Governments will have to reduce greenhouse emissions to zero by 2100 if the impact of climate change is to be kept in check at an affordable cost, a United Nations report said on Sunday.

If rapid steps were not taken to cut the emissions, however, the price could rise considerably, it said, warning that a failure to curb global warming by the end of the 21st century "will bring high risks of severe, widespread and irreversible impacts globally."

Such an "irreversible impact" would occur, for example, if Greenland's vast ice sheets were to melt, which could result in the swamping of coastal regions and cities.

The effects of climate change were already evident in an increase in extremes of heat, heavy rainfall, the acidification of the world's oceans and a rise in sea levels, the study said.

According to the report released by the Intergovernmental Panel of Climate Change (IPCC), the cost of reducing carbon-dioxide emissions in the short term would not brake global growth to any great degree. It said annual projected growth of 1.6 to 3.0 percent a year in consumption of goods and services would be cut by just 0.06 of a percentage point per year if immediate action were taken.

Final assessment

The report is a 40-page synthesis summing up 5,000 pages of climate change studies already published since September 2013.

The studies establish with 95-percent certainty that almost all global warming seen since the 1950s is man-made.

The document, which has been edited for a week by officials from more than 120 governments meeting in the Danish capital, Copenhagen, is to furnish guidelines for a UN deal on global warming scheduled to be struck at an international summit in Paris in late 2015.

Words of hope

Options for limiting the amount of greenhouse gas emissions included improving energy efficiency and moving from fossil fuels to wind, solar or nuclear power, according to the study.

"We have the means to limit climate change," IPCC chairman Rejendra Pachauri said.

"The solutions are many and allow for continued economic and human development. All we need is the will to change, which we trust will be motivated by knowledge and an understanding of the science of climate change," he added.

UN climate negotiations are to continue next month in Lima, Peru, where international delegates are to work on a final draft agreement for the 200-nation Paris summit.

tj/sb (AP, Reuters)