By Pete Inness
We are now just over half way through April, so about half way through meteorological Spring which is defined as March, April and May. Despite the warm weather of the last few days it’s been a fairly cold Spring so far, with the mean temperature across most of the UK being 2 degrees C below normal during March. Easterly winds in late February and March, dubbed the “Beast from the East” in the press, brought cold air across the UK from continental Europe, and there were several spells of snow which caused significant disruption to transport.
To many people the arrival of Spring is not so much set by a date in the calendar but by the occurrence of particular aspects of the natural world. People keep records of events such as the first flowering of particular plants or the emergence of particular insects from hibernation. The Nature’s Calendar website, run by the Woodland Trust, allows people to upload their recordings and produces maps which show how the seasons are progressing in the natural world.
For me personally, the appearance of blackthorn flowers in the hedgerows is a sign that Spring has really started. Blackthorn is a ubiquitous hedging shrub around much of the UK and its white flowers, blossoming before the leaves appear, transform the appearance of the countryside between early March and mid April. In Autumn, blackthorn is also the source of the dark blue sloes needed for the manufacture of sloe gin. With the cold weather in February and March this year it’s not surprising that blackthorn has flowered later than usual. By 1 April last year, with March temperatures a degree or so above normal, there had been many recorded sightings of blackthorn in flower across the whole of the UK, including all of Wales, Northern Ireland and even Northern Scotland (see Figure 1, Top). This year, sightings of blackthorn reported to Nature’s Calendar up to 1 April were restricted to England and South Wales, with no reports at all in Northern Ireland or Scotland and very few reports north of the English Midlands (Fig. 1, Bottom). The very cold Spring of 2013, when March was actually colder than February, led to a very similar pattern of delayed flowering in blackthorn. In 2016, a year with Spring temperatures very close to the long term average, there were many more reports of blackthorn in flower by 1 April, including in Northern Ireland and the central belt of Scotland.
Figure 1. Sightings of blackthorn in flower on or before 1 April, reported to the Nature’s Calendar website. Each dot represents an individual report. (Top) 2017. (Bottom) 2018. Figures downloaded from naturescalendar.woodlandtrust.org.uk.
Year-to-year variations in the dates of particular natural events are not unexpected as the weather has a first order impact on the behaviour or plants, animals, birds and insects. However, climate change has also started to have a discernible impact on long term trends in the natural indicators of the arrival of Spring. A study by Amano et al, published in 2010, used flowering records going back 250 years to create an index of flowering dates for the UK. They found that in the most recent 25 year period in their study, plants flowered between 2.2 and 12.7 days earlier than any other consecutive 25 year period in the study, with flowering occurring, on average, 5 days earlier for each degree of warming.
In some ways this might be good news as a longer growing season for crops in the UK might mean that agricultural productivity will increase. However, there may be downsides in the natural world. Some species may adjust to temperature changes at different rates. To give a simple example, the arrival and nesting of a particular migratory bird species in the UK may be timed to coincide with an abundance of a particular insect larva on which they feed their chicks. However, if that insect starts producing larvae very much earlier than the arrival of the birds then the birds themselves may struggle to feed their young.
In farming too, changes to Spring temperatures may also cause problems. Although Spring is getting warmer on average, there are still examples such as 2013 and 2018 when there can be very cold spells. If blossom on fruit trees has already appeared prior to a cold spell then yields of that particular variety of fruit can be badly hit.
The specific atmospheric conditions that led to the low Spring temperatures this year and in 2013 are now reasonably well understood and there is no indication that these conditions will become less likely to occur in a warmer world. Hence we can continue to expect considerable variations in Spring temperatures from year to year even as the mean climate warms over the next 50 years or so.
Reference
Tatsuya Amano, Richard J. Smithers, Tim H. Sparks, William J. Sutherland., 2010. A 250-year index of first flowering dates and its response to temperature changes. Proceedings of the Royal Society (B).