When an international team of researchers announced in January that climate change could kill a quarter of the species of plants and creatures currently sharing the planet with us, it was a figure so stark that the issue of global warming made headlines around the world.

Scientists had studied six different regions and had constructed computer models to investigate how more than 1,100 different species would survive the temperature changes that will accompany global warming. “If the projections can be extrapolated globally, and to other groups of land animals and plants, our analyses suggest that well over a million species could be threatened with extinction as a result of climate change,” said the lead author of the research, Chris Thomas of the University of Leeds (see end of article for details).

Varieties of flowers in Africa, lizards in Australia and birds in Scotland could be swept away by a climate change triggered by the emissions of “greenhouse gases” into the atmosphere. And the time-scale is not a distant point on the horizon; we could have extinguished many types of life on Earth by 2050.

As a mid-range forecast, researchers found that the effect of climate changes would spell the end for 25 per cent of species - rising to 37 per cent on a more pessimistic projection - and the damage already in progress would lead to the extinction of 18 per cent of species.

Is this apocalyptic vision really going to happen? Is there a scientific consensus? And even if we wanted to limit global warming, is there any realistic chance of international co-operation?

Environmental campaigners in Greenpeace are in no doubt about the extent of the danger, identifying climate change as the biggest single environmental threat to the planet. But spokesperson Ben Stewart says that the scale of the problem is such that people can feel overwhelmed - frozen in the headlights as we watch the juggernaut hurtling nearer: “If we were on another planet - and saw what was going to happen on Earth, you’d think that people would be demanding a re-think about how we behave. But it seems as though, even when people accept that there is a huge problem, we don’t feel we have the ability to do anything.”

But Ben Stewart believes that the message is getting through - and that the international decision taken at the Kyoto conference in 1992 to cut emissions was a cause for optimism, even though the USGovernment has not ratified the deal. If action is taken to cut greenhouse emissions, he says, it won’t be a moment too soon, as the ill-effects are already being felt - and the clock is ticking for even greater damage. He quotes research from the World Health Organisation, published at a conference on climate change last year, which claimed that global warming was already causing the deaths of tens of thousands of people, through destruction of agriculture and the spread of diseases. He also raises the spectre of “climate refugees”, where large numbers of people will be pushed out of their own land because of global warming and will have to look for asylum elsewhere.

But if we’re looking for a reason why there has not been urgent and immediate action to stop such a nightmare vision of the future, then we have to take a step back and see that the arguments on global warming are not cast in black and white. Scientists are far from unanimous about the cause or ultimate effect of global warming. There are also more immediate economic arguments over cutting emissions, that are being held in the balance against scientific forecasts. And if it’s a question of losing jobs tomorrow, or worrying about something that might happen in 50 years, then politicians are under pressure to act in the short-term.

In Britain, Joe Buchdahl, co-ordinator of the Atmosphere, Climate and Environment Information Programme, puts forward the mainstream “official” view. The project, based at Manchester Metropolitan University, is supported by the Department for Environment, Food and Rural Affairs (Defra) to present unbiased and balanced information about global warming.

And Joe Buchdahl says that there is now a “balance of evidence” which confirms the climate is changing - and that, increasingly, it is believed that there is a “human fingerprint” on these changes.

The British government reported to an international conference in Milan last December its belief that global temperature rises “cannot be explained by natural factors” and the evidence suggests that “increasing greenhouse gas levels due to human activities are largely responsible”. As a result, it concluded that Britain in the short term is likely to face hotter, drier summers and warmer, wetter winters. In the long term (perhaps within 200 years) it is predicted the effect will be quite the opposite - global warming may prematurely usher in a new ice age for the UK.

The British Isles draws its geographically misplaced warmth from the Gulf Stream, a “conveyer belt” for warm water and consequently humid air, which originates in the Gulf of Mexico and passes through the North Atlantic. The Gulf Stream relies on a strong undercurrent of cold water - Ja “pump” - cooled by the polar ice cap.

Global warming is causing this continent-sized body of ice to melt, pouring massive amounts of fresh water into the North Atlantic. This weakens the Gulf Stream and, some scientists predict, will push it south and out of Britain’s waters. No warm water would mean, while the rest of the world bakes, the region around the British Isles will literally freeze.

Temperatures could plummet three to five degrees cooler than today. To put this in context, 15,000 years ago, during the last ice age, temperatures were also five degrees cooler than today.

But all of this doesn’t necessarily mean that global warming will have the cataclysmic consequences that some scientists have suggested, says Joe Buchdahl. And he says that best-guesses should not be confused with something that will definitely happen. We don’t know how well or badly humans and other species would adapt to changed climates, he says, and there are too many unpredictable factors to be certain about where global warming might lead.

“Many people want it to be like a weather forecast, where you can say with some accuracy what is likely to happen over the next few days. But it isn’t the same where you’re trying to say what will happen to the climate over the next hundred years,” he says.

This is an international argument - and he says that we have to take seriously how the debate is perceived from other countries. If global emissions are to be cut, then the role of the US as the world’s biggest economy will be vital. And in the US there are powerful lobbyists, such as from within the oil industry, who suggest that threats of global warming are overstated and that any hurried action could damage the economy without delivering any proven benefit.

“The debate has been polarised - and both sides have tended to exaggerate and use the scientific evidence that suits their arguments,” says Joe Buchdahl. “You have to look at what else their agenda might be.”

There are also lobbyists in the US who argue that, rather than cutting emissions, the answer lies in developing cleaner technology - and Joe Buchdahl says this should not be dismissed as just a way of putting off immediate action against pollution. In the long run, we’ll need such cleaner technology if a global solution is to be found, particularly for emerging economic giants such as China: “Is China going to take lectures from the West about limiting their economic growth? Are they going to accept that they can’t use their own coal reserves because we’re worried about global warming? We need to be able to offer them a cleaner alternative.”

But if there is growing agreement that the climate is changing, the next big question is how we should respond. What actions should we take to either delay or to offset the effects? This is the challenge being taken up by a network of 200 academics at various British universities, under the collective banner of the Tyndall Centre for Climate Change Research. Based at the University of East Anglia, and with research in other universities, these academics are working on scientific responses to global warming.

Asher Minns, speaking on behalf of the centre, says that as well as trying to cut emissions, we should be looking at how we are going to adapt to a different type of climate. For instance, if the sea level is going to rise, which parts of the coastline do we want to protect, and which will we abandon? In a relatively affluent country such as Britain, such adaptations can be made and cities beside the sea can be protected. But he says there needs to be advance planning for countries that will be much more vulnerable, such as Bangladesh, where vast numbers of people will be susceptible to sea-level changes.

If there are going to be more violent storms, what will it mean for developed and non-developed countries? Who will pick up the bill for the damage? Who will look after the homeless? What will happen to people living on low-lying islands which become uninhabitable?

There will also be adaptations to be made by agriculture, he says. A warmer climate in Britain could mean a longer growing season and for some produce, the prospect of two crops a year rather than one. But on the downside, milder weather will mean that pests and diseases are not killed off by frost in winter.

But Asher Minns is in no doubt that such climate change is taking place - and that we have to face up to making decisions about a response. Even if greenhouse emissions stopped tomorrow, he says, the heat-trapping gases already in the atmosphere are enough to keep the global temperature rising.

And just a couple of decades ago the Thames Barrier was needed only once a year - now it is perhaps six times a year.

In the past century, there has been an increase in temperature of about 0.6C. The climate change centre is looking at scenarios of a rise of two degrees over the next few decades. It might not sound much, he says, but in climate terms that is a very substantial change.

So where has the argument over global warming reached? In a relatively short space of time, it has become an internationally recognised issue. In Britain, it was only at the end of the 1980s that the topic was first brought to public attention, in a speech by Margaret Thatcher. Since then, there has been a flurry of scientific and governmental activity, with a growing number of global conferences and committees set up to monitor and construct a response.

But a consensus on the extent of the likely impact or causes of global warming remains elusive. The US, the biggest producer of greenhouse gases, remains the strongest sceptic when it comes to adopting an internationally enforced process for reducing emissions. What is certain is that, whatever happens, the heat won’t be going out of this argument for many years to come.

Full report “Extinction Risk from Climate Change”, by Chris Thomas et alwww.nature.comnaturelinks040108040108-1.html

European Environment Agency reportreports.eea.eu.intenvironmental_issue_report_2003_36en

Intergovernmental Panel for Climate Changewww.ipcc.ch

United Nations Environment Programmewww.unep.orgthemesatmosphere

Worldwide Fund for Nature climate change programmewww.panda.orgabout_wwfwhat_we_doclimate_changeindex.cfm

Kyoto Protocol unfccc.intresourceconvkp.html

Tyndall Centre for Climate Change Researchwww.tyndall.ac.uk

Early warning “hot spots” global map www.climatehotmap.orgabout.html

United States Environmental Protection Agency yosemite.epa.govoarglobalwarming.nsfcontentClimate.html

Environment al News Network www.enn.comindepthwarmingindex.aspC02 Science Magazine www.co2science.org

Meteorological office www.met-office.gov.uk

Department for Environment, Food and Rural Affairs www.defra.gov.uk

HAVING AN IMPACT

While individual long hot summers cannot be taken as evidence of any broader trend, the summer of 2003, which was Britain’s sunniest on record, is part of a pattern which has seen a concentration of hot years in the late 1990s and the first years of this century.

Last year had the highest-ever recorded temperature in Britain, reaching 38.5C on August 10 in Kent. But it was only the fifth hottest year overall, behind 1949, 1990, 1999 and 2002.

These rankings by the Meteorological Office are based on temperature readings since 1659, and they show that six out of the seven warmest years in Britain have been since 1990.

Global climate records show a similar picture, with the 10 warmest years on record all occurring since 1990.

Met Office scientist David Parker draws the conclusion that“the warming over the past 50 years is mainly due to human activities, such as the burning of fossil fuels”.

If we’re looking to how our own climate might change in the course of this century, the Met Office’s Hadley Centre for Climate Prediction and Research has computer models showing that the summer months in southern Britain could be 3-5C warmer than at present, with the whole country increasing its annual average temperature by 2-3C.

What is our record on cutting emissions?

In terms of international efforts, the most important agreement has been the Kyoto Protocol, adopted by more than 100 countries since 1998, which seeks to establish specific targets for reducing greenhouse gas emissions.

The countries which have ratified the protocol represent 44 per cent of the industrialised world’s output of greenhouse gases, including Britain’s 4.3 per cent. If these countries can achieve reductions, it will make an impact on global emissions.

But the US, which produces more than a third of all greenhouse gas emissions from industrialised countries, has not ratified the agreement, and neither has Russia. The Kyoto deal has been pushed in particular by the European Union, which collectively produces about a quarter of greenhouse gases. Under the agreement, there are six gases to be reduced, carbon dioxide, nitrous oxide, methane, and three fluorinated industrial gases.

According to a report from the European Environment Agency in December, Britain has been among the most successful of the EUcountries in cutting emissions, and is set to meet its reduction targets.

But the less optimistic news was that the EU overall was failing to meet its targets, with an increase in road traffic blamed.

The EU, along with other Kyoto signatories, is committed to cutting emissions to below the levels of 1990, but the interim report says that current projections show that so far it is unlikely to reach the agreed levels by the target dates of 2008 to 2012.

Are species already disappearing?

Global warming could already be claiming its first victims among mammals, say environmental campaigners the WWF - and it shows how in future other species could be put under pressure by climate change.

Last summer, the WWF reported that the American pika, a relative of the rabbit, was facing extinction in parts of North America due to global warming. The pika is a round, hamster-sized creature that lives in cool mountain areas. But changes in climate have threatened its mountain habitat - and while other types of animal have headed higher and northwards to find a cooler environment, the pika has not. Scientists monitoring its population have found a diminishing number, to the extent that the pika is facing “local extinctions”.

The WWF has also published an extensive report on what it claims are examples of how climate change is already threatening wildlife in national parks and protected areas around the globe. These include concerns about penguins threatened by disappearing sea-ice habitat, and wildlife reserves in the US that are being flooded by the rising sea level.

WHAT IS GLOBAL WARMING?

Global warming is a natural occurrence, as are greenhouse gases (primarily carbon dioxide, methane and nitrous oxide). Without this natural phenomenon, the Earth would be in a permanent ice age with temperatures up to 30C less than current levels. Industrialisation and the burning of fossil fuels has accelerated “the greenhouse effect” because the Earth’s polluted atmosphere acts like a huge greenhouse, sealing in the heat and reducing the natural cooling process. Instead of the Sun’s warmth passing down to Earth and then bouncing back into space, higher than natural levels stay trapped in the atmosphere.

It is claimed that concentrations of carbon dioxide in the atmosphere have risen by almost 30 per cent since the beginning of the Industrial Revolution. A global monitoring organisation, the Intergovernmental Panel on Climate Change (IPCC), has been created to report on what it believes to be the best scientific evidence. This has accepted that the “Earth’s climate system has demonstrably changed since the pre-industrial era, with some of these changes attributable to human activities”.

This deliberately neutral acceptance of the idea of global warming is underscored with evidence, including that the temperature increases in the 20th-century were greater than in any century in the past millennium and that the 1990s represented the warmest decade of the millennium.

But what does it mean if the world’s temperature is turned up a couple of notches? The IPCC data says that the 20th-century saw sea-levels rising by 1-2mm each year.

Other observable changes included ice cover on rivers being cut by two weeks during winter; the thinning of ice cover in the Arctic; 10 per cent less snow cover in the northern hemisphere and the retreat of glaciers. The shape of seasons has already been affected, says the IPCC. In the northern hemisphere, the growing season for crops has lengthened, flowers appear earlier, birds return earlier and their breeding patterns are altering.

All of these changes are believed to result from an average temperature change of less than 1C over the past century. The IPCC projects temperature rises for the next century to be 1.4C to 5.8C, depending on the levels of greenhouse gas emissions. Within this range, the forecast shows that, in Britain, winter rainfall may rise by as much as 20 per cent, and summers become hotter and drier. But Central America and southern Africa could face much drier winters, putting pressure on human populations.

Even if greenhouse gas emissions are stopped immediately, accelerated global warming is already under way, and existing levels of gases - Jsome of which are suspended in the atmosphere for hundreds of years - will mean an ongoing rise in temperature.

With climate changes on the way, we cannot be sure at what point they will cross a point of no return. While a tree might adapt to more arid conditions over several decades, coral could be destroyed by a change in temperature in a single year.

CONFLICTING THEORIES

Global warming and forecasts of climate change driven by human activity are not accepted by all scientists or governments. A range of sceptics cast doubt on the campaigners who say that cutting greenhouse gas emissions is necessary to prevent further environmental damage. Perhaps the most strategically important sceptics are those who approach the subject from an economic as well as a scientific perspective.

In the USin particular, there are influential voices who say that decisions about cutting are premature - and that there is still too little clear-cut evidence of a link between industrial emissions and climate change.

They argue that there are contradictions in the evidence that have still to be resolved. For instance, they claim that there are discrepancies between the temperature data from satellites and from ground stations. One theory is that the apparent increase in surface temperatures is caused by measuring equipment being too near cities, which are warmer than surrounding areas and which can distort readings.

Another theory is that changes in temperature are linked to solar activity - and that this is the driver of climate change.

Industrial lobbyists say that when there is insufficient scientific certainty about global warming, why should they be forced into expensive changes, or face restrictive taxes, which could threaten profits and jobs.

As well as the impact on industry, adopting a greener approach would have consequences for ordinary Americans. There would be little political mileage in depriving Americans of access to cheap fuel and gas-guzzling cars.

There have been claims that reducing emissions as proposed in the Kyoto agreement would push up the price of petrol and cost the US economy hundreds of billions of dollars.

There seems to be a division between the US and Europe. While there is a growing orthodoxy in Europe that greenhouse gas emissions need to be tackled, there are think-tanks and politicians in the US who say that it will take another 25 years before the clear picture on global warming emerges.

As the chairman of the US Senate’s environment committee, James Inhofe, said in January, reiterating his opposition to Kyoto-style targets, there is “more evidence, emerging all the time, that the theory of catastrophic global warming, and policies designed to combat it, are not based on objective science”.

There are also environmental campaigners in the US, and prominent politicians in both the Democrat and Republican camps, who support action against global warming, but the Bush administration is not at present signing up to any compulsory cuts in emissions.