Rich countries accused of carbon 'cheating'
Some rich countries are seeking new rules under the UN climate convention that campaigners say would allow them to gain credit for "business as usual".
Russia, Australia, Canada and some EU countries are among the accused.
The rules relate to land-use change, which can either release or absorb carbon, depending mainly on whether forests are planted or chopped down.
Rich countries, apart from the US, could account for about 5% of their annual emissions through this loophole.
The US is not involved in these negotiations because the proposals fall under the Kyoto Protocol, of which it - alone among developed countries - is not a part.
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By way of comparison, 5% is roughly equal to the total emissions reduction that developed countries pledged to make between 1990 and 2012 under the Kyoto Protocol.
The benefit for some countries, notably Russia, would be much greater.
"This would allow developed countries to circumvent their obligations on reducing emissions," said Melanie Coath, climate change policy office with the UK's Royal Society for the Protection of Birds (RSPB), who has conducted analytical work on the draft text currently being negotiated.
"These are double standards that make us question the legitimacy of the whole process," added Kevin Conrad, lead negotiator for Papua New Guinea and chairman of the Coalition for Rainforest Nations.
"If rich states tell us we have to adopt robust standards (for REDD) and then use forestry as their biggest get-out clause - it's double standards, it's climate fraud."
Diplomats from developing countries have also criticised the proposals, which are under discussion during a fortnight of talks in Bonn under the UN climate convention (UNFCCC).
Some have suggested that rich countries would operate their forestry sectors under looser accounting rules than developing nations would face under the REDD mechanism (Reducing Emissions from Deforestation and forest Degradation).
Several different "fudges" are up for discussion in the draft text that would create the 5% (or 500 megatonnes of CO2) loophole.
One would allow countries to measure emission reductions or increases against a "forward-looking baseline".
In other words, a country would decide how its land-use carbon emission or absorption would be likely to change in future, and then to measure actual performance against that baseline.
By contrast, developed nations have to measure emissions from every other sector of their economies simply for what they are - against a zero baseline.
- A second proposal, from Russia, would mean that countries would not have to count emissions from land-use change until land-use changes across the entire country resulted in net emissions.
Currently, Russia's land-use sector is a big net absorber.
In addition, each governments could decide which aspects of land use change to include in its emission reports - which it would then compile and submit to the UN.
Delegates from some EU countries have suggested that others with large areas of forest - such as Austria, Finland and Sweden - are pushing for lax regulation, along with Russia and Australia.
But the European Commission's chief negotiator, Artur Runge-Metzger, said the EU favoured tighter rules.
"Certainly from the EU side, what we want to see is a system where we have the highest environmental integrity that is possible," he told BBC News.
"And also we don't want to have rules tighter for developing countries than for developed countries."
The UN talks here are due to conclude on Friday, and to set out some goalposts as governments look to the next UN climate summit in Cancun, Mexico, at the end of the year.
Small island states and many of the world's poorest nations are demanding that Cancun must see agreement of a legally binding global treaty, but many others are pushing for a "bottom-up" approach that would seek small but concrete agreements in key areas such as REDD.
Some rich countries are seeking new rules under the UN climate convention, which campaigners say would allow them to gain credit for "business as usual".