Was July 2023 or July 2024 the hottest month on record? Yes.

July saw record heat around the world, including the hottest day on record and several national heat records. But whether it was the hottest month on record depends on who you ask.

Last week, Copernicus – the EU’s climate science service – said July was the second hottest month on record. But this week, NASA and the US National Oceanic and Atmospheric Administration (NOAA) declared it the world’s loudest. The difference masks a remarkable level of agreement on these points – the difference between these standard data sets is only a few hundredths of a degree. But it also sheds light on the difficulty of creating a complete record of global climate and on things we still don’t know about exactly how the planet is warming.

One thing is for sure: last July was hot. Since July is usually the hottest month of the year, that at least means the world just experienced one of the hottest months on record. And it’s the latest in a long line of record months.

Hottest day as fires threaten Mediterranean

The challenge for global temperature analyzers, says Kate Marvel, a research physicist at NASA’s Goddard Institute for Space Studies (GISS), is that there is no way to take a single temperature reading for the entire planet. “We don’t have a single global thermometer that we can use to take the temperature of the entire globe. Instead, we have a bunch of different instruments that we can use to measure different parts of the planet and different aspects of the climate system.”

NASA and NOAA use a slightly different approach than Copernicus. The EU’s flagship climate program relies on a method called reanalysis, based on its ERA5 climate model. It uses that model to create a forecast, which is then reconciled with data collected from surface-level weather stations, satellites and aircraft. This process means Copernicus can produce an almost real-time picture of Earth’s climate, including temperature, wind and precipitation for every 30 square kilometer patch of the planet’s surface.

At NOAA, scientists use observations from land and sea weather stations to calculate the average surface temperature. The researchers perform a statistical analysis to fill in the data gaps that takes into account historical data and nearby observations.

After all those millions of data points were ingested and analyzed, the difference came from the thinnest of margins: NOAA found that July was 0.05°F (0.03°C) warmer than July last year , NASA analysis shows it was 0.02°C warmer, while Copernicus says July was only 0.04°C cooler than July 2023.

Each of these monthly analyzes is only accurate to about 0.08 C, and the rankings can turn up a difference of up to 0.02 C, says Gavin Schmidt, director of NASA GISS. The average difference between Copernicus’ analysis and NASA’s over the past few years is about 0.06 C, he says.

One possible reason for the discrepancy, says Peter Jacobs, a climatologist at NASA’s Goddard Space Flight Center, is that the reanalysis data take the air temperature over the ocean, while the NOAA and NASA observational data take water temperature readings. Water heats and cools more slowly than air, potentially creating a lag.

Another possible reason is what both analyzes do with areas where they don’t have good data, such as the Arctic. Copernicus’ reanalysis can use satellite readings, which are based on infrared and microwave data, rather than just temperature readings, while NASA and NOAA rely on a statistical analysis of likely temperatures to fill in the gaps .

Geographically, however, there is significant agreement between the approaches, notes Berkeley Earth researcher Zeke Hausfather. For example, the dataset shows that the tropical Pacific Ocean is much cooler in July 2024 than in 2023 as El Nino fades. All global temperature data sets are also unequivocal that the planet is warming rapidly.

The real concern isn’t the difference of a few hundredths of a degree between the data sets, but rather the fact that temperatures have stayed so warm, says Hausfather: “The fact that we’re matching them so far this year is a little concerning.” Last year’s record heat streak “set the bar so high. It’s crazy that we’re even tying that.”

Photo: Firefighters and volunteers work to extinguish a fire in Krieza, on the island of Evia, Greece, Tuesday, July 30, 2024. Photo credit: Nick Paleologos/Bloomberg

Copyright 2024 Bloomberg.

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