OSLO, Aug 7 (Reuters) - Deep cuts in greenhouse gas emissions of 40 to 70 percent by mid-century will be needed to avert the worst of global warming that is already harming all continents, a draft U.N. report showed.

The 26-page draft, obtained by Reuters on Thursday, sums up three U.N. scientific reports published over the past year as a guide for almost 200 governments which are due to agree a deal to combat climate change at a summit in Paris in late 2015.

It says existing national pledges to restrict greenhouse gas emissions are insufficient to limit warming to 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial times, a U.N. ceiling set in 2010 to limit heatwaves, floods, storms and rising seas.

Average global surface temperatures have already risen by about 0.8 C (1.4 F) since the Industrial Revolution, the draft said.

"Deep cuts in greenhouse gas emissions to limit warming to 2 degrees C ... remain possible, yet will entail substantial technological, economic, institutional, and behavioral challenges," according to the draft due for publication in Copenhagen on Nov. 2 after rounds of editing.

Cuts in greenhouse gases, mainly from burning fossil fuels, of between 40 and 70 percent by 2050 would be needed from 2010 levels to give a good chance of staying below 2C, according to the Intergovernmental Panel on Climate Change (IPCC) draft.

Such a shift would also require a tripling or a quadrupling of the share of low-carbon energies including solar, wind or nuclear power, it said.

That would be a radical change. Emissions, buoyed by coal-fuelled industrial growth in emerging economies led by China and India, rose to 49 billion tonnes in 2010 from 40 billion in 2000.

The IPCC says it is at least 95 percent probable that human influences are the main cause of climate change, although opinion polls show that many people doubt such findings and reckon that natural variations are to blame.

"Human influence on the climate system is clear, and is estimated to have been the dominant cause of the warming observed since 1950," the draft says.

The draft Synthesis Report, dated April 21, merges data from three previous IPCC studies that focused on the science of climate change, the impacts and possible solutions. It clarifies many findings but does not include new research.

REVISED

IPCC spokesman Jonathan Lynn said the draft obtained by Reuters had already "undergone thorough revision since the authors met at the end of June/beginning of July."

A final draft will be sent to governments at the end of August, he said, before editing at the Copenhagen meeting from Oct. 27. It will round off a year-long cycle of IPCC reports running to thousands of pages by hundreds of experts.

Still, final drafts are often similar to earlier work.

The draft says climate change is causing more heat extremes, disrupting rainfall, harming many crop yields, causing an acidification of the oceans and thawing ice in Antarctica and Greenland that are raising sea levels.

Unchecked climate change was projected to damage economic growth and can even indirectly increase risks of armed conflict by aggravating underlying causes such as poverty, it says.

Costs of strong action to cut emissions would slow consumption growth by a fractional 0.06 percentage point a year this century from an estimated 1.6 to 3.0 percent, it says.

The IPCC focuses on consumption, which is gross domestic product minus investments, in case big investments are needed to shift from fossil fuels to renewable energies. That could stoke GDP and give a misleading impression of economic benefits.

The IPCC says it is impossible to compare costs and benefits of action for any given temperature level. Many factors are hard to quantify - a shift from fossil fuels, for instance, could curb health bills by reducing air pollution.

(Reporting by Alister Doyle; Editing by Sonya Hepinstall)