The commitments made by world governments to reduce greenhouse gas emissions in the next 15 years are not sufficient to stave off the worst effects of climate change, a new analysis by Lord Stern has found.

The study by the author of an influential report on the economics of climate change found that countries' pledges ahead of a crunch UN climate summit in Paris this December would not keep warming below 2C, the level that previous UN negotiations have agreed on.

But Stern said he was still optimistic that a good deal could be reached in Paris.

"There is strong action coming through in many parts of the world," he said, pointing to the rising use of renewable energy and efforts to cut greenhouse gas emissions in countries from Latin America to Asia.

"It is very important to recognise that there is still time before Paris [to strengthen a potential agreement]."

The study looks at plans so far submitted to the UN , and the commitments published by other governments but not yet formally submitted.

US pledges emissions cuts of up to 28% ahead of global climate treaty

All countries are supposed to have drawn up "intended nationally determined contributions" (INDCs), setting out what efforts they will make to reduce their greenhouse gas emissions, or curb the growth in their carbon output, in coming years.