Targets and trust. These are at the heart of a tougher new global climate pact possibly just weeks away.

The bigger the pledged emissions cuts or reductions in growth in carbon dioxide pollution, the greater the need to prove nations meet those targets and curb the pace of climate change.

And proof of emissions reductions over time will help unlock billions of dollars in climate funds for poor nations.

The problem, though, is that it is not yet possible to independently monitor a country's greenhouse gas emissions, such as carbon dioxide (CO2) from burning fossil fuels or deforestation.

"Our system is not good enough right now to be able finger one country versus another. I think the density of observations needs to be cranked up two orders of magnitude," said Pieter Tans of the National Oceanic and Atmospheric Administration in Boulder, Colorado.

Rich nations, he said, weren't going to hand over possibly hundreds of billions of dollars to poorer nations to help green their economies purely on trust.

Which is why measurement, reporting and auditing of nations' greenhouse gas emissions is a key focus of marathon UN climate talks. The world body hopes the negotiations will lead to agreement on a tougher climate pact from 2013 during a meeting in Copenhagen next month.

Rich nations are under pressure from the developing world to sign up to emissions cuts of 25-40 per cent below 1990 levels by 2020 and funnel billions in aid and green technology to the poor.

Big developing nations are also under pressure to curb the pace of their emissions growth. China, India, Indonesia and Brazil are among the world's top carbon polluters.