'''DEFINITION'''

Sea Ice Extent (SIE) is the area covered by sufficient sea ice, that is the area of ocean having more than 15% Sea Ice Concentration (SIC). SIC is the fractional area of ocean surface that is covered with sea ice. SIC is computed from Passive Microwave satellite observations since the 1970s. SIE is often reported with units of millions square kilometers. SIE includes all sea ice, and does not include lake and river ice. The change in sea ice extent (trend) is expressed in millions of km squared per decade. In addition, trends are expressed relative to the 1979-2024 period in % per decade. These trends are calculated (i) for the annual mean values; (ii) for the winter maximum cover (March in the Northern Hemisphere); (iii) for the summer minimum extent (September in the Northern Hemisphere). The annual mean trend is reported on the key figure. The March and September trends are reported in the text below. See also section 1.7 in Samuelsen et al. (2016) for an introduction to this Ocean Monitoring Indicator (OMI).

'''CONTEXT'''

Sea ice is frozen seawater that floats at the ocean surface. This large blanket of millions of square kilometers insulates the relatively warm ocean waters from the cold polar atmosphere. The seasonal cycle of sea ice, forming and melting with the polar seasons, impacts both human activities and biological habitat. Knowing how and by how much the sea-ice cover is changing is essential for monitoring the health of the Earth (Meredith et al. 2019; Fox-Kemper et al. 2021).

'''CMEMS KEY FINDINGS'''

Since 1979, the Northern Hemisphere sea ice extent has decreased in all months. Loss of sea ice extent during summer exceeds the loss observed during winter periods. Over the period 1979-2024, the annual rate amounts to -0.49 +/- +0.02 millions km squared by decade (-4.27% per decade). Winter (March) sea ice extent trend amounts to -0.37 +/- +0.03 millions km squared by decade (-2.45% per decade). Summer (September) sea ice extent trend amounts to -0.79 +/- +0.06 millions km squared by decade (-12.57% per decade). These values are in agreement with those assessed in the IPCC SROCC (Meredith et al. 2019, with estimates up until year 2018). While 2024 started with the 20th lowest mean January sea ice extent, it ended with the lowest mean December sea ice extent, since 1979. September 2024 was the 5th lowest mean September sea ice extent since 1979. The record low mean September was in 2020.

'''DOI (product):'''

https://doi.org/10.48670/moi-00191