Japan, whose economy is steadily recovering from recession, is far behind in its obligations under the Kyoto Protocol to slash gas emissions blamed for global warming by six percent by 2012 from 1990 levels.

Ahead of global negotiations that opened Monday in Bangkok on setting post-Kyoto obligations, Japan proposed shifting the base year for future emissions cuts from 1990 to 2005.

But Yvo de Boer, head of UN climate body leading the Bangkok talks, said that changing the base year only made sense if it came with "much more ambitious" measures to curb global warming.

"To me it's like talking about a starting line of a marathon but not knowing how long the marathon is going to be," de Boer told a news conference.

"If the countries were to decide to change the base from the current base year, 1990, it means that we would be ... measuring the length of a marathon from a different starting point," he said.

"Yes, it's very interesting what base year will be chosen," he said. "But the toughest question -- the most important question, at least to me -- is by how much are rich countries willing to reduce their emissions by 2020."

The week-long talks in Bangkok are meant to sort out practicalities of meeting a UN-backed goal of finishing by the end of next year a new climate change treaty that covers obligations up to 2020.

Japan argues that the 1990 base year is biased towards the European Union, which has been pushing for further steep binding cuts from 1990 levels.

In 1990, some European Union countries were then heavily polluting members of the Soviet bloc. The base year also comes before the full privatisation of Britain's coal industry.

Chief US negotiator Harlan Watson called the Japanese proposal "an interesting idea."

"We're looking into it. There are some people to think that 1990 was advantageous to some parties," Watson told AFP.

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