Paris: Economic recession was the main cause for a drop in United States carbon dioxide emissions between 2007 and 2013 -- not a shift from coal- to gas-fuelled energy as widely claimed, researchers said Tuesday.
US emissions of carbon dioxide, the main contributor to manmade global warming, fell by around 11 percent in the six-year period, according to a paper in the journal Nature Communications.
"Coinciding with the post-2007 decline in emissions, innovations in hydraulic fracturing technology have dramatically increased domestic supplies of gas," said the paper.
"Commentators in the scientific community and media have linked the two trends, celebrating the climate benefits of the gas boom."
But in reality, the change was driven by economic decline, study co-author Klaus Hubacek of University of Maryland told AFP.
"We show clearly that changes in consumption levels, and thus the recession, are mainly responsible," he said by email.
The study entailed close examination of the "sources" of change -- comparing a period of rising emissions from 1997 to 2007, to a period of decline from 2007 to 2013.
The team analysed changes in six different factors: population growth, consumption volume, the types of goods consumed, the labour and materials used to produce goods and services, the type of fuel used, and how much energy is used.
They found that 71 percent of the rise in emissions from 1997 to 2007 was due to a consumption boom.
Conversely, "83 percent of the decrease during 2007-2009 was due to decreased consumption and changes in the production structure of the US economy, with just 17 percent related to changes in the fuel mix" -- the ratio of coal to gas and other fuels used.
From 2009 to 2013, a period of economic recovery, there was a much smaller decrease in emissions of only about one percent.
"We conclude that substitution of gas for coal has had a relatively minor role in the... reduction of US CO2 emissions since 2007," said the paper.
The United States is the world`s second-largest greenhouse gas emitter after China.
Advances in hydraulic fracturing -- pumping liquids into stone to break it up and release the gas within -- and horizontal drilling, have unlocked plentiful supplies in less carbon-intensive natural gas supplies.
The findings have implications for climate policy, said Hubacek.
"If gas is helping to reduce emissions we (the government) should support policies that favour gas," he wrote.
"But if it is other factors such as increase in productivity, lifestyle changes etc, then we should focus on those factors."
The UN has embraced a goal of reducing average global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels.
Nations are negotiating a global climate pact to pursue the target, which will be backed by a roster of national carbon-curbing emissions.
The United States has already submitted its contribution, vowing to reduce greenhouse gas emissions by 26-28 percent from 2005 levels within the next decade.