The warning signs that spring has sprung

Bumblebees in December, swallows in February. Steve Connor looks at how a network of thousands of amateur phenologists - volunteers who note the times of recurring natural phenomena - has tracked the effects of climate change on the British landscape

On 23 December last year a bumblebee flew into the garden of a man living in Isleworth, Middlesex. The following day, another bumblebee was spotted by a woman in Exeter and on Christmas Day someone saw one floating around central London in search of a flower.

On 23 December last year a bumblebee flew into the garden of a man living in Isleworth, Middlesex. The following day, another bumblebee was spotted by a woman in Exeter and on Christmas Day someone saw one floating around central London in search of a flower.

Bumblebees - which are normally associated with lazy, hot summer days - usually hibernate through the winter months, emerging from their nests in the first warm days of March. But something has happened to upset their alarm clock this year - and in previous years.

There were five sightings of bumblebees in December in England and a further 56 sightings in January. In Scotland there was the earliest ever recording of a bumblebee when one was seen flying in the first days of February.

It wasn't always this way. Back in the 1920s no one had ever recorded seeing a bumblebee earlier than in February, even in southern England. It was only in the 1990s that people began to see them in January and now the wake-up time of the bee has moved to December.

The early appearance of bumblebees - the busy pollinators of the garden - is just one illustration among many of how spring has come earlier and earlier as climate change begins to exert a noticeable effect on the British landscape.

Trees are coming into leaf sooner, migrant birds are arriving earlier, frogs are spawning ahead of their usual time and butterflies are flying weeks earlier than normal. It is all direct and visible evidence of climate change, said Sir David King, the Government's chief scientific adviser.

"Farmers are now planting their crops up to three weeks earlier than they were 10 to 15 years ago. This change has been quite dramatic in the past decade," Sir David said yesterday at the launch of National Science Week.

"In terms of the vast amount of evidence that's coming through on climate change, this is one important part of it," he said.

The 20th century saw a steady increase in average global temperatures, a rise which has continued into this century with the hottest years on record.

In Britain during the 1960s the average temperature for the three months of January, February and March was 4.2C, compared to 5.6C for the same period in the 1990s.

This year has seen a milder-than-normal winter. The recent cold snap was, in fact, a reversion to normal temperatures, rather than something unusual, according to Jill Atten- borough of the Woodland Trust, the organisation which is helping to monitor the changing face of the seasons in Britain.

"Spring was really galloping away at the start of the year. Then, in the middle of February, we had a return to normal winter temperatures. We called it a cold snap but it was actually return to normality," Ms Attenborough said.

"That slowed the pace of spring down a little bit. We had reports from people saying that frog spawn was frozen in their garden ponds," she said. "That's a very clear indication of the impact that warmer temperatures and then a return to normal weather conditions can have on something like a frog, which only makes one breeding attempt every year.

One of the problems of spring moving earlier in the calendar is that some species - such as frogs - can be badly caught out when the weather suddenly reverts to type.

"If frog spawn gets frozen the likelihood is that it will be killed and that's the breeding attempt finished for the year," Ms Attenborough added.

The Woodland Trust, working with the British Association for the Advancement of Science and the Centre of Ecology and Hydrology, has set up a network of some 13,500 volunteers throughout the country who keep regular records of the earliest indications of spring, such as the first cuckoo call or the earliest blackthorn blossom.

Phenology - the study of the times of recurring natural phenomena - has a strong tradition in Britain. It began with Robert Marsham, a naturalist who started recording his "Indications of Spring" in 1736.

Then Gilbert White, the author of the Natural History of Selborne, recorded the arrival of summer migrants, noting that swallows arrived on 13 April, swifts by about 27 April and the spotted flycatcher on 12 May.

All three species are now routinely seen weeks earlier, thanks to the work an army of enthusiastic amateurs who have continued in Marsham and White's footsteps.

Over the years they have now built up vast amounts of information on when the first signs of spring are and have produced evidence that the seasons are changing.

Jean Combes, for instance, began to record the times when buds on her local oak trees opened into leaf in 1947. Her detailed records now cover the entire second half of the 20th century.

Mary Manning, another amateur phenologist, began recording natural events in her Norfolk garden in 1965 and has subsequently produced invaluable data on the timing of spring events, said Dr Tim Sparks of the Centre for Ecology and Hydrology.

"Although restricted to her garden, Mary Manning's recording over the past 40 years has filled in an important gap. It is the consistency rather than the scope of the recording that really matters," Dr Sparks said.

Anne Phillips from Walsall has kept diaries of 39 annual events since the 1930s - 27 of them now show a response to the warmer temperatures in Britain today.

Mrs Phillips has shown, for instance, the leaves of horse chestnut and silver birch trees are now opening on average 12 days earlier than when she first started recording, blackbirds are nesting 14 days earlier, and frog spawn and tadpoles are appearing 12 days earlier.

Sometimes the annual appearance of a natural phenomenon can coincide with an important event in someone's life, eventually causing the natural event to move out of synchrony with the calendar. Christine d'Albert from Herefordshire, for example, can recall how her mother used to bake a cake for her daughter's birthday on 22 May and poked bluebells through drinking straws inserted among the candles as an added decoration.

"My memory goes back to 1944 and my mother's practice continued into the early 1950s. In recent years I have watched bluebells flowering much earlier - more like 22 April and as early as 19 April in 2001," Mrs d'Albert said.

Over the past six years the UK Phenology Network has amassed a huge amount of data from the thousands of volunteers who have faithfully recorded changes they have witnessed to the dates of the first signs of spring.

"Even before Christmas we had reports of snowdrops and frog spawn appearing earlier than in previous years in the south west of England and people were having to mow their lawns through the festive period," said a spokesman for the network. "On Christmas Day daffodils were seen flowering in Cornwall and snowdrops were seen in Gloucestershire," he said.

By 5 January, the network had received 24 observations of flowering primroses, 15 of snowdrops, one of frog spawn and 72 people had cut their lawns in December, including on Boxing Day.

Ms Attenborough explained: "It is a stark example of the wonderful thing about monitoring the science of spring and having people of all ages trying to do it wherever they are. It brings climate change into your garden and people can see for themselves the impact it is having on nature."

Sir David said that climate change is melting mountain glaciers, raising sea levels and creating the conditions where more extreme weather events, such as storms and floods, are more likely to occur.

"The Thames Barrier was originally designed to be used once every five years. Now we use it an average of six to 10 times a year," Sir David said.

"We forget that the Thames Barrier is there to protect us. It's a very difficult exercise to say we've got to invest now to prevent a risk that is going to occur in the future," he added.

"But in terms of climate change we have a global problem that requires global action at a political level that we've never seen before. This is a major political challenge," Sir David said.

Changing Climates

PERMAFROST

The temperature in Alaska is rising almost 10 times faster than the world average, forcing the Innuit to abandon traditional ways of life as the ice melts. In the European Alps, disappearing permafrost is threatening landslides as underground temperatures there rise three times faster than last century.

MOUNTAIN GLACIERS

With its glacier on the equator, Mount Kilimanjaro is one of Africa's most beguiling sights. But the mountain's ice fields could completely melt away in the next 20 years. Research mapping has shown that 33% of its ice has disappeared in the last two decades - 82% has gone since 1912.

RISING SEA LEVELS

Low-lying areas are threatened with ecological and human devastation. In the Sunderbans, the 20,000 sq km forest delta in West Bengal, satellite imagery shows the sea level has risen by an average of 3.14cm a year in the past two decades - much higher than the global average of 2mm a year.

WILD WEATHER

During the past decade, El Nino - the name for a pattern of weather generated by warming in the Pacific - has wreaked havoc in several parts of the world, causing floods in some areas and droughts in others. Global warming is also blamed for freak storms throughout Europe, including Britain.