TOKYO – Japanese summers have lengthened by about three weeks over the 42 years from 1982 to 2023, a university research group said Saturday.
The Mie University group said that due to climate change, the number of days classified as summer has increased each year, while the length of winter has remained relatively unchanged, and the average durations of spring and fall have shortened.
The group cited “the rise in sea surface temperatures due to global warming” as the main factor, warning that “the trend toward longer summers and winters will intensify if warming continues.”
The Japan Meteorological Agency defines summer as lasting from June to August, without specifying a temperature range.
The researchers included second-year master’s student Mao Takikawa and Yoshihiro Tachibana, a professor of the university’s Department of Environmental Science and Technology.
In their research, the group divided the area from Japan’s northernmost main island of Hokkaido to the southernmost one of Kyushu, including surrounding ocean areas, into about 200 zones and used the agency’s observation data to calculate the average annual maximum temperature over 42 years.
They defined a “summer benchmark” for each zone based on its temperature range, using the agency’s data on annual highs and lows to determine which days qualified as summer days.
For example, if the highest temperature in an area was 20 C and the lowest was 0 C, the benchmark was 15 C — one quarter down from the maximum — with days above that level counted as summer.
They further analyzed 42 years of temperature data on an annual basis.
The group defined the first day of summer in a year as the date when the summer threshold was exceeded, and the ending date on the last day it was exceeded, with the number of days in between constituting the summer period.
Looking at the average across approximately 200 zones over the 42-year period, the start date of summer advanced by about 12.6 days, while the end date was delayed by about 8.8 days.
The summer period extended by approximately 21.4 days, the group said.
In 1982, the summer period lasted 92 days, from June 29 to Sept. 28. However, by 2023, it had lengthened to 121 days, running from June 11 to Oct. 9.
According to Tachibana, warm air flowing in from the Asian continent once cooled over the oceans surrounding the Japanese archipelago, causing temperatures to rise gradually from spring to summer. However, in recent years, rising sea surface temperatures have prevented this cooling, resulting in an earlier onset of summer.
Sea surface temperatures remain persistently high, making it difficult for temperatures to drop and delaying the end of summer, Tachibana said.
Meanwhile, little change has been observed for the winter period. This is believed to be due to the continued influence of strong cold waves moving in from the continent.
AloJapan.com