Rising urban populations are contributing to expanded demand for meat, adding to nutrition shortages for the world's poor. Increased greenhouse gas emissions from livestock as well as land clearing will make farming more marginal in many regions, especially in developing nations, said Rachel Kyte, World Bank Group Vice President and special envoy for climate change.

"The challenges from waste to warming, spurred on by a growing population with a rising middle-class hunger for meat, are leading us down a dangerous path," Professor Kyte told the Crawford Fund 2014 annual conference in Canberra on Wednesday.

"Unless we chart a new course, we will find ourselves staring volatility and disruption in the food system in the face, not in 2050, not in 2040, but potentially within the next decade," she said, according to her prepared speech.

Agriculture and land-use change account for about 30 per cent of the greenhouse gas emissions blamed for global warming. Feed quality can be so low in arid parts of Africa, where livestock typically graze on marginal land and crop residues, that every kilo of protein produced can contribute the equivalent of one tonne of carbon dioxide - or 100 times more than in developed nations, Professor Kyte said.

A two-degree warmer world - which may occur by the 2030s on current emissions trajectories - could cut cereal yields by one-fifth globally and by one-half in Africa, she said.

The river deltas of Asia, which provide almost two-thirds of the world's rice, will become more vulnerable to sea-level rise and storm surges. By 2050, each hectare of paddy will have to feed 43 people, up from about 27 now, according to a report carried by China Daily.

Professor Kyte said the focus has to turn to so-called "climate-smart agriculture", which contributes to increased productivity of crops, less wastage and a smaller climate change impact.

She cited the example of Ugandan farmers inter-cropping two key cash crops, bananas and coffee. The taller banana trees contributed shade to cool the coffee bushes while securing land from erosion and  building soil carbon levels.

Likewise, work by the CSIRO and the International Rice Research Institute was developing new strains of rice that can absorb more carbon from the atmosphere and more sunlight.

"If researchers succeed in turbo-charging the plant's engine, the new rice variety would need less water and fertiliser but yield 50 per cent more grain than the best current varieties," Professor Kyte said.

The World Bank envoy's speech comes as governments start to receive the final report of the United Nations Intergovernmental Panel on Climate Change's Fifth Assessment series.

The so-called synthesis report finds global warming is already cutting grain production by several percentage points, according to a draft of the report obtained by The New York Times.

The IPCC report also finds that runaway emissions of greenhouse gases are swamping all political efforts to deal with the threat of climate change, raising the risk of "severe, pervasive and irreversible impacts" in coming decades, the Times reported.

"From 1970 to 2000, global emissions of greenhouse gases grew at 1.3 per cent a year. But from 2000 to 2010, that rate jumped to 2.2 per cent a year ... and the pace seems to be accelerating further in this decade," the Times report said.

While technically possible to limit global warming to an internationally agreed upper limit of two degrees above pre-industrial levels, continued political delays for another decade or two would make that unachievable without "severe economic disruption", according to the Times report on the IPCC draft.