* Experts say emission levels should be 44 mln T by 2020
* Says getting harder to stay within 2 degrees C rise
LONDON, Nov 5 (Reuters) - Greenhouse gas emissions in 2020 will be 8 billion to 12 billion tonnes more than the level needed to keep global warming to only 2 degrees Celsius and avoid severe climate change, a United Nations report estimated on Tuesday.
The annual U.N. Environment Programme (UNEP) report analysed countries' current pledges for emission cuts and whether they are enough.
It found the gap between the pledges and emission cuts that scientists estimate are needed by 2020 to avoid the potentially devastating effects of global warming to be 8-12 billion tonnes per year, little changed from last year's estimate of 8-13 billion.
It is "increasingly difficult" to stay on track to limiting temperature rises, and global action is needed to close the emissions gap, the report said.
In 2010, countries agreed to take action to limit temperature rises, but many countries have failed to enact emission cuts to back up their promises.
Delegates from more than 190 countries will meet in Warsaw, Poland next week for a U.N. conference to work on emission cuts under a new climate pact, which will be signed by 2015 but only come into force in 2020.
Scientists have said annual emissions would have to be no more than around 44 billion tonnes (gigatonnes) per year by 2020 to have a good chance of limiting the overall temperature rise to below 2 degrees Celsius.
Total global greenhouse gas emissions in 2010 already amounted to 50.1 gigatonnes, highlighting the scale of the task ahead.
"Delayed action means a higher rate of climate change in the near term and likely more near-term climate impacts, as well as the continued use of carbon-intensive and energy-intensive infrastructure," U.N. Under-Secretary-General and UNEP Executive Director Achim Steiner said in a statement.
Studies have shown that emissions could be reduced by 14-20 gigatonnes/year at a cost of up to $100 per tonne of carbon dioxide equivalent if pledges were more ambitious and were expanded to include all countries and more industries, the report said.
The report cited increased energy efficiency, renewable energy, improved agricultural practices and the reform of fossil fuel subsidies as ways to bring down emissions.
"As we head towards Warsaw for the latest round of climate negotiations, there is a real need for increased ambition by all countries: ambition which can take countries further and faster towards bridging the emissions gap and a sustainable future for all," said Christiana Figueres, executive secretary of the U.N. Framework Convention on Climate Change, in the statement.
The UNEP report involved 70 scientists from 17 countries.
(editing by Jane Baird)