A lack of international will means the chances of bringing climate change under control may already be "slipping out of reach", scientists have warned.
A study by the Swiss science university ETH Zurich shows that without an early and steep cut in greenhouse gas emissions, global temperatures are not "likely" to remain less than 2C higher than pre-industrial levels. The 2C target, which experts say is needed to avert dangerous climate change, was agreed by the UN climate conference in Copenhagen in 2009.
But countries that signed up to the Copenhagen Accord have yet to commit to measures far-reaching enough to meet it, according to experts.
A voluntary agreement hammered out in the dying hours of last December's UN climate talks in the Danish capital is said to fall well short of the cuts required.
The new report, published today in the journal Nature Climate Change, sounds a further loud warning that time is running out. It suggests that for a "likely" chance (more than 66%) of holding warming below 2C by the end of this century, emissions must peak before 2020.
Emission levels will also have to drop drastically to around 44 billion tonnes in 2020, and then keep falling. By 2050, they will need to be well below 1990 levels at around 20 billion tonnes, says the research.
This is an ambitious goal. Last year's emission levels were estimated to be 48 billion tonnes. If no action is taken to reduce global emissions, experts fear they could grow to 56 billion tonnes in 2020.
Authors of the new study, led by Dr Joeri Rogeli, from the Swiss science university ETH Zurich, wrote: "Without a firm commitment to put in place the mechanisms to enable an early global emissions peak followed by steep reductions thereafter, there are significant risks that the 2C target, endorsed by so many nations, is already slipping out of reach."
The scientists base their conclusions on a comprehensive risk analysis of emission scenarios.
The research takes into account results from a number of previous integrated assessment models on climate change. These incorporate data from a range of different disciplines and are designed to inform policy.
Three pathways were identified that could result in a "very likely", or more than 90%, chance of not exceeding the 2C threshold.
All involved a peak during this decade, high post-peak reduction rates, net negative emissions, and heavy use of renewable energy sources and carbon capture technology.
Scenarios showing peak emissions around 2030 were likely to keep warming below 3C, but would miss the 2C target. Another study published in Nature Climate Change suggests that if greenhouse gas emissions continue to rise, the 2C threshold could be crossed between 2040 and 2060. This is well within the lifetime of many people alive today.
The research examines the idea of looking at climate change projections from a different angle, shifting the emphasis from "what" to "when".
Lead author Dr Manoj Joshi, from the University of Reading, said: "It is not just about avoiding potentially dangerous climate change, but also about buying time for adaptation.
"This approach to communicating the impacts and uncertainties of climate change draws attention to rates of change rather than just the change itself. It complements existing methods, and should be employed more widely."