Why a salmon, a horse chestnut and a little old lady in Surrey are the difference between... ...a bank holiday weekend like this, or like that

For the past 50 years or so Jean, a spinster in her seventies who lives in Surrey, has documented a few simple facts about the countryside around her. In particular, she records the dates when four species of tree - oak, ash, horse-chestnut and lime - come into leaf. It's the sort of trivial task that machines aren't much good at, but which humans enjoy. She started doing it as part of a diary. Then she kept on doing it. For decades.

Now, Jean's data is precious, because scientists need it. Her full name and location are closely protected secrets at the Institute of Terrestrial Ecology (ITE), known only to a few scientists who are working on one of its simplest yet most important projects: collating the data to see whether global warming has started to affect British life.

Jean is not alone: the ITE co-ordinates a network of about 500 unpaid volunteers around the country who monitor nature and see how it is changing. Mavis Jones, for example, is a pensioner living in Hillingdon; for the past 10 years she has noted the arrival dates of migratory birds such as swallows and the hatching dates for blackbirds. Has it changed? "It's earlier than it used to be," she says. "Sometimes by days, sometimes weeks." She doesn't have much doubt about the reason. "Oh, it's this so-called global warming."

The results of the work will be announced next month by the Department of the Environment. And though the scientists collating the data are sworn to secrecy, there is little doubt that the 35 disparate pieces of information being monitored - on such topics as tree leafing dates, the arrival dates of swallows in spring, the number of days when skiing is possible in Scotland, and the number of cases of Lyme disease (caused by a tick that flourishes in warm weather) - will indicate that yes, Britain's climate is changing. Spring is happening earlier, birds are taking different migratory routes, "extreme" events such as floods are happening more frequently, average air temperatures are higher, underground chalk reservoirs are drier, and more of the coastline is collapsing into the sea.

To a scientist, the suggestion that the climate is changing presages trouble: more severe weather, such as hurricanes and tornadoes, higher sea levels, changes in biodiversity. But the problem that researchers and policy-makers face is: how do you persuade the man sitting in the car behind the Clapham omnibus that global warming is happening, and that he is contributing to it by driving rather than walking? And how do you persuade him to change his behaviour as a result?

"The only way you'd get people to notice global warming is if it raised the price of beer," comments Tim Sparks, one of the team at the ITE who has been analysing the data for next month's report.

In fact, we are already seeing the price of another liquid - vehicle fuel - rising faster than inflation, through the Government's "fuel duty escalator", whose purpose is claimed to be to encourage people to use less fuel. That has led to the lorry drivers' protests. But the row there is not over whether lorries contribute to the greenhouse effect. The drivers are just angry at the price. The Government certainly hasn't used carbon dioxide emissions as a justification for the duty hike.

Similarly, earlier this month the Government launched a witty ad campaign encouraging us to "do our bit" by recycling rubbish and bottles, sharing car journeys, taking showers not baths, walking instead of driving. Famous names such as George Best, Chris Evans and Ian McCaskill appear in the ads, yet phrases like "global warming" certainly don't. "Clean air", perhaps. "Recycle", certainly. But not the planet. The activists' phrase of a few years ago was "Think global, act local". The Government seems to prefer leaving the first bit out.

Yet there are a few groups of men and women (though perhaps not in the street) who have an inkling that global warming could affect them, and not positively.

The number of "spring salmon" - the large fish that return to spawn after spending more than a year in the sea - has falling by almost 40 per cent in the past five years. As a result, the Environment Agency (which issues salmon fishing licences) has banned the retention of salmon caught in the first five months of the year.

The agency blames changes in ocean currents and temperatures, which in turn affects the spawning and survival rates of the young fish. But, as one of the fisheries officers notes, "There's not much we can do about ocean currents." So it is the would-be fishermen and women who lose out.

Far from the riverbank, even mountaineers have discovered that global warming takes its toll. Last weekend, a husband and wife were descending from the summit of Mt Hood in Oregon, on the west coast of the US. The mountain is only one-third as high as Everest, but Oregon was experiencing temperatures which leapt above 30C for the first time this year. As the pair walked down, they slipped in mushy snow and fell more than 1,000ft to their deaths. In the Alps, summer climbers often note that ice and snow become mushy, and hence dangerous, earlier in the day. It is a subtle change - perhaps only a difference of a quarter of an hour - but one that old hands might note.

The trouble with evidence like that, from a scientist's viewpoint, is that it is anecdotal and short-term. "You need a long series of data to detect climate change," says Dr Sparks. "The man in the street doesn't care about statistical significance, but the scientific community needs that to be convinced." Hence the network of 500 or so "phenologists" (people who measure change) watching nature and passing the information back to the study for the Government. Data like Jean's - which covers a long time series - is especially valuable.

Another network member, Ann King, lives in Norfolk: for the past 10 years or so she has had a rivalry with her father, trying to be the first to hear a cuckoo or spot a swallow. She joined the study network because "I thought it would be interesting - you know, formalise my competition with my father".

Now she in turn hopes to recruit her former employer, a man with a large garden who has fastidiously kept daily rainfall records for the past 20 years. People like him are immensely valuable - especially when formal organisations are cutting back; the Nature Conservancy Council, for example, has reduced its monitoring of butterflies, a subtle indicator of environmental health, owing to funding cuts. People like Mrs King may have to fill the gap.

Even so, as people contemplate a spring that arrives about a week earlier than 20 or 30 years ago, and generally warmer bank holidays, the tendency is to think that there isn't really a downside to global warming; that a global rise in temperature of a few tenths of a degree won't make any difference to our daily lives.

And, indeed, how would you notice the effects of global warming in daily life? What, if anything, would be bad about them? Asked this, many scientists who are happy to talk about ice shelves in Antarctica and gas circulation in the upper atmosphere become stymied.

"There could be increased frequency of precipitation," says Dr John Mitchell, head of climate change modelling at the Meteorological Office's Hadley Centre in Bracknell, Berkshire.

You mean, it would rain more often? "Well, yes." For Dr Mitchell, who has been developing computer models of the entire globe, it is hard to see global warming on an individual's scale.

"In 50 years we could see a warming of 1.5C across the UK," he explains. "Some people think that's just like moving further south, but that's not accurate because the warmth there is due to the position of the sun in the sky, not the general air temperature. From day to day you wouldn't really notice it, because the weather generally is so variable."

Instead, Dr Mitchell deals in probabilities, chances, likelihoods. "A temperature rise like that would reduce the chance of frost by about 50 per cent. Gardeners wouldn't have much to worry about. But there would be 25 per cent more days when the temperature was over 25C." (That is, of course, at the Met Office, rather than John O'Groats.) It would be nice (and certainly convenient) if this were certain, but it isn't. People such as Jean, Mrs Jones and Mrs King can already see the subtle effects: the earlier leafing of trees, different birds, fewer ladybirds.

But as climate change becomes established, some changes could be dramatic. It's possible, for example, that malaria could become endemic in the south east of England. Alternatively, the whole of Britain could turn as cold and icy as Newfoundland on the Canadian coast. The two locations lie on the same latitude; our climate is warmer because of the Gulf Stream in the Atlantic. But some computer models suggest that as the seas warm, the complex system of currents that brings warm water up from the Equator could switch off, plunging us into freezing winters and miserable summers. The trouble is that we don't know for sure, and we don't know when we will know for sure.

But what else might there be? More cases of heat stroke? "With higher sea levels, you would expect more coastal erosion and more frequent severe flooding. But we won't notice it happening quickly. Global warming is one of those things that's subtle but, in the long term, could have massive consequences," says Dr Mitchell.

This is the essential problem with trying to persuade the public about global warming. It is what scientists call the "frog in a saucepan" phenomenon. Drop a frog into hot water, and it will jump out. Drop it into lukewarm water which you then heat, and it will stay, oblivious to changes that are killing it. In this scenario, we are the frogs, blissfully lounging in the ever-heating tub. The question is: what will it really take to make us jump?