Summers may last for nearly six months by the end of this century

Summer could drag out for nearly six months in the Northern Hemisphere by the end of this century, according to a new study published by the American Geophysical Union. The experts report that without efforts to mitigate climate change, a persistent summer season could have far-reaching impacts on agriculture, human health, and the environment.

Beginning in the 1950s, the four seasons arrived in a predictable and fairly even pattern across the Northern Hemisphere. But now, climate change is causing the seasons to become more irregular.

The research shows that the unpredictable lengths and start dates of the seasons will become even more extreme in the coming decades under a business-as-usual emissions scenario.

Study lead author Yuping Guan is a physical oceanographer at the State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, and the Chinese Academy of Sciences.

“Summers are getting longer and hotter while winters shorter and warmer due to global warming,” said Guan. He was inspired to investigate changes to the seasonal cycle while working with undergraduate student and study co-author Jiamin Wang. 

To measure changes in the length and onset of four seasons in the Northern Hemisphere, the researchers analyzed historical daily climate data from 1952 to 2011.

The team defined the start of summer as the onset of temperatures in the hottest 25 percent during that time period, while winter began with temperatures in the coldest 25 percent. Next, the experts used climate change models to predict how seasons will shift in the future.

The analysis revealed that during the study period, summer grew from 78 to 95 days, while winter shrank from 76 to 73 days. Spring decreased from 124 to 115 days, and autumn dropped from 87 to 82 days. Overall, spring and summer arrived earlier, while autumn and winter started later. 

The greatest changes in seasonal changes took place in the Mediterranean region and across the Tibetan Plateau. The researchers predict that without climate change mitigation efforts, winter will last less than two months, and the transitional spring and autumn seasons will continue to shrink throughout the remainder of this century.  

“Numerous studies have already shown that the changing seasons cause significant environmental and health risks,” said Guan. For example, birds are shifting their migration patterns and plants are flowering at different times, creating potential mismatches between animals and their food sources. Seasonal changes can also be very damaging to crops and budding plants when false springs or late snowstorms occur. 

Furthermore, prolonged warm weather extends pollen and allergy season, as well as the length of time that disease-carrying mosquitoes persist and move northward.

Congwen Zhu is a monsoon researcher at the State Key Laboratory of Severe Weather and Institute of Climate System, who was not involved in the study. He noted that a shift in the seasons could result in more severe weather events

“A hotter and longer summer will suffer more frequent and intensified high-temperature events – heatwaves and wildfires,” said Zhu. Additionally, he explained, warmer and shorter winters may cause instability that leads to cold surges and winter storms, much like the recent snowstorms in Texas and Israel.

“This is a good overarching starting point for understanding the implications of seasonal change,” said Scott Sheridan, a climate scientist at Kent State University. “I think realizing that these changes will force potentially dramatic shifts in seasons probably has a much greater impact on how you perceive what climate change is doing.”

The study is published in the journal Geophysical Research Letters.

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By Chrissy Sexton, Earth.com Staff Writer