In the 1950s, the seasons occurred in a predictable and relatively even pattern in the Northern Hemisphere. Flowers bloomed around April. Children planned summer adventures starting in June. Leaves dropped in September. Ski trips began in December.
But recently, the seasons have been out of whack. Over the past seven decades, researchers found high summertime temperatures are arriving earlier and lasting longer in the year because of global warming.
This summer was no exception. In parts of California, which saw its hottest summer on record, unusually warm temperatures arrived in May. Shasta Dam posted its third warmest May on record, a harbinger of a record melt season for the glaciers on the summit of Mount Shasta to its north. Sacramento logged its fifth warmest May.
In the Pacific Northwest, a record-breaking heat wave in late June also occurred much earlier than the region is accustomed to. On June 28, Seattle reached 108 degrees and Portland reached 116 degrees.
“The old rule for our area was typically from the Fourth of July on, you could expect some hot weather,” said Michael Brady, an economics professor at Washington State University. “So not only was the level of the temperatures unprecedented, it was also at least a couple of weeks before you would even expect high temperatures.”
Unusually hot weather also lasted longer than usual in parts of the Lower 48 states as a late-season heat wave scorched the West in the second week of September. Denver hit 99 degrees on Sept. 10, its highest temperature recorded so late in the season. Earlier that week, Death Valley reached 122 degrees, the highest temperature observed so late in the year anywhere on the planet.
As we mark the end of summer and the fall equinox on Wednesday, fall and winter may not offer much reprieve. Research shows summers are expanding while spring, autumn and winter are becoming shorter and warmer with significant impacts for people and the environment.
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In April 2018, a sizable snowstorm blanketed several towns in New Jersey in what seemed like a freak occurrence. In March 2012, unusually warm temperatures in Michigan caused vegetation to emerge from dormancy early, and then be subsequently destroyed by freezing weather in April.
As more unusual weather reports popped up over the years, researcher Yuping Guan was inspired to examine how seasonal cycles were changing. He and his colleagues analyzed 1952-2011 temperature data and found the four seasons no longer occur equally and had irregular onsets.
In the Northern Hemisphere mid-latitudes, the length of summer increased from 78 to 95 days — or about 4.2 days per decade from 1952-2011. Winter contracted from 76 to 73 days, or 2.1 days per decade on average. Spring decreased from 124 to 115 days, and autumn shrank from 87 to 82 days; each shrank about one day per decade. All seasons were warmer. The most obvious seasonal length changes occurred around the Mediterranean and the Tibet Plateau.
Guan wrote in an email that it’s difficult for people to feel the effects of a small rise in global temperature over many decades, but that the changing length of seasons is something “everyone can understand.”
The onsets of all seasons also changed. Spring started 1.6 days earlier each decade, and summer started 2.5 days earlier. Autumn was delayed by 1.7 days per decade, and winter was delayed by 0.5 days. The delayed winter was the most prevalent in the Qinghai Tibetan Plateau. The changes in spring, summer and autumn onset were most pronounced in western Eurasia.
“It’s a hot spring so that it essentially becomes summer,” said Amir Sapkota, who studies extreme weather events and human health at University of Maryland and was not involved in the study. “With the earlier and much longer summer, spring gets shorter and shorter, and it starts early, too. Winter suddenly becomes a lot more compressed.”
The team defined the start of summer when temperatures were in the hottest 25% during the study period, and summer ended when the temperature fell below that threshold. Winter began when temperatures dipped to the coldest 25% of the period. Spring and autumn were the transition periods between the seasons.
Guan said even though his analysis only goes up to 2011, the recent decade is the warmest period on record and the general trend is the same. Other analyses expanded to 2018 and 2019 also showed similar trends in the United States and Australia. In Washington state, the average date of its first 90-degree day has advanced five days, from May 21 to May 16, over the last century.
The primary driver of these seasonal trends is human-caused warming because of greenhouse gas emissions, Guan’s study showed, as opposed to anthropogenic aerosols or natural forcing. The observed changes in seasonal cycles weakened when model simulations were run without the increase of these heat-trapping gases. If emissions do not slow, the seasonal changes will intensify in the coming decades.
Under the current business-as-usual scenario, projections show summer could last six months by 2100, while winter would be less than two months. Spring and summer will start about one month earlier in 2100 than it did in 2011, and autumn and winter would start a half month later.
But perhaps the most alarming aspect of changing seasonal cycles are the impacts on the environment, animals and human health.
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While a longer summer might sound nice for sunbathers, even small seasonal shifts can throw off the balance of our ecosystem from crop production to increased occurrence of mosquito-borne diseases. With the onset of seasons inconsistent recently, predicting and preparing for those environmental changes is difficult.
“When you push ecosystems into a regime that we haven’t seen before, you just end up with a ton more uncertainty in every direction,” said Anna Michalak, a climate scientist at the Carnegie Institution for Science. “That’s much more difficult to deal with from a management standpoint.”
Longer summers could affect water quality in lakes, said Michalak. Warmer waters for longer periods of time could induce more algal blooms, with potentially more toxic algae. Poor water quality not only affects the fish and wildlife that rely on the water but also human activities and tourist industries by the lake.
Prolonged summertime temperatures could extend the season for mosquito-borne diseases, said Sapkota. Tropical mosquitoes would stick around longer as warmer temperatures persist. They would also possibly expand northward and to higher altitudes as temperatures rise in those areas and bring diseases such as dengue fever or West Nile to new areas.
Earlier and shorter spring seasons could also cause flowers and trees to bloom sooner, potentially blossoming out of sync with pollinating insects. Not all species adapt to seasonal changes at the same rate, which could alter the structure of certain ecological systems.
Earlier blooms can also lengthen pollen season, which can cause longer allergen periods for people. North America experiences tree pollen in the springtime, grass pollen in the summer and weed pollen, primarily ragweed, in the fall.
“What’s going to happen is ragweed season is going to extend way into the fall and winter, presumably, and the tree pollen will start way early. So the overall season will become quite long,” said University of Maryland’s Sapkota. He and his colleagues found a very early onset of spring (about 10 days early) was linked with a 17% increase in asthma hospitalizations in Maryland.
As warmer temperatures last longer over the year, extreme weather events will also increase — including heat waves, droughts and fires. This year, summer was tied for the hottest on record with the Dust Bowl in 1936.
“Definitely the longer summers would increase the likelihood of extreme events like heat waves and droughts, but it’s almost like a chicken and egg argument,” wrote Tim Cowan, a senior research fellow at the University of Southern Queensland, in an email. “Greenhouse gases will make heat waves more likely to occur, which in turn would extend summer, adding to the drier conditions and then in turn making heat waves much hotter.”
Longer summers would also increase the length of fires in the Western United States. Not only would the fires affect nearby people, property and ecosystems, but it would also be a source for air pollution both near and far as made apparent over the last several summers.
Perhaps one of the most noticeably affected sectors is agriculture. Growing seasons are expanding — and the change is significant.
“It’s not a couple of days,” said Brady, who has studied the economic impact of various agriculture industries in Washington state. “It’s already gotten longer by a couple of weeks.”
But a longer growing season is not necessarily helpful for unprepared farmers. It means the harvest is typically faster as crops have accelerated growth under high temperatures.
“While the available growing season is longer, the time to maturity or the actual growing season length could be shorter,” said Kirti Rajagopalan, an agricultural researcher at Washington State University. “And that often comes with negative implications for yields unless you’re able to make use of that longer growing season to do something like double cropping.”
Double cropping, or harvesting two crops in a calendar year, is not common in Washington but popular in other Western states like Arizona and parts of California.
But even if temperatures are suitable for earlier planting, other factors also need to line up for a successful crop yield, said Rajagopalan.
“Temperature is not the only thing that drives when you can plant the soil,” said Rajagopalan. “Moisture is really important, too. And there are many parts of the U.S. where the soil is just too wet for you to go into the field and work.”
A lengthening growing season could also increase the number of insects, which could produce more generations in a year and lead to greater resistance to insecticides. The excess nutrients from fertilizers could also increase algal blooms in nearby waterways, spurring the blooms to occur at unusual times.
“It’s just another example of where all these systems are interconnected,” said Michalak. “And so once you start messing with one piece of the system, all the other pieces start changing as well.”