The Hottest July on Earth as Analyzed by MERRA-2
In July 2024, NASA announced that a new record had been set for the hottest day on Earth according to NASA data, including analyses from GEOS products (https://www.nasa.gov/earth/nasa-data-shows-july-22-was-earths-hottest-day-on-record/). The analysis used nearly 45 years of daily, global mean near surface (2 m) temperature data from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), produced by the GMAO. Figure 1 shows a similar analysis with MERRA-2, displaying how global-mean temperatures during 2024 compared to the mean, maximum, minimum, 15th and 85th percentile on each day. Not only was the highest daily, global temperature reached on July 23, 2024, but there were numerous days during the first half of the year that exceeded the prior daily maximum temperature. Here, we take this analysis a step further to identify regions that were large contributors to the record-breaking heat in July as well as the connection to heatwaves. This is of particular importance due to differing impacts of heat to ocean ecosystems and human populated land surfaces.
Figure 2 explores which year over the MERRA-2 record had the hottest July. As seen in the timeseries in Figure 2, more record temperatures have been reached during the month of July in recent years, with the years of 2023 and 2024 standing out as outliers from the general trend seen in the rest of the time series. The inset map in Figure 2 shows that tropical ocean regions were large contributors to the warmest July on record in 2024. Over land, regions that experienced record-breaking high temperatures this July were Antarctica, portions of the Amazon, the desert southwest of the United States, the Sahara Desert, and southern Africa. Despite multiple days exceeding 100F in the northeast United States in 2024, the summers of 1998 and 2020 were on average warmer in the region.
With warmer temperatures, it can be expected there could be more periods of prolonged heat. This is quantified using the Heatwave Frequency defined as the number of days within a month that satisfy heatwave criteria of exceeding the 90th percentile for daily mean temperature at least three days in a row. The global mean Heatwave Frequency was the highest in 2024 out of the 45-year record of MERRA-2. Figure 3 shows a spatial map of the trend in Heatwave Frequency while maps for Heatwave Frequency for individual months and seasons can be found on FLUID at https://fluid.nccs.nasa.gov/reanalysis/extreme_merra2/. Like with the temperature records for 2024, the most pronounced increase in the frequency of heatwaves is over water, particularly the Gulf Mexico and the Caribbean and Mediterranean Seas. There are however statistically significant increases in the frequency of heatwaves over the Amazon, northern Africa, Saudi Arabia, and the western United States.
