The world recorded its hottest day on Sunday, July 21. Just 24 hours later, the record was broken again, and last Monday is likely to be the hottest day in thousands of years.
It might seem impossible for scientists to predict the world’s hottest day, since they don’t have temperature monitors in every corner of the globe and less than a century of relatively comprehensive observations. But they have developed a technique that’s becoming more useful as the planet warms.
This month’s surprising heat finding, announced by the European Union’s Copernicus Climate Change Service, is based on “reanalysis,” a technique that combines temperature data and models to provide a global view of the climate. The center creates a near-real-time picture of Earth’s climate, including temperature, wind and precipitation, for (roughly) every 30 square kilometers of the Earth’s surface.
This reanalysis goes back to 1940, and it allows researchers to state with confidence when a record is broken, whether it’s for a day, month or year. Beyond the new daily heat records, the data also show that 2023 was the hottest year ever recorded and that every calendar month of the past 13 months has been the hottest on record.
Although there aren’t thermometers in every corner of the world, Copernicus receives a huge amount of meteorological data that it uses for its reanalyses. “We have information coming to the center constantly,” says Carlo Buontempo, director of the Climate Change Service, which is part of the European Centre for Medium-term Weather Forecasting (ECMWF).
Scientists at the center receive 100 million readings a day about weather conditions from around the world. The observations come from airplanes, satellites, ships, radar and surface-level weather stations – all of which provide real-time information about temperature, wind, rain and snow, as well as other factors such as air pollution. This information is fed into a model called ERA5, which is already equipped with historical information about the global climate.
There are gaps in these observations because the data sources don’t cover every part of the world. Weather conditions such as cloudiness can also reduce the amount of data coming from sources such as satellites. To fill these gaps, scientists take predictions already made based on the long-term ERA5 model and test them against observations. This means that a forecast predicting a particular temperature at a particular location will be tested against all the data the researchers get about that location and the surrounding weather, as well as broader forces such as ocean currents and air circulation.
This is done repeatedly, while assessing how well the forecast matches the information actually recorded. The model also takes into account any errors in the recorded data, and relies on the laws of physics including weather patterns, currents and air flow that govern the way the global climate works.
In this way, it is possible to build up a complete picture that is as accurate as possible. This is what enables scientists to confidently declare records such as when the world experienced the hottest day in human history.
Globally, five weather services — the U.S. National Oceanic and Atmospheric Administration and NASA, ECMWF, the China Meteorological Administration and the Japan Meteorological Agency — continuously assess global temperatures using this technique. Although their models differ slightly, the five groups have reached similar conclusions about the record heat in recent months and years.
Historical data is difficult to obtain. The longest-running temperature series, Central England Temperatures in the UK, began in the 17th century. Data from before humans began systematically monitoring temperatures comes from sources such as gas bubbles trapped in glacial ice or tree rings. These sources are not as specific as thermometer readings, but Copernicus says it can be said with confidence that recent temperatures are the highest in about 100,000 years.
Meteorologists also have a pretty good idea of when a particularly significant day, such as the hottest day on record, is coming. That’s partly because global average temperatures typically peak between early July and early August. Last year’s hottest day — the hottest ever recorded — occurred in early July, amid a historic marine heat wave. An intense El Niño — a natural global climate phenomenon that typically means warmer temperatures globally — offered another sign that record heat was coming.
For a while, until July, it looked like the world wouldn’t set any new daily records, Buontempo says. “The global average temperature of the oceans started to rise again,” he says. “Some of the people who systematically monitor our forecasts started to ring the alarm bells.” By early last week, they were paying extra attention to the reanalysis and getting ready to make an announcement.
This technology isn’t just useful for declaring the “hottest day ever”: it’s being used to train AI forecasting models, particularly “ensemble” weather forecasts that represent many possible future scenarios. It’s also used by solar energy companies to help homeowners figure out how much energy their panels can produce, and by wind energy companies to plan where to place wind farms.
Copernicus is currently working on a new model, known as ERA6, which will be more accurate – dividing the globe into 14 kilometre squares – and incorporate a number of historical data sources, including early satellite readings from the 1970s.
For Buontempo, the recent extraordinary series of record-breaking months is more important than any single day, because it’s a better indicator of how quickly the world is warming. But pointing to a specific day makes climate change feel more immediate.
“I think we need to make it more concrete, more direct, more visible,” he says. “It’s important that people are informed.”
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
