As Beijing declared its first "red alert" for heavy smog this week, closing schools and curbing car use, Chinese negotiators at UN talks in Paris are being accused of trying to weaken the new global climate accord due to be finalised by Friday.
"It is very frustrating," said one negotiator from a developed country after a meeting where he said Chinese officials had tried to water down efforts to create a common system for the way countries report to the UN on their carbon dioxide emissions and climate change plans.
Another envoy said Chinese delegates were also resisting a measure widely seen as crucial for a successful accord: a requirement for countries to update the pledges they have made to limit their emissions, preferably every five years from around 2020.
China is supporting a general stocktaking review of countries' pledges every five years but wants any updating of the carbon dioxide emissions reduction targets contained in these plans to be voluntary, this envoy said.
Heat-trapping carbon dioxide is an invisible greenhouse gas that differs from the soot and smog choking Beijing, but the two pollutants typically come from the same sources, such as coal-fired power plants.
Beijing's negotiating stance was causing anxiety among many delegates on Tuesday as the Paris talks intensified ahead of Friday's deadline, partly because India was expected to be a bigger obstacle to a successful agreement.
Prime Minister Narendra Modi weighed into one of the most divisive issues of the conference on the eve of the two-week meeting, writing in the Financial Times that wealthy countries that "powered their way to prosperity on fossil fuel" had a moral duty to lead the fight against climate change.
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In contrast, China's President Xi Jinping has garnered public credit for a series of bold climate vows in the run-up to the Paris meeting. He made a rare joint announcement with his US counterpart, Barack Obama, on Beijing's plans to combat global warming late last year and subsequently pledged to launch a national carbon trading system as early as 2017.
In Paris, however, one delegate told the FT that behind closed doors, India has been "quite helpful" on some measures while China's negotiators have been siding with other developing countries on several divisive issues, including a push to provide more information about the volume of each country's emissions.
Under rules dating back to 1992, China has been classed as a developing country that has not been obliged to report to the UN on its carbon emissions as regularly as older industrialised nations, a situation the US and most other developed countries say must be changed in the new Paris climate agreement.
China, the world's biggest carbon emitter, has for example only given the UN two so-called "national communications" reports since 1992, detailing its emissions and what it was doing to limit them. Its latest report was in 2012 but only contained 2005 emissions data.
Wealthy countries are required to produce such reports regularly, along with other data that is then subjected to a review. They argue that because developing countries are responsible for a growing share of global emissions, the reporting and monitoring system needs to be updated.
China accounts for 27 per cent of carbon emissions and India is the fourth biggest emitter with just over 7 per cent of the global total.
"You need to have those basic elements of [emissions] inventories, reporting and review on the developing country side as well," said US climate envoy, Todd Stern, adding the reporting done by developing countries at the moment was "at a pretty rudimentary level".
China does produce domestic emissions data itself, some of which experts question, but is often reluctant to divulge information on the basis of international agreements.
In a sign of the tensions growing in private meetings this week, the EU climate commissioner, Miguel Arias Canete told reporters that although China's President Xi and other world leaders had called for strong climate action at the opening of the Paris talks, "things are much more complex" inside the negotiating rooms.
Meanwhile, a public war of words intensified on Tuesday over another fraught reporting issue in the talks: a 2009 pledge wealthy countries made to deliver $100bn a year by 2020 to help developing countries deal with climate change.
Prakash Javadekar, India's environment minister, told reporters developed countries "have not made much headway" towards the target and questioned a recent OECD study that concluded as much as $62bn in funding from public and private sources was delivered in 2014.
Wealthy countries, meanwhile, said because much more money had been pledged in the run-up to the Paris meeting that the tally for 2020 was now more than $90bn.