometer (EOS AMSR-E) sensor, this underestimation grew from a negligible amount in early January to about 500,000 square kilometers (193,000 square miles) by mid-February (Figure 2). While dramatic, the underestimated values were not outside of expected variability until Monday, February 16. Although we believe that data prior to early January are reliable, we will conduct a full quality check in the coming days.

Sensor drift is a perfect but unfortunate example of the problems encountered in near-real-time analysis. We stress, however, that this error in no way changes the scientific conclusions about the long-term decline of Arctic sea ice, which is based on the the consistent, quality-controlled data archive discussed above.

We are actively investigating how to address the problem. Since we are not receiving good DMSP SSM/I data at the present time, we have temporarily discontinued daily updates. We will restart the data stream as soon as possible.

Some people might ask why we don’t simply switch to the EOS AMSR-E sensor. AMSR-E is a newer and more accurate passive microwave sensor. However, we do not use AMSR-E data in our analysis because it is not consistent with our historical data. Thus, while AMSR-E gives us greater accuracy and more confidence on current sea ice conditions, it actually provides less accuracy on the long-term changes over the past thirty years. There is a balance between being as accurate as possible at any given moment and being as consistent as possible through long time periods. Our main scientific focus is on the long-term changes in Arctic sea ice. With that in mind, we have chosen to continue using the SSM/I sensor, which provides the longest record of Arctic sea ice extent.

For more information on the NSIDC sea ice data, see the following resources on the NSIDC Web site:

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

Ice extent continues to track below normal

February 3, 2009

As is typical during mid-winter, sea ice extent increased overall in January; maximum monthly extent is expected in March. However, January ice extent remained well below normal compared to the long-term record. Ice extent averaged for January 2009 is the sixth lowest January in the satellite record. Also of note is that from January 15 to 26, ice extent saw essentially no increase; an unusual wind pattern appears to have been the cause.

Figure 1. Arctic sea ice extent for January 2009 was 14.08 million square kilometers (5.43 million square miles). The magenta line shows the 1979 to 2000 median extent for January. The black cross indicates the geographic North Pole. Sea Ice Index data. About the data.—Credit: National Snow and Ice Data Center
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Overview of conditions

Arctic ice extent averaged for the month of January was 14.08 million square kilometers (5.43 million square miles). January extent was 760,000 square kilometers (293,000 square miles) less than for the 1979 to 2000 average, and 310,000 square kilometers (120,000 square miles) greater than for January 2007.

During the month of January, Arctic sea ice extent increased by 1.12 million square kilometers (440,000 square miles), an average increase of 36,000 square kilometers (13,900 square miles) per day.

Figure 2. The graph above shows daily sea ice extent.The solid blue line indicates 2008–2009; the dashed green line shows 2006–2007 (the record-low summer minimum occurred in 2007); and the solid gray line indicates average extent from 1979 to 2000. Sea Ice Index data.
—Credit: National Snow and Ice Data Center
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Conditions in context

While ice extent climbed through the month of January as a whole, the period from January 15 to 26 saw almost no increase in ice extent, appearing as a flattening in the line graph. Extent during this pause remained fairly steady at around 14.0 million square kilometers (5.4million square miles).

Figure 3. Monthly January ice extent for 1979 to 2009 shows 2009 as the sixth lowest January on record.
—Credit: National Snow and Ice Data Center
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January 2009 average extent compared to past Januaries

Average ice extent for January 2009 was the sixth lowest in the satellite record. January 2006 had the lowest ice extent for the month; January 2005 claims second place; and January 2007 is in third place. Including 2009, the downward linear trend in January ice extent stands at -3.1% per decade.

Figure 4. This map compares ice extent on January 15 to ice extent on January 26, 2009. Areas in red indicate where ice was present on January 15 but had disappeared by January 26; areas in green indicate where ice was not present on January 15 but had appeared by January 26. Note in particular the regional balance between reduced ice extent in parts of the Arctic and increased extent in others. Sea Ice Index data.

—Credit: National Snow and Ice Data Center
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A balancing act of regional growth and decline

The pause in total sea ice extent change from January 15 to 26 reflects expanding and declining ice extent in different areas of the Arctic. For example, ice extent increased southwest of Greenland but decreased in areas east of Greenland and in parts of the Barents Sea.

This regional variability of sea ice extent as seen from satellites gives the bird’s-eye view of the changeable conditions on the ground that Arctic residents must deal with. One Arctic community may note increased sea ice extent, while at the same time another community not far away may note decreased sea ice extent.

Figure 5. The map of sea level pressure (in millibars) averaged over the Arctic for the period January 15 to 26, 2009, reveals one of the reasons for the pause in ice extent change—the strong low-pressure feature (blue and purple) centered just south of Iceland.
—Credit: From National Snow and Ice Data Center courtesy NOAA/ESRL Physical Sciences Laboratory
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The reason for the pause in January average ice extent change

The pause in ice extent change from January 15 to 26 is somewhat similar to an event that characterized part of December 2008. Both times, the cause for the pause was an unusual pattern of atmospheric circulation.

January 15 to 26 saw very strong low pressure centered just south of Iceland—a very strong Icelandic Low . In accord with Buys Ballot’s Law, strong warm winds from the south and southeast encouraged ice decline in areas east of Greenland and in parts of the Barents sea area. The winds helped compact the ice cover and reduce ice growth. Regional winds from the north explain the increases in ice extent southwest of Greenland.

This strong Icelandic Low was present at the same time that atmospheric pressures were especially high over the subtropical North Atlantic. This large-scale pattern of atmospheric pressure and the regional pattern of changes in ice extent on the Atlantic side of the Arctic are classic signals of the positive phase of the North Atlantic Oscillation (NAO). The negative phase would have a weaker Icelandic Low, and roughly the inverse pattern of sea ice extent anomalies (the red and green in Figure 4 would be approximately reversed). The NAO has climate impacts not just in the Arctic, but in North America and Europe as well.

References

Rigor, I. G., J. M. Wallace, and R. L. Colony. 2002. Response of Sea Ice to the Arctic Oscillation. AMS Journal of Climate Volume 15 Issue 18; 2648–2663.

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

2008 Year-in-Review

January 7, 2009

In this month’s entry, we offer a review of the 2008 year in Arctic sea ice. First, however, we discuss the noticeable pause in ice growth from December 12 to 19, apparently caused by an anomalous atmospheric pressure pattern combined with unusually warm ocean surface temperatures in the Barents Sea.Note: We have updated the timeseries in the daily image update, above, to reflect the new year. The solid blue line shows 2008 to 2009. The dashed green line shows the winter of 2006 to 2007; 2007 went on to reach the lowest summer minimum in the satellite record. The 2008 entries of Arctic Sea Ice News & Analysis are now archived under “Previous Years” in the navigation to the right.

Figure 1. Arctic sea ice extent for December 2008 was 12.53 million square kilometers (4.84 million square miles). The magenta line shows the 1979 to 2000 average extent for December. The black cross indicates the geographic North Pole. Sea Ice Index data. About the data.—Credit: National Snow and Ice Data Center
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Overview of conditions (December 2008)

Average Arctic sea ice extent for the month of December was 12.53 million square kilometers (4.84 million square miles). This was 140,000 square kilometers (54,000 square miles) greater than for December 2007 and 830,000 square kilometers (320,000 square miles) less than the 1979 to 2000 December average.

Figure 2. The graph above shows daily sea ice extent.The solid blue line indicates 2008–2009; the dashed green line shows 2006–2007 (the record-low summer minimum occurred in 2007); and the solid gray line indicates average extent from 1979 to 2000. Sea Ice Index data.
—Credit: National Snow and Ice Data Center
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Conditions in context (December 2008)

Ice growth throughout most of December 2008 fit in with the expected pattern of ice growth in winter months. However, from December 12 to 19 there was almost no increase in ice extent (seen in the flattening of the timeseries line). The week-long pause in expansion of the ice edge is not unprecedented in the satellite record; several Decembers during past decades show similar events. However, such slowing has been unusual in recent years.

Figure 3. Sea level pressure anomalies (in millibars) averaged for the period December 12 through December 19, 2008, reveal one of the reasons for the week-long pause in ice extent change seen in December, as southerly winds and warm sea surface temperatures worked against ice growth. Strong positive anomalies indicate areas of unusually high pressure (orange and red); strong negative anomalies indicate areas of unusually low pressure (blue and purple).

—Credit: From National Snow and Ice Data Center courtesy NOAA/ESRL Physical Sciences Laboratory
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Reasons for December’s pause in ice extent change

December’s week-long pause in expansion of the ice cover appears to have been caused, at least in part, by an anomalous atmospheric pressure pattern. High pressure over Alaska and the European Arctic, coupled with unusually low pressure east of Greenland and over eastern Siberia, brought warm southerly winds over much of the Arctic Ocean. The southerly winds helped keep the ice edge from expanding southward. In addition, warm sea surface temperatures, at least in the Barents Sea, inhibited ice formation.

Figure 4. This timeseries from January through December shows the natural waxing and waning of the Arctic sea ice cover with the seasons. The maximum extent generally occurs in March, the minimum extent in September. Sea ice extent in 2008 (purple) fell well below the 1979 to 2000 long-term average (gray) and was slightly above 2007 (dashed green), in which the lowest summer minimum and the second-lowest winter maximum occurred.
—Credit: National Snow and Ice Data Center
High-resolution image

2008 year in review

Arctic sea ice in 2008 was notable for several reasons. The year continued the negative trend in summer sea ice extent, with the second-lowest summer minimum since record-keeping began in 1979. 2008 sea ice also showed well-below-average ice extents throughout the entire year.

The ice cover in 2008 began the year heavily influenced by the record-breaking 2007 melt season. Because so much ice had melted out during the previous summer, a vast expanse of ocean was exposed to low winter air temperatures, encouraging ice growth. Although still well below average, March 2008 saw slightly greater ice extent at the annual maximum than measured in recent years. However, the ice was also thin: less than a year old and vulnerable to melting in summer. Even the geographic North Pole was covered with thin ice, capturing the imaginations of many in the media and general public.

Would 2008 break the 2007 record low summer minimum extent? Would the geographic North Pole be ice free for the first time in the satellite era? From May through July, cooler temperatures and winds less favorable to ice loss slowed the decline in ice extent. Nevertheless, by August the rate of ice loss was much faster than average—even faster than in 2007—as the effects of a warm Arctic Ocean worked against the thin ice cover. The melt season became a race: waning sunlight versus rapid ice loss.

Ultimately, summer 2008 finished with the second-lowest minimum extent in the satellite record, 9% above the 2007 minimum and 34% below average. A more diffuse ice cover and a thinner pack nevertheless suggested a record-low ice volume (ice area multiplied by thickness) at the end of summer.

As the sun set in the Arctic with the advent of autumn, seasonal ice growth was initially quite rapid, but slowed during early November. Average ice extent in December was well below average and very close to that measured in 2007. Heading into 2009, the Arctic sea ice cover is again young and thin; given this set-up, a continuation of well-below-average sea ice extent in 2009 is a near certainty.

Figure 5. This timeseries is similar to Figure 4, showing the annual cycle of Arctic sea ice extent. However, we also include specfic averages for comparison. Light gray shows the standard 1979 to 2000 long-term average; dark gray shows the 1979 to 2008 average; the dashed yellow line shows the 2002 to 2008 average, a period of steep decline; and solid purple shows 2008.
—Credit: National Snow and Ice Data Center
High-resolution image

A second look at the long-term average

In Arctic Sea Ice News & Analysis updates, we have always used sea ice extent data from 1979 to 2000 to calculate the long-term average. 1979 was the year of the first consistent satellite data on sea ice, so the starting point makes sense intuitively. But some have wondered why the long-term average only includes data through 2000, rather than through the current year. For more details on the pros and cons of using the standard long-term average versus an average including the current year, please see Why do you use the 1979–2000 average for comparisons? in our Frequently Asked Questions.

Figure 5 shows what happens to the long-term average when we include the current year, in this case 2008. The light gray line (1979 to 2000) and the dark gray line (1979 to 2008) are extremely close; including data through the current year does not in fact make a large difference.

To illustrate why the 1979 to 2008 average is slightly less than the standard long-term average, we also isolate the 2002 to 2008 average (dashed yellow line). These recent years show the steepest decline in ice extent over the thirty-year record, which explains why including them brings down the 1979 to 2008 average.

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

An expected paradox: Autumn warmth and ice growth

November 10, 2008

As is normal for this time of year, ice extentincreased rapidly through most of October. However, this year, the increase was particularly fast, which contributed to above-average air temperatures near the surface. A look back at the entire melt season from March through October reveals that the Arctic sea ice is showing some unusual changes in growth and melt cycles.

Note: With the close of the melt season, we have now returned to showing monthly average extent in Figure 1. During frequent summer updates as the crucial melt season progressed, we showed a daily extent value. Daily images are still available in the upper right of the home page. For more information, read our Quick Facts on Arctic Sea Ice.

Figure 1. Arctic sea ice extent for October 2008 was 8.40 million square kilometers (3.24 million square miles). The magenta line shows the 1979 to 2000 average extent for October. 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

Sea ice extent for the month of October averaged 8.40 million square kilometers (3.24 million square miles). As of October 31, ice extent was at 9.27 million square kilometers (3.58 million square miles), more than doubling since the annual minimum of 4.52 million square kilometers (1.74 million square miles) measured on September 14, 2008.

October ice extent was 0.89 million square kilometers (0.34 million square miles) less than for the 1979 to 2000 average, but 1.63 million square kilometers (0.63 million square miles) greater than for October 2007.

Figure 2. The graph above shows daily sea ice extent.The solid light blue line indicates 2008; the dashed green line shows 2007; and the solid gray line indicates average extent from 1979 to 2000. Sea Ice Index data.
—Credit: National Snow and Ice Data Center
High-resolution image

Conditions in context

As autumn and winter take hold in the Arctic, sunlight wanes and days get shorter. The ice pack begins its annual growth cycle. The 2008 growth rate was especially strong in early October but subsequently slowed down slightly.

Figure 3. In this image, near-surface air temperatures show strong warming near the surface in the Beaufort sea region, an area with substantial open water at the end of the melt season. The anomalously high temperatures extend well up into the atmosphere, showing that the ocean is transferring heat to the atmosphere as ice forms. The vertical axis represents altitude measured by pressure, extending from the planet’s surface to 700 millibars (about 3,000 meters or 10,000 feet above sea level). The horizontal axis represents latitude from 50 degrees North (left) to 90 degrees North (right; North Pole).
—Credit: From National Snow and Ice Data Center courtesy NOAA/ESRL Physical Sciences Laboratory
High-resolution image

Higher-than-average air temperatures

Over much of the Arctic, especially over the Arctic Ocean, air temperatures were unusually high. Near-surface air temperatures in the Beaufort Sea north of Alaska were more than 7 degrees Celsius (13 degrees Fahrenheit) above normal and the warming extended well into higher levels of the atmosphere. These warm conditions are consistent with rapid ice growth.

The freezing temperature of saline water is slightly lower than it is for fresh water, about –2 degrees Celsius (28 degrees Fahrenheit). While surface air temperatures in the Beaufort Sea region are well below freezing by late September, before sea ice can start to grow, the ocean must lose the heat it gained during the summer. One way the ocean does this is by transferring its heat to the atmosphere. This heat transfer is largely responsible for the anomalously high (but still below freezing) air temperatures over the Arctic Ocean seen in Figure 3. Only after the ocean loses its heat and cools to the freezing point, can ice begin to form. The process of ice formation also releases heat to the atmosphere. Part of the anomalous temperature pattern seen in Figure 3 is an expression of this process, which is generally called the latent heat of fusion.

In the past five years, the Arctic has shown a pattern of strong low-level atmospheric warming over the Arctic Ocean in autumn because of heat loss from the ocean back to the atmosphere. Climate models project that this atmospheric warming, known as Arctic amplification, will become more prominent in coming decades and extend into the winter season. As larger expanses of open water are left at the end of each melt season, the ocean will continue to hand off heat to the atmosphere.

Figure 4. This animation of sea ice between September 1 and October 30 shows this year’s rapid ice regrowth. Data from AMSR-E sea ice concentration fields from the University of Bremen.
—Credit: From National Snow and Ice Data Center; data courtesy University of Bremen
Open animation in new window (.mov file, 884 KB)
(High-resolution animation available upon request)

Near-record October growth rates

As mentioned above, unusually high air temperatures go hand-in-hand with the rapid increase in ice extent seen through most of the month of October.

At its fastest point on October 15, the 2008 ice growth exceeded the 2007 growth rate on the same date by 92,000 square kilometers (36,000 square miles) per day. The near-record daily growth rate slowed toward the end of the month and has now fallen below the 2007 growth rate. It is important to reiterate this fast rate of growth is not unexpected under current conditions.

Record ice loss over the 2008 melt season

As discussed in our September 16, 2008, post, this year’s annual minimum represented the second-lowest point observed in the satellite record, surpassed only by the 2007 minimum. However, if we look at the total extent of ice lost between the March maximum and the September minimum, 2008 set a new record for total ice loss over an entire melt season. For more on the relatively high levels of ice that started the 2008 melt season and that led to this new record, see our April 7 post.

During the 2008 melt season, Arctic sea ice declined by 10.58 million square kilometers (4.08 million square miles). This was slightly more than the previous record for loss over an entire melt season, set in 2007, which was 10.51 million square kilometers (4.06 million square miles).

Arctic sea ice and climate are behaving in ways not seen before in the satellite record—both in the rate and extent of ice loss during the spring and summer, and in the record ice growth rates and increased Arctic air heating during the fall and winter.

Reference

Serreze, M. C. and J. A. Francis. 2006. The Arctic amplification debate. Climatic Change, 76, 241-264.

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

Arctic sea ice begins autumn freeze-up

September 24, 2008

Arctic sea ice extent, after reaching its seasonal minimum last week, has begun its annual cyclical increase in response to the setting sun. A cooler melt season, retention of first-year ice, and dispersive ice motion set the 2008 melt season apart from 2007.

Figure 1. Daily Arctic sea ice extent for September 23, 2008, was 4.59 million square kilometers (1.77 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 September 23, 2008, was 4.59 million square kilometers (1.77 million square miles), an increase of 77,000 square kilometers (30,000 square miles) above the minimum extent of 4.52 million square kilometers (1.74 million square miles) measured last week.

Conditions in context

Arctic sea ice extent, after reaching its seasonal minimum last week, has begun its annual cyclical increase in response to autumn cooling. The ice will grow over the cold, dark winter months and reach its maximum annual extent sometime next March.

During the intervening time, NSIDC will continue to provide updates on conditions. Also, note that as the season of ice growth progresses, NSIDC will slide the x-axis in Figure 2 to focus on the five-month window around the most current month. In our January entry, we will include a calendar-based full-year timeseries showing January 2008 through December 2008 extent compared to 2007 and to the 1979 to 2000 average.

Figure 3. Comparison of sea ice age during the second full week of September reveals that the Arctic will enter the winter ice growth season with less multiyear ice (bright colors), but far more first-year ice (dark blue) this year than it did in 2007. White indicates water; black indicates land.—Credit: From National Snow and Ice Data Center courtesy
C. Fowler, J. Maslanik, and S. Drobot, University of Colorado at Boulder
High-resolution image

High retention of first-year ice

Perhaps the most interesting aspect of the 2008 melt season was the higher-than-average retention of first-year sea ice (see earlier entries, including April 7). Relatively thin first-year ice is more prone to melting out completely than older, thicker ice. However, more of this year’s first-year ice survived the melt season than is typical. Sea ice age maps from Sheldon Drobot, our colleague at the University of Colorado at Boulder, show that much more first-year ice survived in 2008 than in 2007. This is one of the reasons that 2008 did not break last year’s record-low minimum.

One cause of the high first-year ice survival rate was that this summer was cooler than in 2007. Lower temperatures slowed the melt rate in the early part of the season. While conditions in August favored rapid ice loss, they were not enough to make up for this early-season “cushion.” Furthermore, much of this year’s first-year ice was located at higher latitudes than in 2007, covering even the geographic North Pole. Regions that are far north have lower melt rates because they receive less solar energy than more southerly regions.

Figure 4. Ice motion in 2007 compacted the ice pack; in 2008, ice motion did not have this effect. Average August ice motion
captures the differences between the two years, with black arrows indicating ice motion. Sea ice motion derived using data from the NASA AMSR-E sensor.
—Credit: National Snow and Ice Data Center
High-resolution image

Dispersive sea ice motion

Sea ice motion also helps determine how the ice will fare each melt season. In 2007, a strong northward sea ice motion at the end of the melt season pushed ice floes together, compacting the ice. The tightly packed ice and high temperatures worked together to create a record-low extent.

This year, the wind patterns were different, leading to a less compacted ice cover. This, paired with slower summer melt, helped keep the overall extent larger.

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

Arctic sea ice settles at second-lowest, underscores accelerating decline

September 16, 2008

Update 8:00 am MT October 2: See below.

The Arctic sea ice cover appears to have reached its minimum extent for the year, the second-lowest extent recorded since the dawn of the satellite era. While above the record minimum set on September 16, 2007, this year further reinforces the strong negative trend in summertime ice extent observed over the past thirty years. With the minimum behind us, we will continue to analyze ice conditions as we head into the crucial period of the ice growth season during the months to come.

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 2. The graph above shows daily sea ice extent.The solid light blue line indicates 2008; the dashed green line shows 2007; the dotted line shows 2005; the solid gray line indicates average extent from 1979 to 2000. Sea Ice Index data.
—Credit: National Snow and Ice Data Center
High-resolution image

Conditions in context

Despite overall cooler summer temperatures, the 2008 minimum extent is only 390,000 square kilometers (150,000 square miles), or 9.4%, more than the record-setting 2007 minimum. The 2008 minimum extent is 15.0% less than the next-lowest minimum extent set in 2005 and 33.1% less than the average minimum extent from 1979 to 2000.

This season further reinforces the long-term downward trend of sea ice extent.

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.

National Snow and Ice Data Center

Arctic Sea Ice News and Analysis

Author Archives: admin

Arctic sea ice younger, thinner as melt season begins

Annual maximum ice extent confirmed

Ice extent nears annual maximum

Near-real-time data now available

Satellite sensor errors cause data outage

Ice extent continues to track below normal

2008 Year-in-Review

An expected paradox: Autumn warmth and ice growth

Overview of conditions

Conditions in context

Higher-than-average air temperatures

Near-record October growth rates

Record ice loss over the 2008 melt season

Reference

Arctic sea ice begins autumn freeze-up

Arctic sea ice settles at second-lowest, underscores accelerating decline

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