Arctic sea ice extent declined quite slowly in April; as a result, total ice extent is now close to the mean extent for the reference period (1979 to 2000). The thin spring ice cover nevertheless remains vulnerable to summer melt.

Figure 1. Arctic sea ice extent for April, 2009, was 14.58 million square kilometers (5.63 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 April 2009 was 14.58 million square kilometers (5.63 million square miles). This was 710,000 square kilometers (274,000 square miles) above the record low for April in 2007, and 420,000 square kilometers (162,000 square miles) below the 1979 to 2000 average.

Figure 2. The graph above shows daily sea ice extent as of May 3, 2009. The solid blue line indicates 2009; the dashed green line shows 2007;and the solid gray line indicates average extent from 1979 to 2000. The gray area around 1979-2000 average line shows the two standard deviation range of the data. Sea Ice Index data.—Credit: National Snow and Ice Data Center
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Conditions in context

The decline rate for the month of April was the third slowest on record. The Arctic lost sea ice cover at a rate of 27,300 square kilometers per day (10,500 square miles), compared to an average of 41,600 square kilometers (16,000 square miles) per day for 1979 to 2000. Ice extent was well above normal in the Bering Sea, but below normal in the Barents Sea and the Sea of Okhotsk.

For the past few years, Arctic sea ice extent for most months has been more than two standard deviations below the 1979 to 2000 mean, particularly in summer. Two standard deviations provide an estimate of the expected range of natural variability. Because of cooler than average temperatures, Arctic sea ice extent at the end of April 2009 was within the expected range of natural variability.

Figure 3. Monthly April ice extent for 1979 to 2009 shows a decline of 2.8% per decade. —Credit: National Snow and Ice Data Center
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April 2009 compared to past Aprils

Compared to previous Aprils, April 2009 is near the middle of the distribution (10th lowest of 31 years). The linear trend indicates that for the month of April, ice extent is declining by 2.8% per decade, an average of 42,400 square kilometers (16,400 square miles) of ice per year.

Figure 4. The map of air temperature anomalies for April 1 to 15, 2009, at the 925 millibar level (roughly 1,000 meters [3,000 feet] above the surface), shows cooler-than-usual conditions over much of the Arctic Ocean. Areas in blue correspond to negative (cool) anomalies. Areas in orange and red correspond to strong positive (warm) anomalies.—Credit: National Snow and Ice Data Center courtesy NOAA/ESRL Physical Sciences Laboratory
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Causes of the slow April decline

Cool conditions over the Bering Sea, noted in the April 2009 update, persisted through mid-April. Cool weather also slowed ice loss in the Barents Sea. The cool temperatures resulted from the movement of cold air from eastern Siberia across the central Arctic. After mid-April, the pattern shifted to relatively warmer conditions in the Bering Sea and melt progressed, resulting in the faster decline in the total extent during the second half of the month.

It is difficult to assess how the slow decline through April will affect the summer minimum ice extent. Persistence of cool conditions through the summer could lead to a greater September ice extent compared to that of recent years. However, as discussed in our last post, the spring ice cover is thin and hence quite vulnerable to summer melt. However this summer unfolds, scientists expect to see high year-to-year variability in ice extent embedded within the long-term decline.

Figure 5. This map shows regions of above- and below-average precipitation observed in winter, following summers with very low Arctic sea ice extent.—Credit: From the National Snow and Ice Data Center, courtesy J. Francis, Rutgers University
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Can summer ice extent affect winter weather?

A new study suggests that Arctic ice extent at the end of summer can affect precipitation at lower latitudes the following winter. Jennifer Francis from Rutgers University and colleagues compared winter weather following summers with below-average ice extent, to weather following summers with above-average ice. The researchers found that low summer sea ice extent is linked to drier winters over much of the U.S., Scandinavia, and Alaska, and wetter winters in the northern Mediterranean, Japan, and the Pacific Northwest.

The study showed that extensive ice loss in summer warmed the Arctic atmosphere during autumn. This warmth weakened the storm track that encircles the northern hemisphere, affecting weather patterns far away from the Arctic. As sea ice continues to decline in summer, these influences will become more prominent.

References:

Francis, J. A., W. Chan, D. J. Leathers, J. R. Miller, and D. E. Veron. 2009. Winter Northern Hemisphere weather patterns remember summer Arctic sea-ice extent. Geophysical Research Letters, 36, L07503, doi:10.1029/2009GL037274.

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

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.

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:

• Sea Ice Index
• Why is the Sea Ice Index product used to study sea ice?
• Do your data undergo quality control?

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.