Emission reports from over 40 nations were 10 percent below 1990 levels in 2012, according to data submitted to the United Nations recently.

Due to emissions rising elsewhere, researchers have said the rate of decline was "too slow" to limit the average world temperature rises to 2 degrees Celsius above pre-industrial times, according to Reuters.

Two years ago, "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, according to Reuters. "Europe is a mix with slow GDP growth offset by a shift to coal in some countries."

Emissions as a result of industrialized nations dropped to 17.3 billion tonnes in 2012 from 17.5 billion in 2011, and from 19.2 billion in 1990, which is the base year for the U.N.'s climate change convention.

Emissions in the U.S. fell 3.4 percent in 2012 to 6.5 billion tonnes, representing the lowest drop since 1994, according to Reuters, citing the U.S. Environmental Protection Agency.

The drop was linked to low natural gas prices, a mild winter, and "greater efficiency "in transport.

European Union emissions fell 1.3 percent in 2012 to 4.5 billion tonnes, and were 19.2 percent down from 1990 levels, according to the European Environment Agency.

Corinne Le Quere, professor of climate change at Britain's University of East Anglia, was quoted saying "tougher" action was required in order to reach the target of limiting global warming to 2 degrees C.

"It requires a transformation in the way we use energy," she said, according to Reuters. "In the short term, there are a lot of gains to be made in energy efficiency - in buildings, appliances, transport."

Emissions dropped in Canada in 2012 but increased in other places like Australia, Russia, and Japan.

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