Atmospheric Sciences & Global Change Division
Research Highlights

September 2012

A-Tisket, A-Tasket, The Greenhouse Gas Basket

Ranking methane's impact on future climate change

Results: It's called the global warming potential or GWP for short. A handy "basket" to bundle the impact of all greenhouse gases on future climate change. Researchers from Pacific Northwest National Laboratory, working at the Joint Global Change Research Institute (JGCRI), found that increasing the value of methane's importance in the greenhouse gas basket had relatively small impact on the overall outcome of climate change projections. JGCRI is a partnership between PNNL and the University of Maryland.

Why It Matters: Carbon dioxide (CO₂) gets most of the press when it comes to greenhouse gas. So much so that it's used as a standard by which researchers and policy makers measure the global warming impact of all other greenhouse gases. Though not as abundant, methane, a gas released by human-caused activities and natural emissions, is the second most important anthropogenic greenhouse gas and traps more heat in the atmosphere per pound than CO₂. Considering the shorter lifetime of methane in the atmosphere along with the fact that relatively inexpensive abatement technologies already exist, methane may be an easier emission reduction target to aim for. For these reasons, scientists wanted to know if they upped the ante on methane's importance, would that make a difference in climate change projections. The short answer from this study—not that much.

"Carbon dioxide and methane have very similar physical warming effects on the climate, but it's difficult to put them on a level playing field," said Dr. Steven J. Smith, a senior scientist at JGCRI. "CO₂ lasts thousands of years in the atmosphere, while methane lasts only about ten years. In this research, we took a practical approach to compare them-to see if changing the relative value of methane had an impact on climate change scenarios."

Flares burn off excess methane at oil and gas refineries, landfills, and other industrial plants. Flares are used to control release of methane into the atmosphere but recovery options are also available that capture methane for use as fuel.

Methods: The research team used the Global Change Assessment Model (GCAM), an integrated model of the human and Earth system, to test the importance of the numerical value of the index by which methane is valued economically as compared to CO₂ within a comprehensive climate policy. GCAM, developed at PNNL, examines the evolution of the joint energy-land use system to project climate change outcomes under different greenhouse gas emission scenarios. Physical and economic outcomes were compared when the value of methane was changed relative to CO₂.

The researchers found that the impact of changing the value of methane compared to CO₂ is relatively small and there are several reasons for this small sensitivity. First, the large amount of relatively low-cost available methane abatement options, which tend to be utilized in most cases; and second, the interaction between CO₂ and methane reductions within the integrated system show that when CO₂ is abated, methane is also reduced. This is because reductions in coal or natural gas burning from policies to lower CO₂ emissions also reduce methane emissions.

Finally, the team noted a third cost in changing the metric.

"The GWP index is already embedded in greenhouse gas targets, real-world policies in national and international climate change policy frameworks. Changing this value would have a transition cost, further reducing any benefit a change might provide," said Smith. "If society wants to reduce climate change, then the key for methane is to put in place incentives for reducing emissions. We found that the exact value placed on methane was less important."

What's Next? The study also noted that more detailed studies of the cost and feasibility of methane abatement are needed. Smith is now examining the potential benefit from short-term polices that might focus on methane and black carbon, a warming agent that also has substantial impacts on human health.

Acknowledgments: The research was supported by the Climate Change Division at the U.S. Environmental Protection Agency, with additional support from the U.S. Office of Science Department of Energy and performed by Steven J. Smith, Joseph Karas, Jae Edmonds, Jiyong Eom and Andrew Mizrahi at PNNL.

Reference: Smith SJ, J Karas, J Edmonds, J Eom, and A Mizrahi. 2012. "Sensitivity of Multi-Gas Climate Policy to Emission Metrics." Climatic Change DOI:10.1007/s10584-012-0565-7.