By Nina Chestney

WARSAW (Reuters) - Nitrous oxide (N20) emissions could almost double by 2050 if more aggressive action is not taken, undermining global efforts to curb climate change, the United Nations' Environment Programme (UNEP) said on Thursday.

Commonly known as the "laughing gas", nitrous oxide exists naturally in the atmosphere in trace amounts.

However, it is the third most potent greenhouse gas after carbon dioxide and methane due to human activities such as agriculture, fossil fuel combustion, waste water management and industrial processes.

N2O emissions into the atmosphere are currently around 5.3 million tones a year but this could almost double by 2050 if efforts to cut the gas are not increased, the UNEP report said.

More efficient use of fertilizers, less meat consumption, and improved waste water treatment are some ways to cut N20.

Emissions could be cut by 1.8 million tones a year from 2020 and the benefits could be worth over $160 billion annually across sectors such as agriculture, manufacturing, transportation and electricity production, UNEP said.

The Kyoto Protocol climate treaty includes N20 among the greenhouse gases the world has to reduce to fight global warming but more aggressive efforts are needed, UNEP said.

"We need all hands on deck to combat the serious and significant increases in N2O levels in the atmosphere," said U.N. Under-Secretary-General and UNEP Executive Director Achim Steiner.

"It has a disproportionate impact on global warming because of its radiative properties and long lifetime in the atmosphere, which is 120 years on average," he added.

Delegates from around 195 nations are meeting in Warsaw for a U.N. conference, due to end on Friday, to work on greenhouse gas emission cuts under a new climate pact to succeed an extension to the Kyoto Protocol after 2020.

Atmospheric volumes of greenhouse gases blamed for climate change, including N20, hit a record in 2012, the World Meteorological Organisation said this month.

Although less potent than carbon dioxide or methane, N20 is often overlooked and could undermine efforts to prevent the ozone layer depleting, the report said.

The ozone layer shields the Earth from the sun's harmful rays and has begun to recover from depletion over the past couple of decades due to curbs on chlorofluorocarbons (CFCs) and other halogenated chemicals.

(Editing by David Evans)