It comes amid increasing pressure from local green activists for the government to enact a climate-change policy.

Hong Kong, despite being a highly developed economy, is not obliged to meet any of the mandatory targets in the Kyoto Protocol on climate change, because China as a developing nation has not set any target.

'It is advisable, though not essential, for countries to have targets to achieve, as it can tell how much an economy has to bear,' said Professor Davidson, who will speak at the opening of the International Conference on Climate Change today.

But the scientist, co-chairman of the UN Intergovernmental Panel on Climate Change working group that produced the latest report 'Mitigation of Climate Change', admitted that targets had to be set in a way that took into account 'national circumstances' of each place.

'After all, countries that want to set targets have to see what are feasible or not feasible, and they have to take into account their national circumstances,' he said.

Also speaking at the three-day conference, entitled 'Act On Climate Change - Now or Never', are chairmen of the other working groups under the IPCC, and more than 70 local and international figures from Britain, Germany, China, Australia and the United States.

Chief Executive Donald Tsang Yam-kuen will attend the closing session on Thursday.

Hong Kong produces more than 44 million tonnes of greenhouse gases a year, accounting for 0.2 per cent of the global total, and has often been criticised by local activists for being slow in responding to the global call for action.

California has set its own target even though the US is not part of the Kyoto pact, and London has vowed to become a zero-carbon city.

The mainland has laid down clear targets to boost energy efficiency by 20 per cent and ambitious targets for renewable energy but Hong Kong has no or insignificant targets for both.

Professor Davidson said Hong Kong could get more people onto public transport by 'making it more expensive to drive' and legislate on building energy efficiency.

While he admitted that some technology such as carbon capture and storage might be too expensive for developing countries to adopt, he said it was not always true that mitigation measures would hurt the economy as some sectors, such as clean fuel technologies, might provide opportunities for growth.

On the Kyoto pact, Professor Davidson said that although it carried a modest target of cutting emissions by 5 per cent from 1990 levels, it was at present the only tool that enabled commitment from different nations.

While the post-Kyoto arrangement after 2012 should be one that was fair and equitable for signatories, he said the issues of ensuring that clean technology was available to countries and that developed nations made further emission reduction commitments must be addressed properly.