BONN, Germany (Reuters) - Industrialized nations excluding the United States are planning cuts in greenhouse gas emissions of between 15 and 21 percent below 1990 levels by 2020 under a new U.N. climate pact, official data showed on Tuesday.

The numbers, issued to delegates at August 10-14 U.N. climate talks in Bonn, fall short of cuts of between 25 and 40 percent outlined by a U.N. panel of scientists to avert the worst of global warming such as heatwaves, floods and rising sea levels.

"Emissions ... are expected to be between 15 and 21 percent below 1990 levels by 2020," the U.N. Climate Secretariat said of the figures, compiled from widely differing plans by nations including Russia, Japan, Canada and European Union members.

Overall emissions by the 39 industrialized nations, based on the existing plans, would fall to the equivalent of between 10.71 and 9.86 billion metric tons of carbon dioxide in 2020 from 12.53 billion metric tons in 1990.

The data excludes the United States, the top greenhouse gas emitter after China, which is outside the current Kyoto Protocol obliging all other industrialized nations to cut emissions by an average of at least 5 percent below 1990 levels by 2008-12.

The 2020 numbers include a promise by New Zealand on Monday to cut emissions by 10-20 percent. Environmentalists criticized that goal for hinging on conditions such as agreement at U.N. talks in Copenhagen in December on a strong new climate deal.

The EU, Switzerland, Norway and Liechtenstein are offering the deepest cuts -- some with strings attached -- according to the plans. Canada, Japan, Belarus and Russia are among those planning smaller reductions.

Inclusion of the United States would reduce the overall ambition since President Barack Obama aims to return U.S. emissions to 1990 levels by 2020, a cut of about 14 percent from current levels after a sharp rise since 1990.

BUSH LEGACY

Former President George W. Bush said Kyoto would be too costly and wrongly excluded 2012 targets for developing nations such as China, India and Brazil.

National submissions use widely varying assumptions, such as the use of forests which soak up carbon dioxide as they grow. Including changes in land use and forestry, emissions would fall by between 13 and 20 percent by 2020, the data show.

Yvo de Boer, head of the U.N. Climate Change Secretariat, said earlier 2020 pledges were "miles away" from the ambition needed to meet a goal by Group of Eight leaders at a July summit in Italy to cut emissions by 80 percent by 2050.

He also told Reuters the Bonn talks among 180 nations had "got off to a good start" to address huge tasks such as shortening a contentious 200-page draft text for a global U.N. climate accord to succeed Kyoto in Copenhagen.

The chief climate negotiator for Sweden, which holds the rotating presidency of the European Union, saw signs of hope.

"I think we have a slightly positive dynamic," Anders Turesson told Reuters. "On some issues I can sense some progress."

He welcomed the fact major emitters at a summit last month in Italy, including the G8 and China and India, had backed a broad scientific view that global warming should be limited to below 2 Celsius (3.6 Fahrenheit) above pre-industrial times.