Climate change impacts under current socioeconomic conditions
First, to clarify the impact of climate change alone, we evaluated the number of cities that could host the Olympic marathon in August during the late 21st century (2080–2099) under socioeconomic conditions in 2020, changing only the climatic conditions according to the four RCPs. A total of 238 cities in 62 countries were selected based on (1) socioeconomic conditions as of 2020 (urban population of 1 million or more, national GDP of 100 billion dollars or more (PPP, Int'l $ 2005)), (2) elevation of less than 1,600 m 10 , and (3) availability of meteorological data for WBGT correction using the method of Takakura et al. (2019) 15 .
The WBGT levels as evaluation criteria for cities were set as follows.
WBGT level 1 (Good): There is a greater than 90% probability that the WBGT will fall below 18°C for at least three consecutive hours between 7:00 and 21:00 in August.
WBGT level 2 (Caution): Not applicable to level 1, and there is a greater than 90% probability that the WBGT will fall below 22°C for at least 3 consecutive hours between 7:00 and 21:00 in August.
WBGT level 3 (Warning): Not applicable to level 1 or 2, and there is a greater than 90% probability that the WBGT will fall below 28℃ for at least 3 consecutive hours between 7:00 and 21:00 in August.
WBGT level 4 (Cancel): Levels 1, 2, or 3 do not apply.
We determine that cities with WBGT levels of 1 to 3 can host the Olympic marathon, while cities with WBGT levels of 4 cannot. The WBGT thresholds are set based on Mears and Watson (2015)20, the 90% criterion is set in reference to previous studies on the feasibility of hosting the summer and winter Olympics10,21,22, and the duration (3 hours) and timing (between 7:00 and 21:00 in August) is set based on the general competition time of the Olympic marathon, where all Olympic marathons since the 1980 Moscow Olympics were held, except for the men's and women's marathons at the 1988 Seoul and 2000 Sydney Olympics, the women's marathon at the 1996 Atlanta Olympics, and the women's marathon at the 2020 Tokyo Olympics, which was moved up by an hour the day before to start at 6:0023–28.
As a result, globally, the number of cities that can host the Olympic marathon (WBGT level 1–3) significantly decreases under high emission scenarios in the late 21st century (2080–2099) (Fig. 1). Under historical (1994–2013) climate conditions, all but two cities (236 cities) can host the Olympic marathon, while the number of cities decreases to 229.9 (97%) under RCP2.6 and 181.1 (76%) under RCP8.5 in 2080–2099. The number of safer cities with WBGT levels 1 to 2 decreased from 85 (36%) under historical climate conditions to 77.2 (32%) under RCP2.6 and 40.2 (17%) under RCP8.5. The error bars indicate the range between the maximum and minimum values of the seven GCMs, and the ranges are large enough to be considered. Therefore, under the current situation where it is not possible to judge the superiority of a particular GCM, the results based on various GCMs, as shown in this paper, are more reliable. The detailed number of cities, including the upper and lower limits by the seven GCMs, is given in Supplementary Table. 1. The results for the mid-21st century (2040–2059) are shown in Supplementary Fig. 1 and Supplementary Table 2.
Regionally, there are three patterns in the decrease in the number of cities that can host the Olympic marathon (Fig.
2). The number of cities by country is shown in Supplementary Fig. 2 and the result for the mid-21st century (2040–2059) is shown in Supplementary Fig. 3.
Pattern 1: Asia
The number of cities that can host the Olympic marathon decreases significantly under high emission scenarios. Among the viable cities, the number of cities with relatively low heat risk (WBGT level 1–2) is small and will decrease further with climate change. This result may be because many cities in Asia are in the mid-latitude zone of the Northern Hemisphere and are affected by the Asian monsoon, resulting in hot and humid summers.
Asia: This region contains the largest number of cities (101) out of 238 total cities. Under historical (1994–2013) climate conditions, all but two cities (99 cities) can host the marathon, while the number of cities decreases to 92.9 (92%) under RCP2.6 and 55.4 (55%) under RCP8.5. The number of cities with relatively low heat risk (WBGT level 1–2) decreased from 27 (27%) under historical climate conditions to 22.3 (22%) under RCP2.6 and 9.2 (9%) under RCP8.5.
Pattern 2: North America, Latin America and the Caribbean, and Africa
The number of cities that can host the event (WBGT levels 1–3) does not decrease much even under the highest emission scenario, while the number of cities with relatively low heat risk (WBGT levels 1–2) was small originally and will decrease further due to climate change. This finding may be because many cities are in the high latitudes of the Northern Hemisphere (North America) or the Southern Hemisphere (Latin America and the Caribbean, and Africa) and thus are less likely to be hot and humid in August than Pattern 1 (Asia).
North America: This region contains the third-largest number of cities (47 cities). Under historical (1994–2013) climate conditions, all cities can host the marathon. Under RCP8.5, the number of cities decreases to 41.7 (89%), but the in other scenarios, the number of cities does not change at all or remains mostly unchanged. The number of cities with relatively low heat risk (WBGT level 1–2) decreased from 15 (32%) under historical climate conditions to 10.4 (22%) under RCP2.6 and 2.9 (6%) under RCP8.5.
Latin America and the Caribbean: This region contains the third-smallest number of cities (24 cities). Under historical (1994–2013) climate conditions, all cities can host the marathon. The number of cities decreases to 22.1 (92%) under RCP6.0 and 19.9 (83%) under RCP8.5, but in the other scenarios, the number does not change at all or remains mostly unchanged. The number of cities with relatively low heat risk (WBGT level 1–2) decreased from 9 (38%) under historical climate conditions to 7.4 (31%) under RCP8.5.
Africa: The region contains the second-smallest number of 12 target cities. Under historical (1994–2013) climate conditions, all cities can host the Olympic marathon. The number of cities decreases to 10.9 (91%) under RCP8.5, but in the other scenarios, the number does not change at all or remains mostly unchanged. The number of cities with relatively low heat risk (WBGT level 1–2) decreased from 3 (25%) under historical climate conditions to 2 (17%) under RCP8.5.
Pattern 3: Europe and Oceania
The number of cities that can host the Olympic marathon does not decrease much even in the highest emission scenarios. There were originally many cities with relatively low heat risk (WBGT level 1–2), which will be the case under future climate conditions. This finding may be because many cities are in the high latitudes of the Northern Hemisphere (Europe) or in the Southern Hemisphere (Oceania), where the WBGT is unlikely to be high in August.
Europe: This region contains the second largest number of target cities, 50. Under historical (1994–2013) climate conditions, all cities can host the Olympic marathon, and in all RCP scenarios, the number does not change at all or remains mostly unchanged. The number of cities with relatively low heat risk (WBGT level 1–2) decreased from 44 (88%) under historical climate conditions to 22.3 (45%) under RCP8.5.
Oceania: This region contains the smallest number of target cities, 4. Even under the highest emission scenario, the number of cities that can host the Olympics does not decrease, and all four cities are determined to be relatively safe (WBGT level 1–2) in all RCP scenarios.
Climate change impacts under future socioeconomic conditions
Next, we estimate the number of cities that can host the Olympic marathon in the late 21st century (2080–2099), considering future socioeconomic scenarios (SSP1–5) (Fig. 3). Here, all combinations of the four RCPs and five SSPs are shown to identify the wide range of impacts due to climate change and socioeconomic conditions, but it should be noted that the combinations of RCP2.6 and SSP3 and RCP8.5 and SSP1 to 4 are infeasible19. The number of cities by country is shown in Supplementary Fig. 4–8, and the results for the mid-21st century (2040–2059) are shown in Supplementary Fig. 9–14.
We selected target cities by the same criteria as in the evaluation of 238 cities for current socioeconomic conditions. The numbers of selected cities for the five SSPs are 257 for SSP1 (sustainability scenario), 292 for SSP2 (middle of the road scenario), 299 for SSP3 (regional rivalry scenario), 256 for SSP4 (inequality scenario), and 286 for SSP5 (fossil-fueled development scenario).
Globally, as in the results under current socioeconomic conditions, the number of cities that can host the Olympic marathon tends to decrease under high emission scenarios, especially in Asia. The number of cities under each emission scenario is the highest for SSP3 or SSP5 and the lowest for SSP4. Under all SSPs, the number of possible host cities increases from the current socioeconomic conditions (2020). For example, under RCP2.6, the number of cities ranged from 244.7 (SSP4) to 286.4 (SSP5), while under RCP8.5, the number of cities ranged from 202.1 (SSP4) to 235.2 (SSP5).
Under RCP8.5, which is the highest emission scenario, the number of possible cities is smaller than any of the cases under RCP2.6 and RCP4.5, and the impact of emission scenarios becomes apparent. For example, under the lowest case of RCP4.5 (SSP4), there are 235.6 cities, which is larger than the 235.2 cities under the highest case of RCP8.5 (SSP5). Dozens of emerging cities, mainly in Asia, that could host the summer Olympics due to economic development under the low emission scenarios will not be able to host the Olympics under the high emission scenarios.
Effects of adaptation measures on climate change impacts under future socioeconomic conditions
In this section, we evaluate the change in the number of cities that can host the Olympic marathon in August due to adaptation measures in the late 21st century (2080–2099) (Fig. 4). The four types of adaptation measures in Table 1 are adopted in this study. Adaptation measures 1, 2, and 3 have been adopted or considered for the recent Olympic Games and can be reproduced with the spatiotemporal resolution of this study. Adaptation measure 4 implements 1, 2, and 3 at the same time. The results for all four RCPs in the mid- and late-21st centuries are shown in Supplementary Fig. 15–18.
Table. 1: List of adaptation measures adopted in this study with examples
Adaptation measure 1 (AM1: All day)
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Holding the Olympic marathon between 10 p.m. and 6 a.m. in August.
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Adaptation measure 2 (AM2: October)
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Holding the Olympic marathon between 7 a.m. and 9 p.m. in October.
Example: Since 1980, the men's marathons at the 1988 Seoul Olympics and the 2000 Sydney Olympics have been held in October27.
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Adaptation measure 3 (AM3: By country)
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Holding the Olympic Games in multiple cities in a country and the Olympic marathon between 7 a.m. and 9 p.m. in August.
Example: The Tokyo Organizing Committee of the Olympic and Paralympic Games changed the venue for women’s and men’s marathons and the race walk from Tokyo to Sapporo, according to International Olympic Committee’s strong recommendation concerning the excessive thermal load in Tokyo29.
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Adaptation measure 4 (AM4: All)
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Implementing Adaptations 1 to 3 simultaneously. This measure has not been implemented or considered for the Olympic Games to our knowledge.
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Cf. No measure
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Holding the Olympic marathon between 7 a.m. and 9 p.m. in August. All Olympic marathons since the 1980 Moscow Olympics, except for the men's and women's marathons at the 1988 Seoul and 2000 Sydney Olympics, and the women's marathon at the 1996 Atlanta Olympics, and the women's marathon at the 2020 Tokyo Olympics, which was moved up by an hour the day before to start at 6:00, were held in this time23–27, 30.
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Under RCP2.6, the effects of adaptation measures are limited due to the originally small decrease in the number of host cities (Fig. 4, upper part). Under RCP8.5, the effects of adaptation measures are more pronounced, and the effects of single adaptation measures are greater in the order of AM2, AM3, and AM1 (Fig. 4, lower part). AM2 and AM3 will increase the number of possible cities by approximately 40, while AM1 will increase the number of possible cities by approximately 15, and the majority of the increase occurs in Asia. While AM1 is the most common measure, it is less effective than AM2 and AM3. The effect of AM4, in which all adaptation measures are implemented simultaneously, is the greatest, and more than 95% of the selected cities will be able to host the Olympic marathon even under RCP8.5.
WBGT levels under specific RCP/SSP/adaptation measures in the late 21st century (2080–2099) have been shown for 18 historical host cities of the Olympic Games since 1948 (Fig. 5). Note that we excluded Mexico City, which was the host city in 1968 because its elevation of more than 1,600 m did not meet our conditions for city selection.
If no adaptation measures are taken (NM), the number of cities with high heat risk (WBGT level 3) will increase from 7 in the past climate to 10 to 12 (out of 18 cities) in the future climate. Even for Tokyo, which is assumed to be WBGT level 3 at the time of the 2020 Tokyo Olympics, the venues for marathons and race walks were relocated to Sapporo, a city more than 800 km away in the north, due to concerns about heat31. Thus, it is highly likely that equivalent measures will be required in other future host cities if they are assessed as WBGT level 3.
Among the single adaptation measures, the effect of holding the event in October (Oct) is the highest. In most cities, the WBGT level will be 1 or 2 even under RCP8.5. On the other hand, in Rio de Janeiro and Sydney, which are in the Southern Hemisphere, there are no effects of the measure, or rather, the WBGT level increases. Holding the event in multiple cities in a country (BC) may or may not be effective depending on the cities. Countries with more diverse domestic climates (cooler metropolises) may be advantageous. Notably, this study does not include cities for which sufficient climate data have not been collected, and the actual effect may be greater. The case where all adaptation measures are implemented at the same time (All) is the most effective, with all cities under RCP2.6 and 4.5 and 16/15 cities under RCP6.0/8.5 having a WBGT level of 1. See the Supplementary Information for results for the mid-21st century (Supplementary Fig. 19) and for all 397 cities in the mid- and late-21st centuries (Supplementary Fig. 20–31).