SSMIS sunsets AMSR2 rises

Arctic sea ice extent tracked at near-record low levels through much of June, hitting daily record low levels from June 20 to 26. Sea ice coverage was particularly low in the Barents and Kara Seas, Hudson Bay, and Baffin Bay. There have been unrecoverable gaps in the delivery of sea ice data from the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager/Sounder (SSMI/S) source data. Moreover, NSIDC has been informed that the SSMIS data feed will be ending July 31. We are preparing to switch to the Japan Aerospace Exploration Agency (JAXA) Advanced Microwave Scanning Radiometer-2 (AMSR2) passive microwave sensor as the primary data source. 

With budget cuts from NASA for Sea Ice Today, we will no longer write mid-month analyses in the months leading up to the Arctic sea ice minimum.

Overview of conditions

Arctic sea ice extent averaged 10.48 million square kilometers (4.05 million square miles) in June 2025, the second lowest June average in the satellite record, just 70,000 square kilometers (27,000 square miles) above 2016—the record lowest June (Figures 1a and 1b). Extent was particularly low in the Barents and Kara Seas, both of which were nearly ice free by the end of June. Hudson Bay ice extent was also considerably below average, though not as extreme as last year. The North Water area of Baffin Bay opened up early and as a result, northern Baffin Bay is nearly ice free. Elsewhere around the Arctic, the extent was closer to average. Ice-free areas are opening up in the East Siberian and Laptev Seas, a relatively common occurrence in recent years. This led to an early start to seasonal shipping along the Northern Sea Route, albeit with icebreaker escort.

Figure 1a. Arctic sea ice extent for June 2025 was 10.48 million square kilometers (4.05 million square miles). The magenta line shows the 1981 to 2010 average extent for that month. Sea Ice Index data. About the data — Credit: National Snow and Ice Data Center

Figure 1b. The graph above shows Arctic sea ice extent as of July 8, 2025, along with daily ice extent data for four previous years and the record low year. 2025 is shown in blue, 2024 in green, 2023 in orange, 2022 in brown, 2021 in magenta, and 2012 in dashed brown. The 1981 to 2010 median is in dark gray. The gray areas around the median line show the interquartile and interdecile ranges of the data. Sea Ice Index data. — Credit: National Snow and Ice Data Center

Conditions in context

The sea level pressure pattern for June was characterized by a distinct Beaufort High north of Alaska, similar to the pattern observed in May (Figure 2a). Lower pressure prevailed over the northern North Atlantic and extended across Eurasia. Air temperatures at the 925 millibar level (about 2,500 feet above sea level) were 2 to 3 degrees Celsius (4 to 5 degrees Fahrenheit) above the 1981 to 2010 average in the region of the Beaufort High (Figure 2b). By contrast, it was relatively cool in the Laptev and East Siberian Seas, where temperatures were nearly 4 degrees Celsius (7 degrees Fahrenheit) below average. 

Figure 2a. This plot shows average sea level pressure in the Arctic in millibars for June 2025. Yellows and reds indicate high air pressure; blues and purples indicate low pressure. Credit: NSIDC courtesy NOAA Earth System Research Laboratory Physical Sciences Laboratory — Credit: NSIDC courtesy NOAA Earth System Research Laboratory Physical Sciences Laboratory

Figure 2b. This plot shows the departure from 1991-to-2020 average air temperature in the Arctic at the 925 hPa level, in degrees Celsius, for June 2025. Yellows and reds indicate above average temperatures; blues and purples indicate below average temperatures. Credit: NSIDC courtesy NOAA Earth System Research Laboratory Physical Sciences Laboratory — Credit: NSIDC courtesy NOAA Earth System Research Laboratory Physical Sciences Laboratory

June 2025 compared to previous years

The downward linear trend in Arctic sea ice extent through 2025 for June is 43,300 square kilometers (16,700 square miles) per year or 3.7 percent per decade relative to the 1981 to 2010 average (Figure 3). Based on the linear trend, since 1979, June has lost 1.99 million square kilometers (768,000 square miles) of sea ice. This is equivalent to nearly three times the size of Texas.

“Pond”-erous ice in June

Melt ponds formed by late June, evident as the blue tinge in NASA's Visible and Infrared Imager/Radiometer Suite (VIIRS) imagery (Figure 4). Ponds develop when snow or ice melts and the water collects in depressions on the ice. Visible blue light is preferentially transmitted through the liquid water and reflects from the ice surface back through ponds and into the atmosphere. Ponds have a substantially lower ability to reflect sunlight than whiter snow and ice. The timing of pond formation is a key component in the evolution of the melt season. Earlier pond formation can result in more absorption of solar energy and kick the ice loss into high gear. Later pond formation results in less solar absorption and a relatively slower pace of ice loss. Ponds are only one factor in how the sea ice melt season progresses: winds, ocean temperatures, and clouds also play key roles. 

The news down south

At the beginning of the Antarctic winter, sea ice extent has been above the extreme lows of 2023 and 2024, but still much below average (Figure 5a). Antarctic sea ice averaged third lowest for the month of June, 1.28 million square kilometers (494,000 million square miles) below the 1981 to 2010 average. At the beginning of July, extent was third lowest in the satellite record. Sea ice was particularly low in the Bellingshausen Sea and in the Indian Ocean sector, north of Enderby Land (Figure 5b).

Figure 5a. The graph above shows Antarctic sea ice extent as of July 8, 2025, along with daily ice extent data for four previous years and the record high year. 2025 is shown in blue, 2024 in green, 2023 in orange, 2022 in brown, 2021 in magenta, and 2014 in dashed brown. The 1981 to 2010 median is in dark gray. The gray areas around the median line show the interquartile and interdecile ranges of the data. Sea Ice Index data. — Credit: National Snow and Ice Data Center

Figure 5b. Antarctic sea ice extent for June 2025 was 12.07 million square kilometers (4.66 million square miles). The magenta line shows the 1981 to 2010 average extent for that month. Sea Ice Index data. About the data — Credit: National Snow and Ice Data Center

Revisions to extent estimates

Sea Ice Today uses the National Ocean and Atmospheric Administration (NOAA) Sea Ice Index near-real-time extent estimates, based on daily passive microwave data. For near-real-time data, only limited quality control can be done. The sea ice estimates are refined at NASA Goddard with better calibrated input data and more thorough quality-control filters. In the Sea Ice Index, the Goddard estimates replace the near-real-time values when they become available. Recently, the Sea Ice Index released revised estimates based on NASA Goddard data for 2023 and 2024 (Table 1). These changes are typically small: 30,000 to 50,000 square kilometers (12,000 to 19,000 square miles) and have a negligible effect on the long-term trends and variability. However, even such small changes can affect the ranking of years and the dates of minimums and maximums. For example, in NSIDC near-real-time data, the 2023 Arctic sea ice minimum extent was lower than 2024, but in the final data, the two years were nearly tied, while the date of the minimum shifted two days later in 2024. 

Table 1. This tables shows the changes in 2023 and 2024 Arctic sea ice minimum and Antarctic sea ice maximum extent estimates between the originally published NSIDC near-real-time (NRT) values and the higher quality-controlled estimates from NASA Goddard.

The rise of AMSR2

As reported earlier by NSIDC and other groups, the long data record of the US Department of Defense Meteorological Satellite Program (DMSP) is coming to end on July 31. The Special Sensor Microwave Imager (SSMI) and Imager/Sounder (SSMIS) series of sensors have provided continuous data since July 1987. Combined with the earlier NASA Scanning Multichannel Microwave Radiometer (SMMR), which began in late 1978, NASA, NOAA, and NSIDC have provided the long-term sea ice climate record that Sea Ice Today and many other users have relied upon. 

NSIDC is working on switching our primary data source to the JAXA Advanced Microwave Scanning Radiometer-2 (AMSR2). NSIDC has released a preliminary version. Sensor transitions always require adjustments to keep the derived product as consistent as possible. Switching to AMSR2 is challenging because for the first time in nearly 40 years there is a switch to a distinctly different sensor. AMSR2 has a larger antenna and thus higher spatial resolution as well as slightly different frequencies. The mismatch in resolution with SSMIS in particular requires detailed attention to preserve consistency, particularly for a threshold quantity such as sea ice extent. The intercalibration of AMSR2 data with SSMIS will be improved in the future and we will fully document our methodology.  Initial assessment shows good agreement between extent estimates, but differences are larger during the Arctic summer melt season (Figure 6). This is always a difficult period because of surface melt water effects and the rapidly changing ice edge. 

The view from Qaanaaq, Greenland

The hamlet of Qaanaaq in northwestern Greenland had a front-row seat to the unusually early and widespread opening of the North Water Polynya. Sea ice in the fjord broke out very early, by mid-June, having formed only in early December and remained too thin for hunters to work on until early January.

Further reading

Polashenski, C., D. Perovich, and Z. Courville. 2012. The mechanisms of sea ice melt pond formation and evolution. Journal Geophysical Research, 117, C01001, doi:10.1029/2011JC007231.

Schröder, D., D. Feltham, D. Flocco, et al. 2014. September Arctic sea-ice minimum predicted by spring melt-pond fraction. Nature Climate Change 4, 353–357. doi:10.1038/nclimate2203.

Webster, M. A., M. Holland, N. C. Wright, S. Hendricks, N. Hutter, P. Itkin, B. Light, F. Linhardt, D. K. Perovich, I. A. Raphael, M. M. Smith, L. von Albedyll, J. Zhang. 2022. Spatiotemporal evolution of melt ponds on Arctic sea ice: MOSAiC observations and model results. Elementa: Science of the Anthropocene, 4 January 2022; 10 (1): 000072. doi:10.1525/elementa.2021.000072.