Natural events have an annual rhythm. The buds on trees burst in spring, a succession of plants flower at different times during the year, animals come and go with seasonal migrations. We detect the changes in season through a range of different cues.
We align the astrological seasons with our calendar, the autumn equinox occurs on the same date each year, September 23rd. Astrological seasons, however, are not the same as environmental seasonality.
Throughout the year, the Earth’s position relative to the sun determines day length. The autumn equinox is an astrological phenomenon marking the start of astrological autumn, when the sun rises directly from the east and sets directly to the west, and day and night are approximately equal.
Astrological seasons influence the climate, with temperature and rain- and snow-fall responding to the amount of solar radiation reaching the Earth. Climate and day length in turn affect the appearance and disappearance of different species, and natural phenomena such as autumn colours.
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In a stable climate, the astrological, calendar and environmental seasons may be coupled with predictable timing of environmental events. As the climate changes due to increases in greenhouse gas emissions, the timing of environmental events relative to astrological or calendar time is becoming increasingly disrupted. Natural events may no longer occur at the same time as in the past and may become increasingly unpredictable.
The study of the timing of biological events like fruit production and leaf fall in autumn is called phenology. One of the longest phenological records that we have access to is the timing of cherry blossom season in Japan, with records dating back to the ninth century, and grape harvest times in Burgundy dating back to the 14th century.
These long-term records enable us to assess how much the timing of biological events and shifting seasonality is diverging from historical patterns. Cherry blossom season in Japan is now two weeks earlier than when records began more than 1,000 years ago.
Climate change in seasonal timing does not just affect the timing of life events for individual species. Seasonality is fundamental to how ecosystems work. The timing of the emergence of caterpillars determines the availability of food for birds and, if it does not coincide with the birds’ breeding season, their chicks may not survive.
Seasonality affects how populations of plants and animals grow, reproduce and survive and many species have evolved to synchronise important life events like breeding and migration with predictable, seasonal, environmental changes. Individual species are linked together in complex food webs so a change to the timing of one species’ appearance can propagate through that food web in unpredictable ways.
While scientists have observed how climate change affects the timing of life events for individual species for decades, a recent paper by Daniel Hernández-Carrasco and colleagues show that changing seasonal patterns can generate complex and hard-to-predict ecological impacts.
Complex interactions between species, which are affected differently by changes to seasonal timings, mean that the full effect of climate change-induced disruptions in phenology may not be factored into predictions.
This has implications for the contributions that nature makes to people, such as pollination of food crops and the amount of carbon drawn down from the atmosphere by trees.
For example, pollination failure can occur if a plant flowers before pollinators have emerged, leading to crop failure. However, this response can be unpredictable if there are multiple pollinators which all respond differently to changing seasonality.
Here in Ireland, Prof Astrid Wingler has shown that the start of spring advanced by two and a half days between 1990 and 2018. The onset of autumn, the end of the growing season, was more variable. An earlier spring can lead to an earlier onset of autumn, or changes in temperature and rainfall during the growing season can lead to a later autumn, depending on the species and location.
Earlier spring may be a boon for farmers, who may be able to take advantage of a longer growing season, but the biodiversity that underpins food production and carbon sequestration may have an unpredictable response to changing seasonality.
Yvonne Buckley is co-director of the Co-Centre for Climate + Biodiversity + Water and professor of zoology at Trinity College Dublin
