April sea ice loss in the Arctic proceeded at a near-average rate overall, with the majority of ice losses in the Bering Sea and Sea of Okhotsk. In the Antarctic, sea ice grew faster than average, roughly evenly around the entire continent. Both hemispheres are well below the 1981 to 2010 reference period average, but neither are near record-low extents.
Overview of conditions
The average Arctic sea ice extent for April 2024 was 14.12 million square kilometers (5.45 million square miles), placing it sixteenth lowest in the passive microwave satellite record (Figure 1a and 1b). As of the beginning of May, extent is well below average in the Sea of Okhotsk, and slightly below average in the Bering and Barents Seas and off the coast of Labrador. Ice is near the average position along the eastern coast of Greenland.
Conditions in context
The global average temperature in April was at a record high in many assessments. By contrast in the Arctic, April 2024 temperatures at the 925 hPa level (about 2,500 feet above the surface) were below average by 3 to 5 degrees Celsius (5 to 9 degrees Fahrenheit) along the Siberian coast and northwestern coast of Scandinavia. Markedly warm conditions were the rule over most of Canada and northern Greenland (Figure 2a). The regions near Hudson Bay and along Peary Land on the north coast of Greenland were 3 to 5 degrees Celsius (5 to 9 degrees Fahrenheit) above the 1991 to 2020 climate reference period.
The atmospheric pattern for April featured high sea level pressure centered over the Barents Sea but lower pressure over most of the rest of the Arctic Ocean (Figure 2b). In Hudson Bay and Greenland, pressures were relatively high. The pattern in March that favored faster outflow of ice through Fram Strait did not persist into April.
April 2024 compared to previous years
Including 2024, the downward linear trend in April mean monthly sea ice extent was 36,000 square kilometers (14,000 square miles) per year, or 2.4 percent per decade relative to the 1981 to 2010 average (Figure 3). Based on the linear trend since 1979, April has lost 1.61 million square kilometers (622,000 square miles) of sea ice, which is roughly equivalent to six times the size of Colorado. April 2024 had the highest sea ice extent for the month in 12 years.
Lightening the mood in the Arctic
When sea ice extent shrinks and thins, and there is less snow cover, more light enters the water deeper. Light is a primary driver for sea ice algae and phytoplankton blooms, which form the base of the Arctic marine foodchain. To determine the timing of when enough light is present to initiate an ice algal bloom, NSIDC scientist Julienne Stroeve and others took existing satellite data products and combined them with information on light extinction properties through snow and ice. Light extinction implies the amount of dimming as light passes through ice, beginning at 100 percent (high) and dropping to 10 percent at the base of the ice. This has only been possible recently, with the development of accurate daily sea ice thickness products blended from CryoSat-2, Sentinel-3, and the Ice, Cloud and land Elevation Satellite-2 (ICESat-2).
Figure 4 illustrates the timing of bloom onset from 2019 to 2022, highlighting large year-to-year variability that reflects variability in snow depth over sea ice. In 2019, for example, bloom onset occurred at the end of February in the Beaufort Sea, whereas in 2022 the bloom onset occurred between mid-March to early April. In general, bloom onset starts about a month earlier in the marginal ice zone than it does in the central Arctic Ocean.
Antarctic note
April is the month of most rapid ice growth in the south. Sea ice expanded relatively uniformly around the continent, but remained below the average extent in the eastern Weddell Sea and the Ross and western Amundsen Seas (Figure 5a). Sea ice grew at a slightly above-average rate, totaling about 3.6 million square kilometers (1.39 million square miles) in April, whereas the 1981 to 2010 average ice growth is 3.20 million square kilometers (1.24 million square miles) (Figure 5b).
Above-average temperatures of 3 to 5 degrees Celsius (5 to 9 degrees Fahrenheit) continued to persist in western Dronning Maud Land but below-average temperatures of 4 to 7 degrees Celsius (7 to 13 degrees Fahrenheit) prevailed in the eastern Amundsen Sea and much of the Wilkes Land coast.
Conditions leading to Antarctica’s record low sea ice in 2023
In July 2023, mid-winter Southern Ocean sea ice fell more than 2.40 million square kilometers (927,000 square miles) below the long-term average, a huge shortfall that revised scientific perceptions of what was possible in the Antarctic climate system. A recent paper written by Monica Ionita from the Alfred Wegner Institute Helmholtz Center for Polar and Marine Research placed the cause of the extreme event with a persistent threefold pattern of alternating low and high air pressure centers surrounding the continent. This pattern, known as “zonal wave-3,” transports warmth and moist air toward the Antarctic coast, suppressing sea ice formation and leading to exceptional anomalies in air temperature and ocean temperature.
Ionita, M. 2024. Large-scale drivers of the exceptionally low winter Antarctic sea ice extent in 2023. Frontiers in Earth Science. doi: 10.3389/feart.2024.1333706.
Stroeve, J, et. al. 2024. Mapping potential timing of ice algal blooms from satellite. Geophysical Research Letters. doi: 10.1029/2023GL106486.
