September 2023 smashed the record for the most extreme month for heat in Earth’s history, recording the highest departure from average of any month in analyses dating back to 1850, said NOAA’s National Centers for Environmental Information on October 13. NOAA, NASABerkeley Earth, and the European Copernicus Climate Change Service all rated September 2023 as the warmest September on record, crushing the previous September record by a huge margin. And famed climate scientist James Hansen warned today that the world is on the verge of exceeding the 1.5 degree Celsius warming threshold seen as key to protecting the world’s people and ecosystems — a claim still hotly contested within climate science.

According to NOAA, September global temperatures spiked to a remarkable 1.44 degrees Celsius (2.59°F) above the 20th-century average. The September 2023 global temperature anomaly of 0.46°C (0.83°F) surpassed the previous record-high monthly anomaly from March 2016 by 0.09°C (0.16°F).

Using NASA data, September 2023 was 1.7 degrees Celsius above the temperature of the 1880-1899 period, which is commonly called “preindustrial” (the difference between the 1951-1980 baseline reported on the NASA website and the 1880-1899 period is 0.226°C). This is the first time that a monthly temperature has exceeded 1.5 degrees Celsius above preindustrial temperature threshold in the NASA database.

2023 virtually certain to be Earth’s warmest year on record

The year-to-date period of January-September is the warmest on record globally. According to NOAA’s latest Global Annual Temperature Rankings Outlook and the statistical model it uses, there’s a greater than 99.5% chance of 2023 being the warmest year on record. At the start of this year, few experts foresaw 2023 as being a contender for Earth’s warmest year, as the bulk of El Niño’s warming comes during the second year of each El Niño rather than the first — so it’s possible that 2024 will be even warmer than this year.

Read: What is El Niño?

Berkeley Earth agrees that 2023 is virtually certain to be the warmest year on record. In their database, 2023 has a 90% chance of being over 1.5 degrees Celsius warmer than pre-industrial temperatures (1850-1900). This is the target threshold for the stabilization of the long-term climate over a 20-year-plus period agreed upon in international climate negotiations. This year’s exceedance would likely be just a temporary breach of the 1.5°C threshold, but the planet may start regularly exceeding this mark by the 2030s, especially if major emission cuts are not implemented by then. In databases maintained by NOAA and NASA, 2023 temperatures are likely to remain less than 1.5 degrees Celsius above pre-industrial temperatures.

Figure 2. Global temperatures relative to 1880-1920 in the NASA database. (Image credit: James Hansen)

However, climate scientist James Hansen argued today that 2024 might well see temperatures in the NASA database exceeding the 1.5 degree Celsius threshold, and that global temperatures might never dip back much below that mark.

He and co-authors wrote: “Public discussion has focused on the remarkable magnitude of this monthly anomaly, which exceeds the prior warmest September in the period of instrumental data by about +0.5°C.”

Using the scientific term “anomaly” to refer to the unusual warming, they wrote that the higher-than-average temperatures of the past few months “is probably more important.”

“If this relative anomaly is maintained through this El Nino (through Northern Hemisphere 2024 spring) the peak 12-month mean global warming will reach +1.6-1.7°C relative to 1880-1920.”

Once El Nino peaks, we can expect the average global temperature to fall by only 0.2-0.3°C, the scientists wrote. “The 1.5°C global warming level will have been reached, for all practical purposes. There will be no need to ruminate for 20 years about whether the 1.5°C level has been reached, as [the Intergovernmental Panel on Climate Change proposes. On the contrary, Earth’s enormous energy imbalance assures that global temperature will be rising still higher for the foreseeable future.”

Shockingly warm temperatures virtually everywhere in September

Land and ocean areas each had their warmest September on record in 2023, and September was the sixth consecutive month with record-high global ocean temperatures. North America, Africa, Europe, and South America each had their warmest September on record; Asia had its second-warmest September; Oceania had its third-warmest September. September ranked as the seventh-warmest September on record in the United States.

According to Berkeley Earth, most of the record-high departure from average in global temperatures in September as compared to August (which was itself a record-warm August) was because of record-warm temperatures in polar regions, particularly the Antarctic. In their database, September broke the monthly global temperature record by 0.50 degrees Celsius (0.9°F), and 77 countries had their hottest September on record. Berkeley Earth researchers commented that this “is unprecedented and highly unlikely. An examination of CMIP6 climate models used to simulate global warming would lead us to estimate the chance of this occurring at ~1 in 10,000,” and that “it suggests that models may not be fully representing recent climate changes.”

The record-smashing heat was not limited to Earth’s surface. In the upper atmosphere, September had the largest departure from average since satellite measurements began (see Tweet below).

map of the world showing warmer than average temperatures across virtually all of the world
Figure 1. Departure of temperature from average for September 2023, the hottest September for the globe since recordkeeping began in 1850. Record-warm temperatures covered 20% of the world’s surface, which is the highest percentage of any month on record. (Image credit: NOAA/NCEI)

Australia had its driest September on record, which led to severe wildfires. Total rainfall was 70.8% below the 1961-1990 average for September, influenced by a positive Indian Ocean dipole, El Niño, and the long-term human-caused global warming. However, on a global basis, the month’s record-warm temperatures fostered extra evaporation from oceans, and that led to atmospheric moisture setting a new September record in 2023, both in terms of dewpoint temperature and precipitable water (see Tweets below).

Climate change’s share of the tab for extreme weather events: $143 billion per year

Climate change is responsible for $143 billion of the cost of extreme events every year, according to a study released in September, The global costs of extreme weather that are attributable to climate change. The authors combined data from extreme event attribution studies with “data on the socio-economic costs of these events” to estimate the global cost of extreme weather attributable to climate change in the last twenty years. They found that 63% of the climate change cost is from loss of human life. “Our results suggest that the frequently cited estimates of the economic costs of climate change arrived at by using integrated assessment models may be substantially underestimated,” the authors said.

El Niño at the threshold of the “strong” category

El Niño conditions continued over the past month in the eastern tropical Pacific and were at the “strong” threshold, according to NOAA’s October 12 discussion. Sea surface temperatures in the Niño-3.4 region in the week ending October 12 were 1.5 degrees Celsius above average; a “strong” El Niño event is defined when these sea surface temperatures are in excess of 1.5 degrees Celsius above average. NOAA gave a 75-85% chance of the current event being defined as a strong El Niño for the November-January period, and a 30% chance of a “historically strong” event rivaling 2015-16 and 1997-98, with sea surface temperatures in the Niño-3.4 region at least 2 degrees Celsius above average during November-January. The forecasters gave an 80% chance that El Niño conditions would continue into March-May 2024.

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Arctic sea ice: eighth-lowest September extent on record

Arctic sea ice extent during September 2023 was the eighth-lowest in the 45-year satellite record, according to the National Snow and Ice Data Center. The annual minimum extent occurred on September 19 and was the sixth-lowest minimum extent in the 45-year satellite record.

The shipping channels through the southern route of the Northwest Passage through Canadian waters opened to ice-free navigation during September. The Northeast Passage (or Northern Sea Route) along the northern coast of Russia was also open to ice-free navigation. The northern route of the Northwest Passage had the lowest ice coverage of any year except for the record-low year of 2011 but remained closed to ice-free navigation during September. The Arctic had its warmest September on record, according to NOAA.

Antarctic sea ice: lowest on record

Antarctic sea ice extent in September was by far the lowest on record, the fifth consecutive month with a record low. The annual maximum extent occurred on September 10, setting a new record for lowest maximum extent by a wide margin. Air temperatures over Antarctica were an astounding 5 degrees Celsius (9°F) or more above average over large portions of the continent in September, contributing to much of the record global warmth for the month (see Tweets below).

Notable global heat and cold marks for September 2023

The information below is courtesy of Maximiliano Herrera. Follow him on Twitter: @extremetemps

  • Hottest temperature in the Northern Hemisphere: 50.1°C (122.8°F) at Omidieh, Iraq, September 11;
  • Coldest temperature in the Northern Hemisphere: -40°C (-40°F) at Summit, Greenland, September 21;
  • Hottest temperature in the Southern Hemisphere: 44.5°C (112.1°F) at Villamontes, Bolivia, September 17; and
  • Coldest temperature in the Southern Hemisphere: -80.6°C (-113.1°F) at Vostok, Antarctica, September 8.

Major weather stations in September: 20 all-time heat records, no all-time cold records

Among global stations with a record of at least 40 years, 20 set, not just tied, an all-time heat record in September, and no stations set an all-time cold record:

St. Croix (U.S. Virgin Islands) max. 36.1°C, September 9: New territorial record high for the U.S. Virgin Islands (reliably recorded);
Ducos (Martinique) max. 36.6°C, September 15: New territorial record high for Martinique;
Nagahama (Japan) max. 36.0°C, September 27;
Parnaiba (Brazil) max. 39.4°C, September 20;
Za Doca (Brazil) max. 38.4°C, September 20;
Cobija (Bolivia) max. 39.8°C, September 24;
Magdalena (Bolivia) max. 40.3°C, September 25;
Belo Horizonte Airport (Brazil) max. 38.6°C, September 25;
Sete Lagoas (Brazil) max. 38.8°C, September 25;
Balsas (Brazil) max. 41.9°C, September 25;
St. Laurent do Moroni (French Guiana, France) max. 38.8°C, September 25: New territorial record high for French Guiana;
Januaria (Brazil) max. 41.8°C, September 26;
Manaus (Brazil) max. 39.3°C, September 26;
Grantley Adams Airport (Barbados) max. 34.0°C, September 26;
Ebini (Guyana) max. 40.1°C September 26: New national record high for Guyana;
Tingo de Ponaza (Peru) max. 41.4°C, September 27: New national record high for Peru;
New Amsterdam (Guyana) max. 37.0°C, September 29;
Mabaruma (Guyana) max. 36.0°C, September 30;
Timehri Airport (Guyana) max. 38.0°C, September 30; and
Kourou (French Guiana) max. 35.3°C, September 30.

Twenty (plus six) all-time national/territorial heat records set or tied in 2023

As of the end of September, 20 nations or territories had set or tied an all-time national heat record in 2023; six of these records were set in September. Five nations or territories — Laos, the U.S. Virgin Islands, Chad, Saba, and French Guiana — beat or tied their old all-time heat record twice in 2023; Laos beat their previous all-time heat record an astounding four times in 2023. According to Herrera, the record for most national/territorial all-time heat records is 24, set in 2019. Here are the ones set so far in 2023:

Thailand: 45.4°C (113.7°F) at Tak Agromet, April 15;
Laos: 42.7°C (108.9°F) at Luang Prabang, April 18; beaten one day later with 42.9°C (109.2°F) at Sayaburi, April 19; beaten again on May 6 and May 7 with 43.5°C (110.3°F) at Luang Prabang;
Vietnam: 44.1°C (111.4°F) at Hoi Xuan, May 6; beaten again with 44.1°C (111.4°F) at Tuong Duong, May 7;
Singapore: 37.0°C (98.6°F) at Ang Mo Kio, May 13 (tie);
Chad: 48.0°C (118.4°F) at Faya, May 25; tied again on June 16;
China: 52.2°C (126°F) at Sabao, July 16;
Vatican City: 42.9°C (109.2°F) at Roma Macao, July 18;
Cayman Islands: 35.3°C (95.5°F) at Owen Roberts Airport, July 22;
Albania: 44.0°C (111.2°F) at Kucova, July 25;
Morocco: 50.4C (122.7°F) at Agadir, August 11;
U.S. Virgin Islands (USA): 35.6°C (96.1°F) at St. Croix, August 14 (tie); beaten on September 9 with 36.1°C (97°F) at St. Croix;
Dominica: 36.6°C (97.9°F) at Canefield Airport, August 27;
Aruba: 36.5°C (97.7°F) at Queen Beatrix Airport, August 28 (tie);
Saba: 34.4°C (93.9°F) at Juancho Yrausquin Airport, August 29; tied again on September 8;
Martinique (France): 36.6°C (97.9°F) at Ducos, September 15;
St. Barthelemy (France): 35.5°C (95.9°F) at Gustavia, September 15 (tie);
French Guiana (France): 38.1°C (100.6°F) at Grand Santi. September 15; beaten on September 25 with 38.8°C (101.8°F) at St. Laurent do Moroni;
Guyana: 40.1°C (104.2°F) at Ebini, September 26;
Peru: 41.4°C (106.5°F) at Tingo de Ponaza, September 27; and
Suriname: 38°C (100.4°F) at Zanderj Airport, September 30 (tie).

Three all-time national/territorial cold records set or tied in 2023

As of the end of September 2023, three nations or territories had set or tied an all-time national cold record:

Myanmar: -6.0°C (21.2°F) at Hakha, Jan. 17 (tied);
China: -53.0°C (-63.4°F) at Jintao, Jan. 22; and
Cyprus: -12.8°C (8°F) at Trodos Mt. Station, Feb. 8 (tied).

Eighty-four additional monthly national/territorial heat records and four additional monthly cold records beaten or tied

In addition to the 20 all-time heat records listed above (plus four, for the records set in two different months in Laos, Chad, Saba, and the U.S. Virgin Islands), 84 additional monthly all-time heat records have been set in 2023, for a total of 108 all-time monthly heat records:

  • Jan. (13): Czech Republic, Liechtenstein, Netherlands, Denmark, Poland, Belarus, Lithuania, Latvia, Romania, Moldova, Ukraine, Cyprus, Nigeria
  • Feb. (4): Chile, Taiwan, Pakistan, Cyprus
  • March (3): Botswana, Vietnam, Taiwan
  • April (11): Cabo Verde, Botswana, Turkmenistan, Mauritius, Antigua and Barbuda, Spain, Morocco, Portugal, Andorra, Saba, St. Barthelemy
  • May (9): Mauritius, Solomon Islands, Botswana, Cambodia, Cocos Islands, Panama, Saba, Maldives, French Guiana
  • June (13): Botswana, Vietnam, Tuvalu, Hong Kong, Mauritius, Aruba, Saba, Senegal, Costa Rica, China, Solomon Islands, Morocco, French Guiana
  • July (10): Mauritius, Liechtenstein, US Virgin Islands, Dominica, Italy, Malta, El Salvador, Tanzania, St. Barthelemy, Martinique
  • August (16): Qatar, Niger, Mauritius, Chile, St. Barthelemy, Turkey, Thailand, Botswana, France, Bolivia, Paraguay, Martinique, Chad, Suriname, French Guiana, Kenya
  • September (5): Mauritius, Chad, Norway, St. Barthelemy, Djibouti

In addition to the three all-time cold records listed above, six nations or territories have set a monthly all-time cold record in 2023, for a total of nine monthly cold records:

  • Feb. (1): Montenegro
  • March (2): St. Eustatius, Martinique
  • June (1): Finland
  • August (2): French Polynesia, Montenegro

Hemispherical and continental temperature records through September 2023

Lowest temperature reliably recorded in January in the Southern Hemisphere: -51.2°C (-60.2°F) at Concordia, Antarctica, Jan. 31;
Highest temperature ever recorded in April in Europe: 38.8°C (101.8°F) at Cordoba, Spain, April 27;
Highest minimum temperature ever recorded in Africa for any month: 39.6°C (103.3°F) at Adrar, Algeria, July 6;
Highest temperature ever recorded in July in Europe: 48.2°C (118.8°F) at Jerzu and Lotzorai, Italy, July 24;
Highest minimum temperature ever recorded in July in Europe: 36.2°C (97.2°F) at Palermo, Italy, July 24;
Highest temperature ever recorded in Africa in August (tie): 50.4°C (122.7°F) at Agadir, Morocco, August 11;
Highest temperature ever recorded in the Southern Hemisphere in August (tie): 45.0°C (113°F) at Villamontes, Bolivia, August 23;
Highest minimum temperature ever recorded in Oceania and in the whole Southern Hemisphere in August: 28.8°C (83.8°F) at Funafuti, Tuvalu, August 31, beating the record of 28.7°C set at August Nui, Tuvalu, on August 14; and
Highest minimum temperature ever recorded in South America and the Southern Hemisphere in September: 30.6°C (87.1°F) at Base Aerea Jara, Paraguay, September 3.

Bob Henson contributed to this post.

Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...