Emissions from more than 40 nations were 10 percent below 1990 levels in 2012, according to a Reuters compilation of national data submitted to the United Nations in recent days that are the main gauge of efforts to tackle global warming.
Still, with emissions rising elsewhere, experts said the rate of decline was too slow to limit average world temperature rises to 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial times, a ceiling set by almost 200 nations to avert droughts, heatwaves and rising seas.
In 2012 "the success story is the declining emissions in the United States," said Glen Peters, of the Center for International Climate and Environmental Research in Oslo. "Europe is a mix with slow GDP growth offset by a shift to coal in some countries."
Total emissions from industrialised nations fell to 17.3 billion tonnes in 2012 from 17.5 billion in 2011 and compared with 19.2 billion in 1990, the base year for the U.N.'s climate change convention.
U.S. emissions fell 3.4 percent in 2012 to 6.5 billion tonnes, the lowest since 1994, the U.S. Environmental Protection Agency said on April 15. The fall was linked to low natural gas prices, helped by a shale gas boom and a shift from coal, a mild winter and greater efficiency in transport.
In the European Union, emissions dipped 1.3 percent in 2012 to 4.5 billion tonnes and were 19.2 percent down from 1990 levels, the European Environment Agency said.
Road transport emissions declined in some EU nations such as Italy, Spain and Greece that are suffering prolonged economic downturns. Emissions rose against the trend in Germany and Britain, with more coal used to generate electricity.
Among other major nations, emissions dipped in Canada in 2012 but rose in Russia, Japan and Australia.
The overall decline in emissions by industrialised nations is not enough to offset a rise in world emissions, driven by emerging economies such as China, India, Brazil and South Africa which are using more energy as their populations get richer.
Global emissions surged to 49 billion tonnes in 2010 from 38 billion in 1990, according to the U.N.'s Intergovernmental Panel on Climate Change (IPCC).
Governments aim to agree a pact to slow climate change by the end of 2015 to succeed the U.N.'s Kyoto Protocol, which binds only some developed nations to cut emissions until 2020.
The IPCC says that it is at least 95 percent probable that human activities, rather than natural variations in the climate, are the dominant cause of warming since the mid-20th century. Even so, opinion polls show that many voters are doubtful.
Corinne Le Quere, professor of climate change at Britain's University of East Anglia, said far tougher action was needed to reach the target of limiting global warming to 2 degrees C, with global cuts of about 3 percent a year.
"It requires a transformation in the way we use energy," she said. "In the short term, there are a lot of gains to be made in energy efficiency - in buildings, appliances, transport."
Industrialised nations' emissions have fallen since 1990 partly because many manufacturers had shifted operations abroad to emerging economies with lower costs, she said, meaning there was no overall reduction in emissions.
Counting greenhouse gases emitted to make products consumed in rich nations - from cars to washing machines - emissions by industrialised nations had risen an estimated 6 percent since 1990, she said. Data submitted to the United Nations, however, only cover emissions inside each country.