Autumn’s arrival in the UK has been noticeably delayed in 2023, as the days grow shorter, temperatures become cooler, and leaves transition to their vibrant autumn hues. This delayed autumn is not an isolated occurrence but part of a broader trend where the transition from summer to winter is happening later in the year. Research conducted over the past 13 years points to climate change as the likely driving force behind this shift in seasons.
One of the most striking impacts of climate change is the alteration of vegetation seasonality patterns. This includes changes in the timing of critical biological events, such as bud burst, leaf appearance, flowering, and leaf fall. These events play a pivotal role in signaling the onset of spring and autumn.
Traditionally, documenting the timing of these events required meticulous record-keeping year after year. The earliest records of spring events in the UK date back to 1736, thanks to the work of naturalist Robert Marsham.
Today, satellite data has become an indispensable tool for monitoring changes in vegetation seasonality. This data allows researchers to estimate vegetation vigor, an indicator of the health, strength, and lushness of vegetation. Changes in vegetation vigor can be used to identify the start and end of each growing season.
Research shows that spring has advanced by approximately 15 days, while the onset of autumn has been delayed by a similar duration, resulting in an extension of the growing season by about one month over the past three decades.
This shift in season timing is particularly pronounced at higher latitudes, with vegetation situated over 55° north of the equator experiencing an extended growing season, increasing by up to one day per year.
While a longer growing season can theoretically enhance carbon uptake through extended photosynthesis, it also exposes plants to risks such as spring frosts and summer drought. An early spring in central and northern Europe in 2018, for instance, led to increased vegetation growth, causing soil to dry out rapidly and exacerbating summer drought conditions.
The primary driver of this shift in season timing is rising global temperatures, with the global mean temperature increasing by 0.18°C per decade since 1981. However, the impact of temperature on growing season duration varies depending on the type of vegetation.
In forest-dominated ecosystems, a warmer climate can promote increased vegetation productivity. On the other hand, a warmer climate increases water evaporation, leading to soil drying out, which can adversely affect plants with shallow roots, such as grasses and herbaceous plants.
Climate change also leads to more frequent droughts during the growing season, causing water stress for plants, premature leaf shedding, and a phenomenon referred to as a “false autumn.”
The UK experienced these conditions in August 2022 when an extreme heat wave led to early leaf fall and leaf browning.
Furthermore, a longer and drier growing season heightens the risk of forest fires. A US study from 2006 found a significant surge in wildfire activity in the northern Rockies’ forests, closely linked to increased spring and summer temperatures and earlier spring snowmelt.
While climate change’s impact on vegetation growth and seasonality is clear, its severity varies depending on the type of plant and its location. The availability of satellite data spanning the past 50 years is a valuable resource for capturing changes in the duration of the vegetation growing season. This data helps scientists quantify the scale and consequences of these changes, providing insights into how plants are responding to our warming climate.