By Alister Doyle
COPENHAGEN, Oct 31 (Reuters) - - A draft U.N. guide for slowing climate change says world greenhouse gas emissions may have to fall to a net zero this century but is vague about what each nation should do now.
About 500 delegates, including scientists and government experts, are meeting in Copenhagen to edit the report, which is meant to guide policymakers in setting national goals for a global climate deal at a U.N. summit in Paris in late 2015.
The draft synthesis report by the U.N.'s Intergovernmental Panel on Climate Change (IPCC) says rising world emissions will have to peak soon and then fall fast to limit risks of what could be "irreversible" damage.
"Somewhere after the middle of this century human-caused emissions will have to come down to a net zero," Achim Steiner, head of the U.N. Environment Programme, told Reuters.
"That in essence is going to design the arithmetic of national actions," he said of efforts to limit rising temperatures to avert desertification, mudslides, heatwaves, more powerful storms and rising sea levels.
Net zero means that any emissions, for instance from burning fossil fuels, would be balanced by other measures such as extracting carbon dioxide from the air and burying it.
The report indicates that net zero emissions would give a strong chance of achieving a U.N. goal of limiting a rise in average temperatures to 2 degrees Celsius (3.6F) above the temperatures in pre-industrial times.
Governments feel more comfortable setting long-term goals for the planet than targets for themselves. "No one wants to admit how much they will have to do," to meet the 2C target, said Alden Meyer of the Union of Concerned Scientists.
GOING TO ZERO
On Sunday, U.N. Secretary-General Ban Ki-moon and leading scientists will issue the final synthesis. About 30 pages long, it will sum up three reports of more than 1,000 pages each issued since September 2013 about the science, impacts and solutions associated with climate change.
One chapter of the earlier reports indicates rich nations would have to halve their emissions by 2030 from current levels to get on track for 2C. Asian nations as a group would need to cap their soaring emissions around current levels by 2030.
Few governments are considering such tough goals.
Last week the European Union agreed a 40 percent cut in greenhouse gas emissions from 1990 levels by 2030. [ID:nL6N0SI6IK] President Barack Obama wants U.S. power plants to cut emissions by 30 percent from 2005 levels by 2030.
China, the top emitter, has indicated it wants to cap its rising emissions, but has not said when or at what level.
"The main message for me from this report is the idea of going to zero" net emissions, said Niklas Hoehne of climate consultancy Ecofys, an IPCC author of studies looking at regional responsibility.
Since the IPCC issued its last set of reports in 2007, developing nations have often pointed to a chapter suggesting rich nations should cut emissions by between 25 and 40 percent below 1990 levels by 2020, deeper than almost all cuts actually planned.
The IPCC says it is at least 95 percent sure that manmade emissions of greenhouse gases, rather than natural variations in the climate, are the main cause of warming since 1950.
(Reporting by Alister Doyle; editing by Andrew Roche)