Surface air temperature for September 2021

September 2021 was a month with contrasting temperature anomalies over western and eastern Europe.  It was much warmer than the 1991-2020 average over most western regions. The United Kingdom reported its second warmest September on record, based on the countrywide average temperature for the month, while France reported its highest ever daily maximum September temperature for the country as a whole. It was correspondingly cooler than the 1991-2020 average in the east, though not cool enough to be record-breaking in a long-term context. Helsinki, for example, had a cooler September in 2021 than in any year from 1997 to 2020, but eleven earlier Septembers in the period from 1961 to 1996 were cooler still.

Elsewhere, below average temperatures occurred in a region extending eastward from eastern Europe to Lake Baikal. High August temperatures over Greenland were followed in September by much-below average temperatures over all but the far north-east of the country. It was also notably cooler than average over easternmost Russia.

September was warmer than average over most other land areas. This was particularly so over central regions of the USA and Canada, northwestern Africa, an arc from Iran through central Asian countries to southern China, parts of Siberia, drought-hit central South America and much of the Antarctic.

Air temperatures continued to be below average over much of the tropical and sub-tropical eastern Pacific Ocean. They were predominantly above average over central and eastern parts of the mid-latitude North Pacific, but below average east of Japan. Temperatures were also well above average over a region of the extratropical South Pacific east of the dateline. It was warmer than average over almost all of the Atlantic and Indian Oceans. Conditions were mixed elsewhere over the oceans of the southern hemisphere.

Globally, the latest three Septembers are the warmest three in the data record. September 2021 was:

• 0.40°C warmer than the 1991-2020 average for September.
• estimated to be the second warmest September, after September 2020, but only limited significance can be attached to the rankings of the four warmest Septembers (which include 2016 and 2019) as the four differ by less than 0.08°C in global average temperature.

European-average temperature anomalies are generally larger and more variable than global anomalies. The European-average temperature for September 2021 was the coolest September since 2013, being 0.2°C below the 1991-2020 average. It followed a relatively cool August.

Temperatures averaged over the twelve months to September 2021 were:

• above average over most land areas and the majority of the ocean surface
• most above the 1991-2020 average over the far north of Siberia and the adjacent Arctic seas,  northeastern Canada and Greenland, northwestern Africa, the Middle East and the Tibetan Plateau, and over the far south of South America and the Antarctic Peninsula
• above average over much of Europe, more so in the east, but slightly below average over a few parts of the continent
• below average over several other land areas, over West Antarctica in particular
• below average over the eastern equatorial Pacific, where the La Niña event that peaked in late 2020 dominates the 12-month average
• also below average over the North Atlantic west of Ireland and several oceanic areas in the southern hemisphere.

Averaging over twelve-month periods smooths out shorter-term variations in regional- and global-average temperatures. Globally, the twelve-month period to September 2021 was 0.28°C warmer than the 1991-2020 average. This twelve-month average anomaly is well below the 0.46°C average for the twelve-month periods ending in September 2016 and May and June 2020, the three warmest periods in this data record. The warmest calendar year is 2016, with a temperature 0.44°C above the 1991-2020 average. 2020 was on a par with 2016, being cooler by less than 0.01°C, well below the spread among multiple datasets. The third warmest calendar year is 2019; its temperature was 0.40°C above average.

0.82°C should be added to the value relative to the 1991-2020 reference period to relate recent global temperatures to the pre-industrial level defined in the IPCC Special Report on “Global Warming of 1.5°C”. The average global temperature for the twelve months to September 2021 is 1.1°C above that level.

There is more variability in average European temperatures, but values are more certain due to the relatively dense observational coverage of the continent. The average for the twelve-month period to September 2021 is 0.5°C above the 1991-2020 average. 2020 is the warmest calendar year on record for Europe by a clear margin, with a temperature 1.2°C above the 1991-2020 average. Surface air temperature for September 2021 | Copernicus