SOON the swallows will come swooping back to Scotland from South Africa, a traditional sign that winter has ended. It may seem as if all is as nature intended, but it is not. Swallows are now arriving 20 days earlier than they did in the 1970s - and it’s because of global warming caused by climate pollution.

It’s not just swallows whose habits are changing as temperatures rise. A series of scientific studies have found numerous signs that spring - which officially begins tomorrow with the vernal equinox marking the first day of Spring - is coming much sooner than it used to.

Orange tip butterflies are taking to the air 13 days earlier than they did a century ago, while queen wasps are buzzing about six days sooner. Daffodils at Nethy Bridge in the Cairngorms are flowering five days earlier than they did a decade ago.

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Fish and frogs are spawning sooner, trees are budding earlier and lawns are being mowed more often. Insects are appearing two weeks ahead of when they did in 1970, while plants are emerging ten days earlier.

The impacts of rising temperatures are not simple and they can vary, but scientists are clear that most signs of spring are advancing. “A warming climate appears to be disrupting the sequence of events that we have grown up with,” said Tim Sparks, professor of environmental change at Coventry University.

“Early naturalists thought there was a natural order to events. We now know that they were incorrect in this assumption."

Sparks provided evidence on how the average behaviour of seven selected plants and insects had changed between 1891-1947 and 2000-2016. Horse chestnut trees, for example, were now flowering on April 29 rather than May 6, while hawthorn were flowering on April 29 rather than May 11.

According to the Met Office, the average spring temperature has risen from 7.1 degrees between 1961 and 1990 to 8.1 degrees between 2006 and 2015. Scientists blame the rise on carbon pollution from vehicles, factories, farms and homes.

One of the UK’s leading experts on shifting seasons, known as phenology, is Dr Stephen Thackeray, from the government-supported Centre for Ecology and Hydrology (CEH) in Lancaster. “Over the last few decades many familiar UK seasonal events, such as flowering, breeding and migration, have shifted earlier in the year,” he told the Sunday Herald.

“Many seasonal events - our traditional signs of spring - have changed in association with changing air and water temperatures. Species have responded very differently over time, and this could affect the way in which they interact with each other.”

Scientists working for the government Joint Nature Conservation Committee (JNCC) and CEH have complied a “spring index” to show how wildlife is being put under pressure by climate change. It is based on historical data, along with observations by 50,000 volunteers in recent years as part of a major project called Nature’s Calendar.

The index is founded on four events: the first flowering of hawthorn and horse chestnut, the first recorded flight of an orange-tip butterfly and the first sighting of a swallow. Though timings have varied over the years, it suggests that since 1999 the annual average dates for these events have been six days earlier than they were in the first part of the 20th century.

The JNCC’s biodiversity indicators manager, Dr James Williams, pointed out that the spring index advanced more rapidly when the average temperature in March and April was seven degrees or higher. Complex changes could harm some species, he suggested.

“Differential responses among species may cause problems for life cycles such as pollinating insects emerging out of synchrony with flowers opening in spring,” he said. “This could increase vulnerability to extreme events such as late frosts, disrupt food webs, and change the balance of competition between species.”

Nature’s Calendar has also provided evidence that spring is moving faster from south to north every year. In 2015 spring took nearly three weeks to travel from the far south west of England to the far north east of Scotland, travelling at about three kilometres an hour. Between 1891 and 1947, it moved north at 1.9 kilometres an hour.

Nature’s Calendar is coordinated by the conservation charity, Woodland Trust. “There is no doubt that over the last 40 years spring has been arriving earlier,” said the trust’s spokesman in Scotland, George Anderson.

“We believe climate change is the biggest single threat to what little remains of our ancient woodland heritage. The more data we can collect, the more evidence we will have that climate change is a problem, as there are still those who need convincing of that.”

The more information the trust gathered, the better it could also manage the impacts of climate change, Anderson argued. “Our Nature’s Calendar volunteers make a valuable contribution to understanding climate change and to protecting species from it.”

A naturalist in the Cairngorms, Roy Turnbull, has been charting changes at Nethy Bridge in Strathspey since the 1980s. There, average March temperatures have been rising by 0.7 degrees a decade, he said.

Frogs are spawning 1.3 days sooner every ten years, while bumblebees are appearing 1.3 days earlier and wood anemone flowering two days earlier. Turnbull has seen silver birch leaves growing 7.4 days sooner every decade, and the first daffodils 4.8 days earlier.

Friends of the Earth Scotland highlighted the need to do more to cut carbon pollution. “Nature is sending us very clear signals that the climate is changing and that major disruption is on the way,” said the environmental group’s director, Dr Richard Dixon.

“No one can seriously deny there is a big problem. We need to work much harder to reduce climate change emissions from factories, traffic, farming and homes if we are to save our precious wildlife and, ultimately, our own society.”

The government’s wildlife agency, Scottish Natural Heritage, agreed that climate change was having an effect. “So far, there is evidence of impacts of reduced mountain snow cover on montane plants and of temperature increases on montane butterflies,” said the agency’s climate change manager, Mary Christie.

“It’s very difficult to predict how the many species across the country will be affected – some may actually benefit, but many may have trouble adapting to changing conditions. Some new species will appear and other existing species disappear in Scotland as temperatures rise.”

12 signs Spring is coming sooner

swallow: arrives 20 days earlier than in the 1970s

sand martin: arrives 25 days earlier than in the 1970s

orange tip butterfly: appears 13 days earlier than a century ago

hawthorn: flowers 12 days earlier than a century ago

horse chestnut tree: flowers eight days earlier than a century ago

daffodil: flowers five days earlier than ten years ago

queen wasp: appears six days earlier than a century ago

perch: spawn earlier than they used to

plants: appear 10 days earlier than in 1970

insects: appear two weeks earlier than in 1970

frogs: spawn five days earlier with each degree rise in temperature

butterflies: emerge six days earlier with each degree rise in temperature