The summers are getting longer

Our summers are getting longer

Climate change is changing our seasons: in the past all four seasons were about the same length, now summer is taking up more and more time. A study reveals that it is 17 days longer today than it was in 1950. Spring, autumn and winter, on the other hand, are getting shorter and shorter in almost the entire northern hemisphere. If this trend continues, the summer of 2100 could last almost half a year - with considerable consequences for people, nature and the weather.

Climate change not only increases global temperatures, it also changes the location of the climate zones and affects nature. Studies show that plants sprout and bloom earlier and earlier in spring, and in many places the overall vegetation period has lengthened as a result. In some regions the rainy seasons are shifting and shortening, in others summer heat phases are increasing.

Seasons trends compared since 1952

Researchers led by Jiamin Wang from Lanzhou University in China have now investigated in more detail what these developments mean for the typical seasons in the northern hemisphere. To do this, they evaluated weather data from 1952 to 2011 for the middle latitudes of the northern hemisphere and used several climate models to analyze seasonal trends. In the next step, they forecast the development up to the year 2100.

In order to make it possible to compare the delimitation of the seasons despite different local climatic conditions, the scientists used the relative temperature: As summer, they defined the time when the daytime temperatures were among the warmest 25 percent of the year. Winter was characterized by temperatures in the bottom 25 percent. Spring and autumn lay in between.

Summer is becoming more and more dominant

The result: the seasons have shifted significantly since 1952. Summers are getting longer, while spring, autumn and winter are shortening. Specifically, the summer was extended from 78 to 95 days on average, and at the same time it starts 2.5 days earlier today. As a result, the summers in the northern hemisphere have become longer by 4.2 days per decade, as the researchers report.

The remaining three seasons, on the other hand, have been shortened: Winter now only lasts 115 days - it has been shortened by an average of 2.1 days per decade. At the same time, the beginning of winter moves back by around half a day per decade. Spring and autumn have been shortened by around one day per decade since the 1950s. The shift in the seasons is most evident in the Mediterranean and Tibet, according to Wang and his colleagues.

Cooler spring, hotter summer

The temperatures of the different seasons have also changed: "The summers are getting longer and hotter, the winters shorter and milder," reports co-author Yuping Guan from the Chinese Academy of Sciences. Summer temperatures have risen by 0.089 degrees per decade, winters have warmed by 0.26 degrees per decade. “At the same time, the frequency, duration and intensity of heat waves in summer have increased significantly since the 1950s,” the team said.

In contrast, spring has gotten colder. "An increasingly cool and earlier spring means an increased risk for plants," explain the scientists. Because there can be more frequent frosts that damage the plants that sprout prematurely. A mild winter can further encourage the plants to start growing too early.

Climate change as a driving force

Scientists see climate change as the main cause of this shift in the seasons. Their climate models show that the observed trend does not materialize if global warming since 1950 is excluded from the simulations. "This suggests that the anthropogenic greenhouse effect dominates these changes in the length and beginning of the four seasons," state Wang and his team.

If climate change continues, the seasons will continue to shift. The researchers used their climate simulations to determine how strong. In the most extreme scenario without effective climate protection, the summer of 2100 could be an average of 166 days - almost half a year. The winter, on the other hand, would be shortened to just 31 days. Spring and summer then start around a month earlier, while autumn and winter start a month later.

Far-reaching consequences

These shifts will not remain without consequences for nature and humans, as Wang and his team emphasize: "The changing seasons disrupt agriculture and the biorhythm of species," they explain. In spring, many animal species lose the synchronization of their courtship and breeding season with the vegetation cycle, in winter there is an increasing lack of frost as a climate signal.

"In addition, hotter, longer summers also favor heat waves, storms and forest fires, which also increase the risks for humanity," the scientists say. The short, mild winters, on the other hand, could encourage winter storms and also short cold spells like in Texas. (Geophysical Research Letters, 2021; doi: 10.1029 / 2020GL091753)

Source: American Geophysical Union

March 10, 2021

- Nadja Podbregar