Analysis: Climate change influenced unusual October heat in Japan

During the first week of October (October 1-6, 2024), over 120 million people in Japan experienced unusual heat made at least 3 times more likely by human-caused climate change. 

How unusual was this heat? 

• At least 74 Japanese cities recorded temperatures of 30°C or higher in the first week of October.

• Conditions in these 74 cities were 5°C to 8°C above normal for this time of year (above the 1991-2020 average). 

How did climate change influence this heat? 

• Daily average temperatures reached Climate Shift Index (CSI) level 3 in 259 cities. 

• A CSI level 3 indicates that human-caused climate change made this heat at least 3 times more likely, signifying a very strong climate change influence. 

The table below shows a selection of cities most impacted by this climate change-driven heat. For instance, Kusatsu reached a high of 31.5°C on October 1, which was 7.3°C above normal. The daily average temperature in Kusatsu on October 1 reached a CSI level 5 — this indicates that human-caused climate change made this excessive heat at least 5 times more likely, signifying an exceptional climate change event.

Use the Climate Shift Index global map to see CSI levels in your city and region, and see our FAQs to learn about the CSI. 

Table 1. Select cities with significant maximum temperatures, temperature anomalies (departure from normal temperatures) and climate change signals on certain dates during October 1-6, 2024. The CSI level is based on the average daily temperature for that city. 

Recent impacts

• A heat wave in Tokyo this summer claimed a record 252 lives between June and early September, mostly among the elderly who lacked air conditioning or refrained from using it due to cost concerns.

• Extreme summer heat in Japan has led to a significant drop in agricultural output, severely affecting vegetable, dairy, and fish farms.  The record-breaking heat is already causing food prices to rise, particularly due to a short supply of eggplants, bell peppers, and cucumbers. 

• Warmer-than-average temperatures are expected to delay peak fall foliage in some regions, especially in northern and eastern Japan. Some regions, such as Sapporo and Nagano, may experience red leaves over 10 days later than usual.

Table 2. Japanese cities that experienced hot summer days (days with temperatures at or above 30°C), and days with a very strong climate change influence (days at CSI 3 or higher) over October 1-6, 2024. 

How do we know climate change is influencing this heat?

The Climate Shift Index uses peer-reviewed methodology and real-time data to estimate how climate change has increased the likelihood of a particular daily temperature.

We compare the likelihood that these temperatures would occur in a world without carbon emissions released by humans, versus in today’s world with decades of carbon emissions building up in the atmosphere. This is an established scientific method to determine how much climate change has or has not affected individual extreme weather events.

For this analysis, temperatures come from both ERA5 and the National Center for Environmental Prediction’s Global Forecast System.

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