The world is in danger of spending its carbon budget by about 2025 and risks temperatures rising beyond 2 degrees Celsius unless nations adopt a flexible carbon accounting system, conservation group WWF says in a report.

The report by Dutch energy consultancy Ecofys and commissioned by WWF, says rich nations must cut emissions by 80 percent by 2050 and global emissions of greenhouse gases must be cut by 30 percent from 1990 levels by 2030.

Failure to do so would lead to rapid warming.

The report, Sharing the effort under a global carbon budget was released on Friday on the sidelines of U.N. climate talks in Bangkok that are trying to find ways for nations to sign up to a broader effort to fight climate change.

The U.N. climate panel says that to limit the average rise in global temperatures to 2 deg C by 2050, emissions need to peak or stay below 450 parts per million.

To reach that limit, rich nations would need to cut emissions at home and help poorer nations curb the pace of their emissions.

The report lays out three methods to help nations distribute the sharing of the budget by helping developing nations to continue to grow their economies and to recognize financing of clean-energy programs by rich nations in poorer states.

If we relax on the trajectory of one country, another country needs to pick up the bill, the report says.

It says the carbon budget can be defined as the amount of tolerable global emissions over a period of time to limit the average global temperature increase to 2 deg C.

The budget is from 1990 to 2050, Stephan Singer, WWF's director of global energy policy, told Reuters.

Within the past 18 years, we have used up approximately 40 percent of the budget of 1990-2050. If we keep on doing what we're doing now and including the increase of emissions, we have wasted, not only time, we've spent our budget by about 2025.

According to the report, the cumulative global greenhouse gas emissions excluding land use change and forestry is forecast to be 1,660 billion tons between 1990 and 2050. Between 2010 and 2050, it is 970 billion tons.

But the report, looking at the budget out to 2100, says because mankind has already increased its global emissions substantially since 1990, the remaining net cumulative budget between 2009 and 2100 is limited to 870 billion tons of carbon dioxide-equivalent.

This translates to an allowable global annual emissions on average for the next 91 years of no more than 9.5 GtCO2-eq (billion metric tons CO2-equivalent), or about 20 percent of today's net global emissions, the report says.

It says in order to stay within the boundary of the global emissions budget, sometime from 2060 net global emissions must be negative, with little emissions from energy use and greater soaking up of CO2 from forests and other methods.

(Reporting by David Fogarty; Editing by Sue Thomas)