Arctic sea ice naturally grows and shrinks in an annual cycle given seasonal differences in average temperature. It reaches its annual maximum in March and minimum in September.
Arctic sea ice area and volume have trended downward over the past 40 years. NASA found that September ice extent between 1979 and 2020 has declined by an average of 13.1% per decade.
This decline is driven by increasing global temperatures from human-generated greenhouse gas emissions. While a 2017 study suggested that between 30% and 50% of the observed melting is due to natural causes, one of its authors stated that “anthropogenic forcing is still dominant,” whereas “natural variability...helped accelerate this melting.”
The decline is turning reflective sea ice into absorptive ocean water, causing the Arctic to warm about three times faster than the global average.