Last September was one of the hottest on record, while the most contrasting weather patterns were seen in Europe, where record-breaking temperatures were recorded alongside colder than average temperatures in the east.
This year the month ranked in second place following last year's September, and it is followed by 2019 and 2016, according to reports from the Copernicus Climate Change Service.
"This ranking has limited significance, as the difference between the four warmest years in terms of global average temperature is less than 0.08ºC," the service said in its climate bulletin published on Thursday.
It was much warmer than the 1991-2020 average over most western regions in Europe, but it was cooler than the 1991-2020 average in the east. However, it was not cool enough to be record-breaking in a long-term context.
For example, Finland's capital Helsinki experienced a cooler September in 2021 than in any year from 1997 to 2020, but during 11 September months in the period from 1961 to 1996, colder temperatures were recorded.
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"European-average temperature anomalies are generally larger and more variable than global anomalies," the service stated in its climate bulletin, adding that the European average temperature for September 2021 was the coolest September since 2013, and 0.2°C below the 1991-2020 average.
So far, 2020 is still the warmest calendar year on record for Europe by a clear margin, with a temperature of 1.2°C above the 1991-2020 average, while 2016 was the warmest globally, with a temperature 0.44°C above the 1991-2020 average.
It was also drier-than-average in large parts of southern Europe during September this year, but in certain places, including the west of France and along the eastern coast of the Black Sea, it was wetter than usual.
Fluctuating sea ice levels
When it comes to sea ice levels, September saw an 8% decrease in Arctic sea ice, which reached its annual minimum at 5.6 million square kilometres, 0.5 million square kilometres below the 1991-2020 average for September, making it the 12th lowest value recorded in 43 years.
The Antarctic sea ice extent reached 18.9 million km2 on average, only 0.3 million km2 above the August average value, however, this small month-on-month growth can be explained by the fact that the extent reached its annual maximum in late August/early September and has been slowly decreasing since then.
On a regional level, the sea ice extent reached a record minimum in the Greenland Sea, while a 15-year high was recorded in the Beaufort-Chukchi Sea, shared by Canada, Russia and the United States.