Satellite image of the Paris 2024 Olympic Games Marathon course taken on June 7, 2024 by the Operational Land Imager-2 on Landsat 9.

Over the last century, Paris has become significantly warmer, which poses a real challenge for the marathon runners at the 2024 Olympic Games.

The course features difficult terrain and historic landmarks, while athletes also face the effects of urban heat, exacerbated by limited green space and dense development. Innovative satellite imagery has improved monitoring of these conditions, providing important data to mitigate heat-related risks for competitors.

Climate change and Olympic challenges

Since Paris hosted the Olympic Games a century ago, temperatures in the city have risen by 3.1 degrees Celsius (5.5 degrees FahrenheitHot days with maximum temperatures above 30 °C (86 °F) are almost three times more common today than in 1924.

That kind of climate change could pose a problem for 2024 marathon competitors, who already face unusually hilly terrain that some commentators have called one of the most “grueling” and “brutal” Olympic courses ever. On August 10 and 11, runners had to tackle two long, grueling hills while completing a 42-kilometer course rich in historical significance and views of iconic architecture.

Extreme heat can pose a particular risk to endurance athletes competing in outdoor events. At the 2020 Summer Olympics in Tokyo, a British runner collapsed during the 10,000-meter race in sweltering heat, as did a Japanese athlete during the 50-kilometer walk. Fortunately for marathon runners in Paris, the heatwave that caused problems during the first week of competition has mostly subsided. Recent weather forecasts predict temperatures will remain below 24 °C (75 °F) for both the men’s and women’s marathons.

Land surface temperatures of the Paris 2024 Olympic Games marathon course, acquired on August 6, 2024, by the Visible Infrared Imaging Radiometer Suite on the NOAA-NASA Suomi NPP satellite.

Advanced monitoring of marathon routes

The map above provides a basic perspective on how the thermal environment changes along the Marathon route on a typical summer day. It shows the land surface temperatures (LSTs) measured by the VIIRS (Visible Infrared Imaging Radiometer Suite) on the NOAA–NASA Suomi NPP satellite at 13:40 local time on August 6, 2024, a clear day when air temperatures were 28 °C (82 °F). Scientists at NASA’s Jet Propulsion Laboratory “sharpened” the thermal data using machine learning methods and surface reflectance data from the European Space AgencySentinel-2 satellite. The technology improved the resolution of the data from 375 meters per pixel to about 20 meters per pixel.

Warmer land surface temperatures are shown in red; cooler areas appear blue. Note that parks and other green infrastructure – such as the tree-lined avenues leading from the Arc de Triomphe – are cooler than buildings, roads and parking lots. Urban spaces like these are typically built from materials that have high thermal capacity and absorb and re-emit the sun’s heat more than living landscapes like trees, grass and gardens.

This effect, known as an urban heat island, can be particularly intense in Paris. This is partly because the city has fewer trees than many other European cities and has many zinc roofs, which are particularly vulnerable to heating.

Note that the map shows land surface temperatures, not air temperatures. Heat islands are visible in both land surface temperatures and air temperature data, although differences tend to be more pronounced in land surface temperatures. When the image was taken on August 6, land surface temperatures varied from highs of 43 °C (110 °F) near the start line to lows of 25 °C (77 °F) in dense forest areas.

Urban heat and its effects on athletic performance

The other image (top of the page), taken by the OLI-2 (Operational Land Imager-2) on the Landsat 9 satellite, shows the limited tree cover in central Paris. The gray-looking zinc roofs can reach temperatures of up to 90 °C (194 °F) on hot summer days.

An analysis of satellite data by the European Environment Agency found that green infrastructure – urban forests and parks – covers only 26 percent of Paris. That’s significantly less than the 41 percent average in other European capitals. Such differences can add up. In a recent analysis of urban heat island intensity for 100 European cities, Paris came out on top.

The men’s Olympic marathon begins at 8 a.m. local time on August 10, one day before the women’s marathon. Races start on wide, cobbled boulevards in one of Paris’ densest neighborhoods. Runners pass several of the city’s famous landmarks, including the Hôtel de Ville, the Palais Garnier opera house, Place Vendôme, the Louvre and the Trocadéro.

Adapting to the heat in the city during marathons

The sparse vegetation and dense development in this part of the city will raise temperatures and increase the heat stress runners face, says Ariane Middel, a climatologist at the University of Arizona and co-author of a study detailing how microclimate variations affect marathon runners’ heat stress during the 2020 Tokyo Olympic Marathon. “However, the tall buildings on both sides of the course and some tree-lined streets provide some shade for runners – an important factor in reducing heat stress on the human body on a sunny day,” she said. “Running on grass instead of asphalt does not reduce heat stress on the body as much as running in the shade.”

Middel said runners’ heat stress should ease somewhat as the course runs through tree-lined boulevards along the Seine for several kilometers and then descends even further as runners pass through the Fausses-Reposes forest, a former royal forest near Versailles. Likewise, the Bois de Meudon (also called the Forét domaniale de Meudon), another forest near Versailles, should help runners cool off as they run back to the race start area, she said.

The route follows the route of the 1789 Women’s March on Versailles, a key moment during the French Revolution when thousands of women and allies marched through Paris to Versailles. Once there, they laid siege to the palace and put pressure on King Louis XVI to ratify the Universal Declaration of the Rights of Man and of the Citizen.

Even as temperatures rise in Paris and elsewhere, Middel pointed to several ways race planners can mitigate the risks of extreme heat: “Marathons should start early in the morning to benefit from cooler temperatures and more shade, plant more trees along the courses and install temporary sun protection structures.”

NASA Earth Observatory images by Wanmei Liang and Michala Garrison, using VIIRS land surface temperature data, a marathon route from Github, and Landsat data from the US Geological Survey.