After an unusually sunny September, autumn has finally arrived in the UK. The days are getting shorter, the temperature is dropping, the leaves are changing color. The late start to fall 2023 is not unique. In fact, this is part of a broader trend in which the transition from summer to winter occurs later in the year. My own research over the last 13 years points to climate change as a possible cause.

One of the most visible effects of climate change is the change in the seasonal patterns of the vegetation around us. This includes the timing of important biological events such as bud break, first leaf emergence, flowering and leaf fall.

In general, the appearance of the first leaf indicates the arrival of spring, and the fall of the leaves indicates the beginning of autumn. The timing of these events varies, especially in the Northern Hemisphere, where spring begins earlier and autumn is delayed.

Traditionally, monitoring the seasonality of vegetation requires careful documentation of these seasonal events from year to year. The earliest records of spring events in Great Britain date back to 1736, when naturalist Robert Marsham began recording the timing of spring events in Norwich, England.

Today, satellite data has become an important tool for tracking changes in the seasonality of vegetation. Based on these data it is possible to estimate the strength of the vegetation (indicator of the condition, strength and magnificence of the vegetation). The changes can then be used to determine the beginning and end of each growing season.

A longer growing season

Climate scientists now have nearly fifty years of satellite observations. Analysis of this data shows that spring advances by approximately 15 days and autumn by the same amount. The overall result has been to extend the growing season by as much as a month over the last three decades.

The change in the seasonal schedule is especially evident in high latitudes. Vegetation located more than 55° north of the equator, such as the larch forests of northern Russia, tends to lengthen the growing season by one day per year.

A longer growing season isn’t necessarily a bad thing. This means a longer period of photosynthesis, which could theoretically increase net carbon uptake, although there is no concrete evidence yet.

But starting the growing season early puts plants at risk of damage from spring frosts and increased vulnerability to summer drought. Research has shown that early spring in central and northern Europe in 2018 contributed to increased vegetation growth. This led to rapid loss of soil moisture, exacerbating summer drought conditions.

The role of climate change

Temperature is one of the main factors affecting the growth of vegetation in high northern latitudes. Therefore, the early start of spring and late start of autumn are likely due to the increase in global average temperature. Since 1981, the average global temperature has increased by 0.18°C per decade.

However, the effect of temperature on the duration of the growing season may vary depending on the type of vegetation. In ecosystems predominantly dominated by forests, a warmer climate can lead to greater photosynthesis and increased vegetation productivity.

On the other hand, in warmer climates, more water evaporates from the Earth’s surface, causing the soil to dry out. This can negatively impact the growth of shallow-rooted plants such as grasses and herbs.

Another consequence of climate change is the increase in the frequency of droughts during the peak growth period. Drought conditions cause severe water stress in plants, resulting in premature leaf drop or discoloration, a phenomenon often referred to as ‘false autumn’.

The UK faced conditions such as premature leaf fall and browning as it battled an extreme heatwave in August 2022.

A longer, drier growing season could also increase the risk of wildfire. A 2006 US study found a significant increase in wildfire activity in northern Rocky Mountain forests since the mid-1980s. This change was closely associated with higher spring and summer temperatures and early spring snowmelt.

Climate change clearly affects vegetation growth and seasonality. However, the degree and severity of its effect varies depending on the type of plant and where it grows. The availability of satellite data over the last 50 years is a valuable resource for recording changes in the length of the growing season. These data help scientists measure the scale and consequences of these changes, providing insight into how plants are responding to our warming climate.