The cost of meeting the world's target for global warming could rise by half if current pledges under the UN flag to cut carbon emissions are not improved, the OECD said on Thursday.

Delay in reducing greenhouse-gas emissions in the coming years will hand future generations the bill for limiting warming to a safer two degrees Celsius (3.6 degrees Fahrenheit), the Organisation for Economic Co-operation and Development (OECD) said.

"We must act now to reverse emission trends," it said in a 90-page report issued ahead of the next round of UN climate talks, opening in Durban, South Africa on Monday.

"The further we delay action, the costlier it will be to stay within 2.0 C (3.6)," it said, referring to an objective laid down in the 2009 Copenhagen Summit and endorsed at UN talks in Cancun, Mexico last year.

That goal "is still achievable, but the costs are rising every day, month and year that passes to compensate for the increased emissions," the report said.

"Delayed or only moderate action up to 2020 -- such as implementing the Copenhagen/Cancun pledges only, or waiting for better technologies to come onstream -- would increase the pace and scale of efforts needed after 2020.

"It would lead to 50-percent higher costs in 2050 compared to timely action and potentially entail higher environmental risk."

The study compares the costs and benefits of three pathways for cutting emissions that would, by century's end, yield a 50-50 chance of preventing carbon dioxide concentrations above 450 parts per million (ppm).

The 450 ppm barrier is roughly equivalent to capping temperature increases at 2.0 C (3.6 F) compared to pre-industrial levels.

Under even the most ambitious of these scenarios, an overshoot of the 450 ppm target "has now become inevitable in the middle of the century," before falling again, the report says.

Higher spending in the coming decade to accelerate the shift toward a low-carbon global economy will yield saving benefits in the long run, and provide a greater buffer to the environment, it concludes.

Continuing along the current "business-as-usual" path would drive greenhouse gas concentration to about 685 ppm, nearly twice the current level, and far above the 450 threshold corresponding to the 2.0 C (3.6 C) limit.

Under this scenario, "global average temperature is likely to exceed this goal by 2050, and by 3.0 to 6.0 C higher than pre-industrial levels by the end of the century."

The report was published in the runup to the high-level talks in Durban, running from November 28 to December 9, under the UN Framework Convention on Climate Change (UNFCCC).

The OECD suggests policy actions such as boosting carbon-reduction pledges; creating a regulatory environment that will boost the market price of carbon; adapting to climate change impacts already in the pipeline; and accelerating the development of low-carbon energy sources and technology.

"In the context of tight government budgets, finding least-cost solutions and engaging the private sector will be critical to finance the transition," the report concludes.