Arctic: summer sea ice shrank by 35%, winter ice by just 7%
1980s versus 2010s. In September, at summer’s end, ice covered 7.2 million km² on average, later 4.7; in March, at winter’s end, 15.8 and 14.7 million km².
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The seasonal high and low did not shrink by the same proportion. The September decadal mean fell more than the March mean.
All four bars share one scale, million km². Each averages ten September or March values, one from each year of the specified decade.
- September: decadal means million km²
7.21 (1980–1989) → 4.65 (2010–2019)
- March: decadal means million km²
15.84 (1980–1989) → 14.66 (2010–2019)
Separately average ten March and ten September values in each decade. Decline = 100×(1 − 2010s mean / 1980s mean). This compares the specified decades, rather than estimating a linear trend.
Fetterer et al. (2025), Sea Ice Index v4, DOI 10.7265/a98x-0f50. Northern/Southern Hemisphere are not countries. Extent counts whole grid cells with ice concentration ≥15%; area weights by concentration. Monthly satellite estimates in million km², not volume or land glaciers. Recent points may be preliminary. The published annual mean is calendar-day weighted and retained only with all 12 months present. Hemispheres are not summed into a universal climate index.
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