DOI: 10.25923/mmxf-0r86

W. N. Meier¹, A. Petty², S. Hendricks³, A. Bliss⁴, L. Kaleschke³, D. Divine⁵, S. Farrell⁶, S. Gerland⁵, D. Perovich⁷, R. Ricker⁸, X. Tian-Kunze³, and M. Webster⁹

¹National Snow and Ice Data Center, Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA
²Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD, USA
³Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
⁴Cryospheric Sciences Laboratory, Goddard Space Flight Center, NASA, Greenbelt, MD, USA
⁵Norwegian Polar Institute, Fram Centre, Tromsø, Norway
⁶Department of Geographical Sciences, University of Maryland, College Park, MD, USA
⁷Thayer School of Engineering, Dartmouth College, Hanover, NH, USA
⁸NORCE Norwegian Research Centre, Tromsø, Norway
⁹Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, WA, USA

Headlines

• The lowest annual Arctic maximum extent in the 47-year satellite record occurred in March 2025, followed by the 10th lowest minimum extent in September 2025.
• In the first Arctic Report Card, 2005 was reported as the record lowest minimum extent, but that 2005 extent now ranks 20th lowest in the 47-year record.
• The last 20 years have been characterized by lower extent and a younger and thinner ice cover than in previous decades.
• Changing sea ice extent and thickness is allowing increased marine traffic and prompting reevaluations of national security concerns.

Introduction

Arctic sea ice forms the frozen interface between the polar ocean and atmosphere. Its high albedo due to its bright, near-white surface reduces the absorption of solar energy, and as a physical barrier sea ice modulates the heat and moisture transfer between the atmosphere and ocean. Sea ice plays a key role in polar ecosystems, providing an essential habitat for marine life and is an essential component of life and culture in Indigenous communities of the North. Changes in sea ice extent and seasonality have already impacted the Arctic ecosystem and peoples who rely on resources from the ocean (e.g., George et al. 2020). Additionally, the presence of sea ice historically limited economic and other activities in the Arctic; as the ice declines, maritime traffic is increasing and driving a reevaluation of resource extraction and national security activities in the Arctic (Zhao et al. 2024).

The Arctic sea ice environment has substantially changed since the publication of the first Arctic Report Card in 2006, which reported on 2005 sea ice conditions. At the end of summer 2025, the ice cover was younger, thinner and 28% less extensive than in 2005. The profound changes in sea ice since 2005 are opening the Arctic to more human activity and bringing to the fore concerns about safety, security, and the environment.

Sea ice extent

Sea ice extent, derived from sea ice concentration fields and defined as the total area covered by ice of at least 15% concentration, is an iconic indicator of long-term Arctic sea ice conditions. The primary source of extent observations is the 47-year record derived from satellite-borne passive microwave sensors, beginning in 1979.

After the annual minimum in September 2024, extent increased more slowly than average through mid-October, particularly in the Beaufort and Chukchi Seas on the Pacific side of the Arctic, as well as in the Barents and Kara Seas on the Atlantic side. Sea ice developed rapidly in the East Siberian and Laptev Seas in late October when the Siberian coast was iced in. Freeze-up was very slow in Hudson Bay, with record or near-record low extent from November through January. Extent was also near-record low in the Bering Sea and the Sea of Okhotsk in February and March (Fig. 1). This corresponded with record and near-record high surface air temperatures (see essay Surface Air Temperature). Other regions of the Arctic saw less extreme anomalies (relative to the 1991-2020 average) but were nonetheless lower than average throughout the winter. Arctic-wide, monthly extent was either lowest or second lowest from December through March, culminating in a record low annual maximum extent in March 2025 (Fig. 2; Table 1).

However, the record low maximum extent was followed by a slow start to the melt season with very little ice loss through April. Ice loss then accelerated with early open water areas developing in the Laptev and Kara Seas, resulting in record low ice extent in May in the Laptev Sea. The Bering Sea ice retreated rapidly in May. Compared to recent years, the Beaufort Sea was relatively slow to lose ice early in the melt season, with above-average extent into July; however, in late July and throughout August, ice rapidly melted in the Beaufort Sea. In contrast to record low extent in 2024, ice conditions in the Canadian Archipelago and Northwest Passage were lower than normal, but not extreme. Overall, ice loss throughout the summer season led to the 10th lowest minimum extent in September 2025 (Fig. 2). In the first Arctic Report Card, 2005 was reported as the record lowest minimum extent. That 2005 annual minimum extent now ranks 20th lowest in the 47-year record (Table 1); only 2006 was higher in the 20 years of the report.

Melt onset of sea ice

The summer melt season begins when liquid water first appears on the sea ice surface—often within the overlying snowpack—and is known as the melt onset date. Following melt onset, more advanced stages of surface melt, such as melt ponds, develop, further reducing surface albedo. In the spring and early summer when solar radiation nears its annual peak, the timing of melt onset becomes an important control on the absorption of heat in the ice-ocean system throughout the summer. In the Arctic, melt onset generally varies with latitude, occurring in March at lower latitudes and in early to mid-June in the central Arctic near the North Pole. Melt onset is sensitive to weather conditions, including cyclones and intrusions of warm, moist air, which contribute to interannual variability of melt onset timing at the regional scale.

In 2025, the average melt onset date for the marine areas of the Arctic was 21 May 2025 (Fig. 3a), two days earlier than in 2024. Compared to the 1991-2020 average, melt onset dates in 2025 were anomalously early in the Canadian Arctic Archipelago, much of the Laptev Sea, and parts of the Atlantic sector (Fig. 3b). In contrast, an anomalously late melt onset occurred in the central Arctic surrounding the North Pole and in much of the Pacific sector, including the East Siberian Sea, which is a region where sea ice extent survived the melt season in many recent years. Overall, Arctic melt onset dates are occurring earlier at a rate of 4.1 days per decade. The 2025 melt onset date occurred two weeks earlier than the melt onset observed at the beginning of the satellite melt onset date record in 1979.

Sea ice age

For sea ice, age is a proxy for thickness because multi-year ice generally grows thicker through successive winter periods. As in the last several years, multi-year ice (i.e., ice that has survived at least one summer melt season) in 2025 was largely constrained to the region near the north coast of Greenland and the Canadian Archipelago (Fig. 4). The Arctic sea ice today is far younger than in the 1980s and 1990s and is even substantially younger than 2005, with 47% less multi-year ice. There is almost no old (>4 years) multi-year ice remaining in the Arctic, just 95,000 km² at the September 2025 minimum. This is a reduction of 72% from the average of 326,000 km² over the Arctic Report Card period, 2005-24, and a reduction of 95% from the average of 1.72 million km² for the earlier 20 years, 1985-2004.

Sea ice thickness

Quantitative estimates of sea ice thickness based on satellite altimetry began in 2010 and have been used to directly track this important metric. At the end of the winter growth season, the April 2025 thickness (Fig. 5a) in the Arctic had the typical pattern of thicker ice along the northern coast of Greenland and the Canadian Archipelago due to deformation and advection across the pole, though this also corresponds to the region of older ice noted above. The thickness anomaly pattern for April 2025 shows a substantial amount of spatial variability (Fig. 5b). The sea ice growth season began with the Laptev, Kara, and Barents Seas having thicker ice than the 2011-21 average and other regions being thinner than average (Fig. 5c). Ice growth through the winter was robust and by April all regions were either average or thicker than the 2011-21 average. However, there were areas within some regions with thinner than average ice.

Methods and data

Sea ice extent values are from the NSIDC Sea Ice Index (Fetterer et al. 2025), based on passive microwave derived sea ice concentrations from the NASA Team algorithm (Cavalieri et al. 1996), though other high-quality products exist (e.g., Lavergne et al. 2019). For 2025, the algorithm was adapted for data from the JAXA Advanced Microwave Scanning Radiometer 2 (AMSR2) to create a consistent NASA Team product (Stewart et al. 2025).

Sea ice age data are from the EASE-Grid Sea Ice Age, Version 4 (Tschudi et al. 2019a) and Quicklook Arctic Weekly EASE-Grid Sea Ice Age, Version 1 (Tschudi et al. 2019b) archived at the NASA Snow and Ice Distributed Active Archive Center (DAAC) at NSIDC. Age is calculated via Lagrangian tracking of ice parcels using weekly sea ice motion vectors. Only the oldest age category is preserved for each grid cell.

Melt onset dates were derived from passive microwave satellite observations using the Advanced Horizontal Range Algorithm (Drobot and Anderson 2001; Bliss 2023). A sharp increase in the daily time series of brightness temperatures occurs when liquid water develops on the frozen sea ice surface, with the date of this brightness jump indicating the melt onset date.

Weekly CryoSat-2 estimates have been combined with thin ice (<1 m) estimates from the ESA Soil Moisture Ocean Salinity (SMOS) instrument and Sentinel-3 A/B data to obtain an optimal estimate across thin and thick ice regimes (Ricker et al. 2017) on a 25-km resolution EASE2 grid. Optimal interpolation is used to fill data gaps in the weekly CryoSat-2 fields and to merge the CryoSat-2 and SMOS estimates. The results here are from Version 206 (ESA 2023).

Acknowledgments

W. Meier thanks the NSIDC DAAC and the NASA ESDIS projects for support.

References

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December 10, 2025