The UK is experiencing a "false autumn", according to the Met Office, as leaves turn brown and berries ripen weeks ahead of the official start of the season. Trees and plants began showing autumnal signs in August due to "the stress of extreme summer conditions", the weather agency said.
When does autumn officially begin? Traditional "folk" markers for the start of autumn include leaves turning brown, hawthorn berries ripening, swallows migrating and, more recently, coffee chains launching pumpkin spice brews.
More scientifically, when a season begins can be defined in three ways: meteorological, astronomical and phenological. Across the Northern Hemisphere, meteorological autumn begins on 1 September and ends on 30 November. Using set calendar periods makes it easier for meteorological and environmental agencies to compare data over years.
The astronomical date varies between 21 and 24 September, in time with the autumn equinox, when day and night are of equal length, and ends with the winter solstice in December.
The phenological definition relies on the natural world and its response to weather and climate. In the UK, autumn starts when observable events such as leaves falling from trees and the migration of birds begin. Weather and climate conditions mean it varies from year to year.
How is climate change affecting the seasons? Global warming has changed both the length and timing of the seasons, with Europe and the rest of the Northern Hemisphere experiencing a "much more pronounced change in seasonality" than that in the Global South, Jadu Dash, an environmental sciences professor at Southampton University, told Polytechnique Insights. Autumn has started to arrive, on average, two weeks later, while spring arrives a fortnight earlier.
Met Office analysis found that for every 1C increase in October temperatures, the date at which trees lose all their leaves is delayed by two or three days. But, said the agency, the "ongoing impacts" of climate change can also speed up the arrival of other autumnal conditions such as ripening berries, leading to more frequent occurrences of a false autumn as ecosystems "struggle to adapt to extreme weather patterns".
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