M. -L. Timmermans1 and Z. M. Labe2
1Department of Earth and Planetary Sciences, Yale University, New Haven, CT, USA
2Climate Central, Princeton, NJ, USA
Headlines
- In the marginal seas of the Arctic Ocean’s Atlantic sector, August 2025 mean sea surface temperatures (SSTs) were as much as ~7°C warmer than 1991-2020 August mean values.
- Anomalously cool August 2025 SSTs (~1-2°C cooler) were observed in parts of the marginal seas of the Arctic Ocean’s Pacific sector.
- August mean SSTs show warming trends for 1982-2025 in almost all Arctic Ocean regions that are ice-free in August, with mean SST increases of ~0.3°C per decade in the region north of 65° N.
- Warming Arctic SSTs alter local ecosystems and accelerate sea-ice loss, with wide-ranging global climate and societal consequences.
Introduction
Arctic Ocean sea-surface temperatures (SSTs) in the summer are primarily influenced by the amount of incoming solar radiation absorbed by the sea surface and by the flow of warm waters into the Arctic from the North Atlantic and North Pacific Oceans. Solar warming of the Arctic Ocean surface is influenced by the sea-ice distribution (with greater warming occurring in ice-free regions), cloud cover, and upper-ocean stratification. Inflows of relatively warm Arctic river waters can provide an additional heat source in coastal regions.
Arctic SST is a key indicator of the strength of the ice-albedo feedback during a given summer melt season. As the brighter, more reflective sea-ice retreats, more solar energy is absorbed by the darker ocean surface, which in turn warms the water and accelerates further ice melt. In addition, higher summer SSTs are associated with delayed autumn sea-ice freeze-up and increased ocean heat storage throughout the year. Marine ecosystems are also influenced by SSTs, which affect the timing and development of primary production cycles, available habitat, and other factors, such as the occurrence of harmful algal blooms (see essays Primary Productivity and Warming of the Bering and Chukchi Seas). These ecological changes influence fisheries, food security, and human health, particularly for Arctic communities that rely on marine resources (e.g., Quakenbush et al. 2024; Schoen et al. 2023). More recently, Arctic marine heatwaves have emerged as a significant concern, further stressing already vulnerable ecosystems (e.g., Gou et al. 2025).
2025 sea surface temperature
The SST data analyzed span June 1982 through August 2025, with 1991-2020 used as the climatological reference period (“normal”) (see Methods and data). Here, we focus most closely on August 2025 mean SSTs in context with the climatological record. August mean SSTs provide the most appropriate representation of Arctic Ocean summer SSTs because sea-ice extent and concentration are near a seasonal low at this time of year, and there is not yet the influence of surface cooling and subsequent sea-ice growth that typically takes place in the latter half of September.
August 2025 mean SSTs were as high as ~12°C in parts of the Barents and Kara Seas (within the Arctic Ocean’s Atlantic sector) with somewhat cooler values ~ -1-7°C in other Arctic basin marginal regions (Fig. 1a,b). August 2025 mean SSTs were anomalously warm compared to the 1991-2020 August mean in the Barents, Kara, and Laptev Seas: SSTs were around 1-4°C higher, with SST anomalies as high as ~7°C in the Kara Sea (Fig. 1c). In the Arctic’s Pacific sector, the Beaufort and northern Chukchi Seas had anomalously cold SSTs (August 2025 mean values ~1-2.0°C lower than the 1991-2020 mean), while the southern Chukchi Sea and the Bering Sea were anomalously warm (SSTs ~1-2.0°C higher). This general pattern of August 2025 SST anomalies is consistent with regional patterns of anomalously warm surface-air temperatures in the Kara and Laptev Sea regions and cold surface-air temperatures in the Beaufort Sea region in summer 2025 (see essay Surface Air Temperature).
In most Arctic regions, August 2025 SSTs ranked among the warmest recorded in the 1982-2025 period (70th to 100th percentile; Fig. 1d), although regional SST anomalies differ significantly from year to year. The Kara and Chukchi Seas had considerably higher SSTs in August 2025 compared to August 2024, with differences of up to 9°C (Fig. 1e). By contrast, August 2025 SSTs were up to a few degrees cooler than 2024 SSTs in the Barents and southern Beaufort Seas.
SST patterns in August 2025 were indicated by developments in June and July. Above-normal SSTs in the Arctic’s Atlantic sector were also observed in June and July 2025 (Fig. 2). This is consistent with relatively warm summer 2025 surface-air temperatures in the region (see essay Surface Air Temperature). Below-normal SSTs in the Bering Sea in June and July (Fig. 2a,b) were consistent with the relatively late ice retreat from the Pacific sector (see essay Sea Ice), suggesting a weaker ice-albedo feedback that can be initiated by warm Pacific inflows to the Arctic. The transition from below-normal to above-normal SSTs from June to August in the Bering Strait region corresponded with the transition from anomalously cool surface air temperatures in spring to warm in summer (see essay Surface Air Temperature).
The Arctic Ocean has experienced significant mean August SST warming from 1982 to 2025, with statistically significant (95% confidence interval) linear trends in almost all regions (Fig. 3a). Mean August SSTs for the entire region of the Arctic Ocean north of 65° N exhibit a linear warming trend of 0.3 ± 0.1°C/decade, matching the Northern Hemisphere trend (Fig. 3b). Similarly, the North Pacific and North Atlantic (between 50° N and 65° N) show warming trends of 0.4 ± 0.1°C/decade over the same period. The global mean SST trend is also not statistically distinct, with mean August SSTs exhibiting a linear trend of 0.20 ± 0.02°C/decade over the 1982 to 2025 period. In the context of these long-term trends, August 2025 mean Arctic Ocean SST was the second warmest on record, exceeded only in August 2007. Globally averaged SSTs for August 2025 were the third warmest on record, following August 2023 and August 2024 (Mercator Ocean International 2025).
Regionally, the Kara and Laptev Seas show the strongest warming trends in the Arctic Ocean (Fig. 3a), with August SST linear trends in these seas of 0.12 ± 0.03°C/yr and 0.08 ± 0.02°C/yr, respectively (Fig. 4a,b). In the Kara Sea, August 2025 mean SSTs were the warmest on record. Statistically significant linear trends (0.06 ± 0.02°C/yr) in August SST are also observed in the Barents Sea (Fig. 4c). On the other hand, there is no statistically significant trend in the Chukchi Sea, which is notably influenced by anomalously cool August SSTs in recent years (Fig. 4d). Beginning around 2007, there is some indication of an intensification of the warming trends in the Barents, Kara, and Laptev Seas (Atlantic sector of the Arctic Ocean; Fig. 4a-c). However, changepoint analysis of these regional SST records yields variable results across the seas, and no statistically robust shift is detected to date. A longer time series will be necessary to determine whether these apparent changes represent a sustained shift in the warming trajectory.
Methods and data
The SST data presented here are from the 0.25° × 0.25° NOAA Optimum Interpolation Sea Surface Temperature (OISST) Version 2.1 product, a blend of in situ and satellite measurements (Reynolds et al. 2002, 2007; Huang et al. 2021a,b); https://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.highres.html (NOAA 2024). The datafile “sst.mon.mean.nc” (comprising monthly means from the daily data) was retrieved from https://downloads.psl.noaa.gov/Datasets/noaa.oisst.v2.highres/ (accessed 3 September 2025). Note that in January 2023, OISST Version 2.1 replaced the 1° × 1° NOAA OISST Version 2, which was analyzed in Arctic Report Cards before 2023; while overall SST trends and patterns are similar between versions, the difference merits caution when comparing Arctic Report Cards across years (for further details, see Timmermans and Labe 2023).
For setting a proxy SST in sea-ice covered regions, OISST Version 2.1 sets SST equal to the freezing temperature (computed using a climatological sea-surface salinity) where ice concentrations are greater than 35% (see Banzon et al. 2020). Therefore, uncertainty in inferring SSTs (and SST trends) may be significant in the vicinity of the sea-ice edge, which varies in location each year, and when sea ice covers a significant portion of the region of interest.
Sea-ice concentration data are the Near-Real-Time NOAA/NSIDC Climate Data Record of Passive Microwave Sea Ice Concentration, Version 3 (https://nsidc.org/data/g10016/versions/3) (Peng et al. 2013; Meier et al. 2021a,b). The 1991-2020 median ice edge is derived from the NOAA/NSIDC Climate Data Record of Passive Microwave Sea Ice Concentration, Version 5 (https://nsidc.org/data/g02202/versions/5) with a threshold of 15% concentration defined as the ice edge.
Boundaries shown in Fig. 3a for the regional time series (Fig. 4) are as follows: Kara Sea: 70-80° N, 50-100° E; Chukchi Sea: 65-75° N, 180-202° E; Laptev Sea: 70-80° N, 100-139° E; Barents Sea: 66.5-80° N, 20-60° E.
Acknowledgments
M. -L. Timmermans acknowledges support from the National Science Foundation Office of Polar Programs and the Office of Naval Research. We thank Dr. Kristina Dahl (Climate Central) for reviewing the essay. NOAA OI SST V2 High Resolution Dataset data provided by the NOAA PSL, Boulder, Colorado, USA, from their website at https://psl.noaa.gov.
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November 11, 2025