MANY PEOPLE FIND it hard to believe that a small animal species - our own - could change anything as vast as the sky and the atmosphere. It was only recently that the relationship between the quantity of greenhouse gases in the atmosphere and average global surface temperature was fully established. Some still want to deny this, but there is now a commanding consensus that human activities have a significant effect on the planet.

The root cause lies in the way we produce and use energy. Energy is at the core of human activities and their impact on the natural world. It has contributed to the enormous changes on the surface of the Earth since the last ice age: clearance of forests, agriculture, settled communities and cities, exploitation of resources, and loss of biodiversity. The human species continues to multiply with demands for yet more energy, and an industrial society based on fossil fuels remains the ideal to which nearly all humans aspire.

How we generate energy in the future is fundamental to humanity's existence. Yet climate change is not the only threat: we have to reckon with the effects of population increase, degradation of land and accumulation of wastes, water pollution and supply, and destruction of biodiversity. Then there is the way in which we treat each other. There is a widening gap between the world's rich and the world's poor, and a disproportionate consumption of the Earth's resources.

All these issues are interlinked: all relate to energy and climate, and all concern the future of humanity. We are pushing Gaia beyond her limits. Of all the ways in which we are doing so, the most important is climate change. At present the trend is unquestionably towards global warming, but the biggest danger in an overcrowded world is change itself, which by all reckonings is accelerating. On a global scale the effects include different patterns of rainfall and drought, more extreme events along the boundaries between climatic zones, and changes in ocean currents and sea levels. All natural ecosystems will be affected as living organisms try to adapt themselves to new circumstances.

There are also two jokers in the pack: there is a possibility that an increase in fresh water in the North Atlantic and Arctic oceans may weaken the Gulf Stream that brings warmth to Western Europe. There are indications that this process may already have started. Thus warming could produce cooling. The other possibility is that the melting of tundra could release vast quantities of methane and methane hydrate. These are powerful greenhouse gases, and global temperatures could rise as a result.

NOT SURPRISINGLY THE scale of the disruptions that climate change could bring has brought the world together as no other environmental issue has. Governments have realised the need to work together. There were the World Climate Conferences of 1979 and 1990. Following the report of the Bruntland Commission on Environment and Development in 1987, the Intergovernmental Panel on Climate Change was set up. It has since reported in 1990, 1995 and 2001, and its next report is due in 2007. One of the main achievements of the UN Summit Conference on Environment and Development at Rio in 1992 was the Framework Convention on Climate Change. Its objective was - and is - to stabilise "greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system".

How this should be done has been discussed at nine successive meetings at the Parties to the Convention. Recent meetings have concentrated on the implementation of the Kyoto Protocol agreed in December 1997. This was never more than a modest beginning. It committed thirty-eight industrial countries, including the United States, to a global carbon dioxide emissions reduction target of 5.2% between 2008 and 2012. Since then the United States has refused to ratify the Protocol.

Until recently the rest of the world, including India and China, regarded the problem as one for industrial countries. But increasingly such countries have realised how much their own future welfare is involved. China, with its massive population, is soon likely to overtake the United States as the world's largest carbon emitter. In fact the Chinese, unlike the Americans, have actually reduced their carbon emissions in real terms over the last five years as a product of the reorganisation of Chinese industry and attempts to reduce their dependence on fossil fuels.

All this may look positive. But even if the Kyoto commitments are met, greenhouse-gas emissions would still be some thirty per cent up on 1990 by 2010. But none of this has been accepted by the biggest polluter of all, the United States: with less than five per cent of the world's population it emits around twenty-four per cent of global greenhouse gases. With American society still based on cheap energy and with vested interests so close to the current US Administration, it is no surprise that President Bush has refused to ratify the Protocol. Nevertheless a combination of several individual US states, both Republican and Democrat, have taken steps to reduce carbon emissions and improve energy efficiency.

The European Union countries, including Britain, ratified the Protocol in New York in May 2002. The Union now has an overall emissions target of eight per cent below 1990 levels for the period between 2008 and 2012. The British government has decided to do better still and has adopted a legally binding target of twelve and a half per cent. Since then, as global warming seems to be proceeding faster than expected, the government has also adopted a voluntary target of a twenty-three per cent reduction by 2010. More recently it has set itself a still more ambitious target in line with the recommendations of the Royal Commission on Environmental Pollution: a sixty per cent reduction by 2050.

SO WHERE DOES all this leave energy policy in Britain and elsewhere? How can these various targets be met? There is a two-pronged approach. The first is to mitigate the effects of the use of fossil fuels, and the second is to promote alternative sources of energy. Obviously the two are closely linked, and involve almost all sectors of the economy. In both there is conflict between a powerful array of private interests and the public interest, and between the short term and the long term. This makes for very tricky politics within countries as well as between them.

It is already clear that reserves of fossil fuels are limited. As reserves diminish, so prices will rise and the incentive to look for other sources of energy and improve efficiency will increase. There is no way in which the world economy could rapidly switch from fossil fuels; hence the new interest in technologies to mitigate their effects. They range from coal gasification to geological sequestration of carbon dioxide.

Technology is vital but can be double-edged following the law of unintended consequences. The story of chlorofluorocarbons (CFCs) is a case in point. Initially thought harmless, they were later identified as a major cause of ozone depletion and a powerful greenhouse gas. But it does not mean that we should not take the high- rather than the low-technology route in the future.

How best to promote the switch to alternative sources of energy is equally difficult. Here governments have the major responsibility. The British government has made some efforts in the right direction. In 2003 it published an Energy White Paper to lay out policy for the next half-century. It was notably coy about how targets were to be reached, and generally fudged the nuclear issue. It also contained inconsistencies. Current transport policy, particularly over aviation, seems to be going in the wrong direction. But the paper did include such measures as doubling energy generation through combined heat and power, setting a ten per cent target for electricity suppliers, promoting a national emissions trading scheme, and calling for greater energy efficiency.

The European Union has a wider trading emissions scheme coming into force this January, into which the present British scheme will fit. The scheme imposes requirements on the largest individual emitters of carbon dioxide to monitor and account for their emissions. The installations include all forms of electricity generation, oil refineries, iron and steel producers, the minerals industry, and paper, pulp and board manufacturing. Together the installations covered by the scheme cover about half of all British carbon dioxide emissions.

Making the cuts will inevitably involve a new mix of energy sources and technologies. The difficult task for government is to know where to fix priorities and which technologies to push. Research into non-conventional fossil fuel sources and cleaner coal seems unlikely to lead far, while renewable technologies which already exist, from solar and wind to biomass and tidal power, and even perhaps geothermal energy, deserve much more support than they have so far received.

Then there is the 'nuclear' question. As chairman of a Chatham House group looking into future prospects for the industry, I simply say this: first, I reproach this government and its predecessor for not putting more effort and resources into coping with the problems of high-level waste. Next, I reproach them for fudging nuclear issues. One of the conclusions of the Chatham House study was that public opinion was persuadable if anyone wanted to persuade it. The present failure to confront the issues has unfortunate effects on recruitment of new generations of scientists and engineers.

Next, I believe that we should be investing in new fusion technology. The problems of true cost, safety, proliferation, security, risk and the rest should be examined in a complete overall assessment of nuclear against other forms of renewable energy to lay a proper foundation for debate and future policy.

Another project on the horizon is the development of hydrogen technology. Beginning with transport, President Bush has announced a US$1.2 billion hydrogen fuel initiative which aims to make it practical and cost-effective to use clean hydrogen-powered vehicles by 2020. To achieve widespread use of hydrogen it must obviously be produced cost-effectively in large plants or in smaller facilities near vehicle fuelling stations, and the technology must avoid creating carbon dioxide in the process of refining hydrogen.

IN ALL DEBATE on future energy policy, there is one central conclusion. The world is never going to run out of energy. Our friendly neighbourhood hydrogen bomb - the Sun - will not run out of fuel for hundreds of millions of years to come. The question is how we move from one means of generating energy to another. To achieve this transition there must be a wider recognition in society of the scale of the changes which are needed. Every individual must feel that he or she can do something and take increased responsibility for his or her actions. But a sixty per cent reduction in carbon use will require a real change in lifestyle. All over the world people have to change their ways and remodel their thinking. Otherwise Nature will do what she has done to over 99% of species that have ever lived, and do the job for us.

Sir Crispin Tickell is Chancellor of the University of Kent.