The chance of limiting global temperature increases to two degrees Celsius this century are swiftly diminishing, a new United Nations report warned Tuesday, ahead of the body's annual climate talks next week.

Global greenhouse gas emissions will be eight to 12 billion tonnes higher than target levels in 2020, even if surveyed countries stick to existing emission-reduction agreements, according to the UN Environment Programme (UNEP) report.

Scientists say avoiding the upper threshold of two degrees Celsius (3.6 degrees Fahrenheit) would avert the worst affects of climate change, but according to the report would likely require a slashing of emissions by 14 percent in 2020.

"The window of opportunity of trying to meet this two-degree target threshold is, in a sense, becoming ever-more elusive," said UNEP chief Achim Steiner.

UNEP officials presented the report at a news conference in Berlin Tuesday.

The annual report projected 2020 emissions of so-called "carbon dioxide equivalents" would be about 59 billion tonnes, a billion tonnes higher than the estimate in last year's report.

"Achieving the two-degree goal is with every year less possible. Emissions always rise, even though they need to sink sharply," German Institute for International and Security Affairs researcher Oliver Geden told AFP.

Geden has called the two-degree target "patently unrealistic" and advocated for it to be dropped or modified.

The UNEP, however, said the goal was still technically possible.

The report highlights the agricultural sector, which contributes about 11 percent of global greenhouse emissions, as an area largely untapped for emissions savings.

It named reducing tilling on farm fields and increased planting of trees and shrubs on agricultural land as among the strategies to eliminate up to four billion tonnes of greenhouse gases in 2020.

Another important factor, Steiner said, is international financial support for developing countries to build renewable energy sources.

"If Africa is by default forced to go down the fossil-fuel path, we are adding an entire China economy to the global carbon budget, to the global oil and gas markets, in just the next 20 or 30 years," Steiner said.

As it becomes less and less likely emissions will be brought down to target levels in 2020, the report warned, more difficult, riskier and expensive steps will be needed later to stave off effects of global warming.

"The minute we have to begin to deploy technologies that the market simply cannot sustain, it is the taxpayer who will have to step in," Steiner said.

The report was issued ahead of annual UN climate talks starting Monday in Warsaw.

More than 190 countries will meet to set a timetable towards a new binding, global climate agreement to be signed in 2015 and go into effect in 2020.