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Emission Reduction Assumptions for Carbon Dioxide Overly Optimistic, Study Says

April 03, 2008

Reducing global emissions of carbon dioxide (CO2) over the coming century will be more challenging than society has been led to believe, according to a research commentary appearing this week in the journal Nature.

The authors, from the University of Colorado at Boulder, the National Center for Atmospheric Research (NCAR) in Boulder, and McGill University in Montreal, said the technological challenges of reducing CO2 emissions have been significantly underestimated by the Intergovernmental Panel on Climate Change (IPCC).

The study concludes the IPCC is overly optimistic in assuming that, even without action by policymakers, new technologies that will result in dramatic reductions in the growth of future emissions will be developed and implemented.

Titled "Dangerous Assumptions," the Nature commentary is co-authored by scientists Roger Pielke, Jr., of CU-Boulder, Tom Wigley of NCAR and economist Christopher Green of McGill University.

"This welcome commentary is an indication that the science policy discussions have shifted from 'is there global warming?' to 'how does society respond?'" said Cliff Jacobs of the National Science Foundation (NSF)'s Division of Atmospheric Sciences, which funds NCAR.

"In the end, there is no question whether technological innovation is necessary--it is," write the authors. "The question is, to what degree should policy focus explicitly on motivating such innovation?"

"The IPCC plays a risky game in assuming that business-as-usual advances in technological innovation will carry most of the burden of achieving future emissions reductions, rather than focusing on those conditions that are necessary and sufficient for those innovations to occur."

Recent changes in "carbon intensity"--CO2 emissions per unit of energy consumed--already are higher than those predicted by the IPCC because of rapid economic development, says lead author Pielke. In Asia, for instance, the demands of more energy-intensive economies are being met with conventional fossil-fuel technologies, a process expected to continue there for decades and eventually move into Africa.

Atmospheric CO2 levels are currently about 390 parts per million. A commonly cited goal is to stabilize concentrations at roughly 500 parts per million or less.

Because technical innovation is ongoing, the IPCC authors assume that most of the needed reductions will occur automatically. According to calculations by the Nature authors, the IPCC assumes that 57 to 96 percent of the total carbon removed from the energy supply will occur automatically through such routine technological progress.

The reason for the IPCC's underestimate of carbon intensity changes lies partly in the way the IPCC emissions scenarios partition future emissions changes into those that will occur spontaneously and those that are policy-driven, say the scientists.

This partitioning underestimates the full magnitude of the technology challenge associated with stabilizing the amount of carbon dioxide in the atmosphere.

"According to the IPCC report, the majority of the emission reductions required to stabilize CO2 concentrations are assumed to occur automatically," says Pielke. "Not only is this reduction unlikely to happen under current policies, but we are moving in the opposite direction right now. We believe these kinds of assumptions in the analysis blind us to reality and could potentially distort our ability to develop effective policies."

Stabilization of atmospheric concentrations of CO2 and other greenhouse gases was the primary objective of the 1992 United Nations Framework Convention on Climate Change approved by almost all countries, including the United States, notes Wigley.

"Stabilization is a more daunting challenge than many realize and requires a radical 'decarbonization' of energy systems," says Wigley. "Global energy demand is projected to grow rapidly, and these huge new demands must be met by largely carbon-neutral energy sources--sources that either do not use fossil fuels or that capture and store any emitted CO2."

Unlike the IPCC authors, who built in assumptions about future "spontaneous" technological innovations, the Nature commentary authors began with a set of "frozen technology" scenarios as baselines--scenarios in which energy technologies are assumed to stay at present levels. "With a frozen technology approach, the full scope of the carbon-neutral technology challenge is placed into clear view," says Green.

The National Science Foundation (NSF)