How can they say that? Discussing the claim of hottest July on record

The devastating headlines have been all over the place this summer. Heat records on land and in the ocean are being set in multiple locations around the globe. Unusually large, long-lasting, and deadly wildfires in Canada and Hawaii, along with mass coral bleaching events off the coast of Florida, add tangible consequences to the otherwise abstract concept of record-breaking heat. 

Bleached and diseased coral in the Cheeca Rocks Marine Sanctuary, off the coast of the Florida Keys.
Photo from NOAA AOML.

A report released by NASA on August 14th states that last month was the hottest July in recorded history, and that it will likely go down as the hottest month ever measured, with records dating back to 1880. The map below shows temperature departures with a 30-year average used as a baseline. The corresponding graph shows that same anomaly, with the same baseline, plotted through all the years in NASA's period of record. 

The return of El Niño last Spring (which I discussed in this blog post) may have exacerbated the temperature spike this summer. But it doesn't explain all the extra heat. After all, there have been many El Niño events before this one, and the ENSO cycle creates temperature swings of less than 5°F, even in the most extreme cases. NASA GISS's analysis shows that July 2023 was .43°F hotter than any other July on record, including all the previous El Niño episodes since 1880. But still, how can scientists be sure that these temperatures are unprecedented? It's a question that climatologists take very seriously, and they use rigorous processes to ensure their analyses are trustworthy and reliable. 

A Cotton Region Shelter at an official National Weather Service observation site

In the 19th Century, before remote sensing existed, all our weather observations were in situ, or taken on site. These observations primarily came from thermometers near the ground (the proper way to measure temperature at ground level is through a Cotton Region Shelter, often referred to as a Stevenson Screen), and in buoys and ships on the water. To ensure that modern temperature records are consistent with previous methods of measuring temperature, NASA's temperature analysis includes a quality control process to account for the Urban Heat Island effect and other possible impacts, like inconsistent spacing of observation sites; for example, there are fewer weather sensors in Siberia than in the Tri-State area, and NASA GISS's analysis corrects for these differences. The areas that have experienced the most warming, as it turns out, are not in major metropolitan areas like NYC. Instead, NASA's report shows that nearly uninhabited reaches of the Arctic and Antarctic are among the areas that have warmed the most. 

Other reports earlier this summer indicate that we may be experiencing the hottest summer in human history. This claim is understandably tough to comprehend. How can scientists know the temperature records of the past if no one had a thermometer? As it turns out, some plants and animals can only survive in very narrow temperature ranges, and they have left clues behind in layers of rock and sediment. Climatologists utilize these ancient climate records to calculate climates of the past. They also utilize resources like ice cores, tree rings, and even fossilized pollen to create a full picture of past environmental conditions. You can learn more about the fascinating science of paleoclimatology here.

Scientists at Woods Hole Oceanographic Institution examine an ice core containing paleo climate artifacts 

such as ancient air samples, pollen grains, and ash particles.
Photo from WHOI

In this blog post from June, I talked about the difference between daily heat records and those records that have a longer time span. A more consistent, steady heating process will produce warmer years, not just warmer days. After all, the term "average" means that any extremes within the dataset are being evened out. And the same is true on a spatial scale, not just a time scale. It's easier for a heat record to be broken in one individual city than it is for the entire Earth to reach a new temperature extreme. And yet, that's exactly what we saw in July, and it's an ongoing trend in the 21st Century.