"Gobsmackingly" warm September was Earth's hottest on record

The planet has shattered heat records in recent months, but even by the standards of a sizzling summer, September's temperature anomaly stands out.

The big picture: Following the hottest June through August on record, and the globe's hottest ever month in July, last month's preliminary data has astonished climate researchers who anticipated such extremes eventually.

• The figures show a temperature anomaly of about 0.9°C (1.6°F) above the 1991-2020 average. Converted to the preindustrial era, this amounts to a departure from average of 1.7°C (3.06°F), temporarily exceeding the Paris Agreement's temperature target of 1.5°C compared to preindustrial levels.
• The Paris Agreement, however, refers to long-term average temperatures over a few decades, not a single month or year.
• In data from the Japan Meteorological Agency, September 2023 beat the previous warmest September by 0.5°C (0.9°F). Typically, monthly records are beaten by fractions of a degree, with such narrow margins that different climate centers around the world can rank them differently.
• "We've never seen a record smashed by anything close to this margin," climate scientist Zeke Hausfather told Axios. "It's frankly a bit scary."

Between the lines: The data comes from computer modeling systems that use separate, well-tested methods (known as reanalysis) to estimate the planet's weather and climate conditions during the previous month.

• Data from three different reanalyses show that the planet spiked a far hotter fever in September, all but ensuring that 2023 will be the warmest year on the books.
• "This month was, in my professional opinion as a climate scientist – absolutely gobsmackingly bananas," Hausfather posted on X.com.
• September featured numerous extreme weather events, including heat waves in Europe, devastating and deadly flooding in Greece and Libya from an unusually powerful Mediterranean storm system, record heat in Japan and continued anomalously large wildfires in Canada.
• In the U.S., the month ended with a record-breaking deluge on New York City and surrounding areas, bringing parts of the region to a halt.

The intrigue: The extreme heat this summer and fall is the result of a combination of factors, starting with long-term, human-caused climate change. In addition, an increasingly intense El Niño event is underway in the tropical Pacific Ocean, which is drawing more heat out of the deep oceans.

• Yet global oceans are setting warming records for a variety of natural and human-caused reasons, sending global temperatures even higher.

• Scientists are also investigating what role additional water vapor in the stratosphere is playing, along with a decrease in certain types of shipping emissions as a result of new marine shipping pollution rules.
• The water vapor was vaulted into the stratosphere during 2022's explosive eruption of the Hunga Tonga-Hunga Ha'apai undersea volcano in the southern Pacific Ocean.
• Also on the list of suspects is the high speed with which the tropical Pacific shifted from a cooling La Niña to an El Niño, which jerked the climate into more rapid warming.

What they're saying: "While a fraction of a degree might not initially strike us as significant," Texas A&M climate professor Andrew Dessler told Axios via email, "Even small increments in global temperatures have far-reaching consequences."

• Dessler noted that during the last ice age, Earth was only 6°C (10°F) cooler than it is today.
• He also emphasized that climate change impacts are not linear, so "The harm caused by each additional tenth of a degree of warming is much larger than the harm caused by the previous tenth of a degree."

What's next: Additional monthly data, including some stemming from global surface observations, will come out from the Copernicus Climate Change Service, NASA and NOAA during the next two weeks; however, the September temperature rankings are not expected to be different.