Overview

• An analysis of 1982–2023 data shows summer now starts about 12.6 days earlier and ends about 8.8 days later on average.
• The average summer period expanded from 92 days in 1982 (June 29–Sept. 28) to 121 days in 2023 (June 11–Oct. 9).
• The Mie University team divided Japan and surrounding seas into about 200 zones and used Japan Meteorological Agency observations with zone-specific temperature benchmarks.
• The researchers attribute the longer summers chiefly to rising sea surface temperatures and warn the trend will strengthen if warming persists.
• Winter length showed little change, which they link to recurring continental cold waves, while spring and autumn have shortened.