In the UK, it’s often difficult to feel too worried about a longer summer. Although recently, perhaps, they’ve come with a disquieting feeling – a sense that something is amiss. Now, a new study by researchers at the University of Chinese Academy Sciences in Beijing has confirmed it. As the world has warmed, the seasons have shifted, bringing longer, hotter summers, shorter winters and more frequent and damaging ‘false springs’.
The researchers utilised historical daily climate data from 1952 to 2011 to produce the study, which has been published in Geophysical Research Letters. They found that, on average, global summers have elongated from 78 days in 1952 to 95 days in 2011. At the same time, on average, spring has contracted from 124 to 115 days; autumn from 87 to 76 days; and winter from 76 to 73 days.
Shifting seasons are directly linked to warmer global temperatures: a slight change in temperature brings an earlier spring thaw and delays the first frost until later in autumn. The researchers also found that average seasonal temperatures have increased: summers have heated at a rate of 0.089°C per decade and winter by 0.26°C per decade.
Anthropogenic alterations to seasonal cycles have consequences. Colder and earlier springs pose a greater risk of ‘false springs’, which cause plants to emerge prematurely from dormancy. In 2012, North America recorded its earliest spring ever; buds burst and leaves emerged, but plants were then exposed to subsequent frosts. Damage to fruit and vegetable crops in Michigan alone totalled half a billion US dollars. Another false spring occurred in the southeastern USA in 2007, causing US$2 billion in agricultural losses. ‘False springs are becoming more common with anthropogenic change to the seasons; this is also a huge problem in China,’ says the study’s lead author, Yuping Guan. One 2019 study demonstrated that false springs have increased in frequency in recent decades in China. The authors predicted that, under a ‘business as usual’ scenario in which emissions remain high, spring blooms will appear 23 days earlier by 2100.
‘Studies have shown that changing seasons bring environmental risks,’ says Guan. Birds, he says, often time their migrations and breeding cycles to coincide with the emergence of plants as food sources. The mismatch can shorten time windows for migration and breeding. Arctic seabirds, for example, have shown shorter breeding cycles over the past 35 years.
The findings allowed the team to make predictions about how the seasons will continue to change as the climate warms. As the temperature rises, the team predicts that spring and summer will arrive 3.3 days and 4.6 days earlier per decade, respectively. ‘As a result,’ they say, ‘there is good potential for a 166-day summer and 31-day winter by 2100.’ The findings corroborate earlier warnings: in 2015, a team of researchers predicted that spring will be, on average, 23 days earlier in the western USA and Great Plains by 2100. Guan and his team point to the fact that policy-making for the agricultural, ecological and environmental sectors will be impacted by this adjustment of the seasonal clock: ‘Seasonal-related topics involving ecology, the ocean and the atmosphere may need to be revisited because seasons are the basic time parameter for a wide range of natural phenomena.’