By Jonathan Amos
Science reporter, BBC News, San Francisco
Tidal and other renewable technologies would lower the CO2 burden
If growth in carbon dioxide emissions is to be constrained and even reversed then the world cannot afford a coal renaissance, scientists have said.
Some commentators have argued that falling reserves of oil and gas will automatically limit CO2's rise.
But at an American Geophysical Union meeting, researchers said reserves of coal dwarfed those of other fuels.
It was even possible oil's demise could trigger an acceleration in emissions through more coal use, they added.
"We can replace oil with liquid fuels derived from coal," said Ken Caldeira from the Carnegie Institution at Stanford University in California.
"But these liquid fuels emit even more carbon dioxide than oil, so the end of oil can mean an increase in coal and even more carbon dioxide emissions to the atmosphere, and even more rapid onset of dangerous climate change."
Professor Caldeira's group has used climate and carbon cycle models to look at how future emissions and temperature projections would be altered by different fuel strategies.
The team tried to work out the maximum effects that would arise from replacing oil either entirely with coal-based liquid substitutes or entirely with renewable energy sources.
The assessment found that if coal-derived liquids are adopted, the Earth would achieve a 2C rise in temperature from pre-industrial times (a figure sometimes quoted as being a desirable ceiling to stay beneath in order to avoid "dangerous climate change") by 2042. This is three years faster than a business as usual future with oil.
If the renewables strategy is adopted, the 2C figure is not reached until 2056.
"Clearly, to address the climate issue we have to address the coal issue," Professor Caldeira told BBC News.
Tar sands excavation would lead to a dirtier world, researchers say
His assessment was shared by Pushker Kharecha from Nasa's Goddard Institute for Space Studies (Giss).
"We cannot move into things like coal-to-liquids and unconventional fossil fuels such as methane hydrates, tar sands, oil shale and so forth," he said.
"If they become large-scale substitutes for oil and gas, that would worsen things because they are much dirtier than oil and gas because they produce more emissions per unit energy delivered."
Dr Kharecha presented details of recent research from the US, UK and France looking at the feasibility of not only constraining the growth of CO2 emissions but actually reducing its concentration in the atmosphere to 350 parts per million by volume (it is currently up at about 385ppmv).
The group found it was possible, but only with a prompt moratorium on new coal use that does not capture CO2, and a phase out of existing coal emissions by 2030.
Reforestation together with improved agricultural practices could help draw down CO2.
"Efficiency and conservation have huge potential to offset emissions in the near term," Dr Kharecha told BBC News.
"And then in the mid-term and long-term we can focus on moving to alternatives such as renewable energies, and possibly a balanced look at nuclear because it does provide many benefits in addition to the numerous problems that it poses."
Mining for the most polluting of fossil fuels
A new analysis presented here puts the total available global coal reserves at 662 billion tonnes.
The figure is substantially lower than the ones used in assessments by the Intergovernmental Panel on Climate Change (IPCC) to gauge possible future emissions scenarios.
"This is a radically different number from what is conventionally assumed," said Professor David Rutledge from the California Institute of Technology, who led the analysis.
"The IPCC assumes that about five times as much coal is available for burning."
But the scientists at this meeting said that if burnt, even this smaller amount of coal would radically alter the climate unless all the emissions were captured and stored.
"There is far more than enough currently useable coal and other fossil fuels to push us past the threshold beyond which we would not want to go with the climate," Dr Kharecha said.
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