Linking climate and air pollution: Methane emission controls yield a double dividend

An important area of research at GFDL is investigating the contribution of methane to surface ozone pollution, and quantifying the potential benefits to air quality and climate from controls on methane emissions. Methane is both a greenhouse gas and an important contributor to background levels of ozone. Tropospheric ozone, a significant greenhouse gas and the primary constituent of photochemical smog, provides an obvious link between air quality and climate.

Figure 1: Historical growth in atmospheric methane abundances (left axis; ppb) and the associated radiative forcing (right axis; W m-2). Excerpted from Figure 2a in the IPCC TAR 2001 Summary for Policymakers. (larger image)

Figure 2: Global mean radiative forcing (W m-2) of the climate system for the year 2000, relative to 1750. Taken from the IPCC TAR 2001 Summary for Policymakers. Radiative forcing is a metric used to gauge the relative climatic implications resulting from a perturbation to atmospheric composition; a positive forcing will yield a surface warming whereas a negative forcing will decrease surface temperature. (larger image)

Figure 3. Weighted global mean annual surface methane in dry air (ppb) sampled at 42 NOAA sites where a minimum of eight years were available. The global means are calculated by averaging within the latitudinal bands 30-90S, 30-0S, 0-30N, 30-60N, 60-90N and then weighting by area. Results are taken from a recent study by NOAA scientists including Arlene Fiore (GFDL), Larry W. Horowitz(GFDL), Edward Dlugokencky(ESRL/GMD) and Jason West(GFDL/Princeton). Please see Fiore et al., Geophys. Res. Lett., 33, L12809, 2006, doi:10.1029/2006GL026199 for more details.

The atmospheric abundance of methane has risen dramatically over the past century, more than doubling (Figure 1). In the troposphere (the layer of the atmosphere where weather occurs, extending from the surface to approximately 12 km), ozone concentrations are also thought to have increased by at least a factor of two over the past century.

Increases in the atmospheric abundance of methane and ozone have implications for climate since they are the second and third most important anthropogenic greenhouse gases, respectively, behind carbon dioxide; together they contributed roughly two-thirds as much radiative forcing as carbon dioxide did, from 1750 to 2000 (Figure 2).

Global methane emissions can be constrained to within ~30% by measurements of atmospheric methane abundances (Figures 1 and 3) and knowledge of the major methane sink – chemical destruction in the troposphere, which can lead to ozone production. The distribution of methane emissions is even more uncertain, with estimates on some sources varying by more than a factor of 2. Anthropogenic sources (coal mining, natural gas extraction and distribution, landfills, wastewater, ruminants, rice cultivation, and biomass burning) are estimated to contribute 60% to total methane emissions. The natural source is likely dominated by wetlands, with smaller contributions from termites, wildfires, and ocean reservoirs, although the recent discovery of methane emissions from plants may result in a downwards revision of previous wetland emission estimates.

Figures 1 and 3 show that methane abundances continued to increase over the last century before leveling off in the late 1990s. In simulations with a global atmospheric chemistry model, GFDL scientists Arlene Fiore, Larry Horowitz, Jason West, and collaborator Edward Dlugokencky (ESRL), found that this flattening of the methane trend could be driven by changes in meteorology that increased the methane sink in 2000-2004 relative to 1991-1995. Other recent studies, however, attribute the trend to changes in emissions. Clearly, more work is needed to reconcile these different explanations.

When methane is chemically destroyed in the atmosphere in the presence of sufficient nitrogen oxide, ozone is produced. In the United States, ozone in surface air is regulated as a pollutant under the Clean Air Act due to its detrimental impacts on human health and vegetation. Ozone pollution has been a particularly challenging problem since it is not emitted directly, but instead is produced during a complex series of reactions involving nitrogen oxides, hydrocarbons and sunlight. When these ingredients combine in stagnant, humid air masses, ground-level ozone can rapidly accumulate on a time-scale of hours. Ozone concentrations in excess of the national ozone standard tend to occur most often in summer, when meteorological conditions are particularly conducive to ozone production, leading to pollution episodes which typically last a few days at most.

Until recently, methane was considered irrelevant for addressing surface ozone pollution because its long atmospheric lifetime (8-9 years) prevents it from contributing to the rapid photochemical production which leads to high ozone episodes. Rather, methane plays a role in contributing to background tropospheric ozone. Increases in methane will thus raise the baseline ozone level in air globally, including at the surface. Ozone episodes, fueled by the traditional short-lived ozone precursors that are regulated in the United States (nitrogen oxides and non-methane hydrocarbons), then build on top of this baseline.

GFDL model output shows a decrease in annual mean daily maximum 8-hour surface ozone mixing ratios

Figure 4: Decrease in annual mean daily maximum 8-hour surface ozone mixing ratios (ppbv) in the MOZART-2 model due to a 20% reduction in anthropogenic methane emissions. For more details, please refer to a recent study by GFDL researchers: West et al., Proc. Natl. Acad. Sci., 103 (11) 3988-3993, 2006. (larger image)

Figure 5: Methane emission reduction potential in 2010 in North America (green), the rest of Annex I (pink; Annex I refers to all nations in Annex I of the United Nations Framework Convention on Climate Change), and the rest of the world (blue), as estimated by IEA for five industrial sectors (top bar of each pair) and EPA for four industrial and one agricultural sector (lower bar of each pair). The EPA estimates are smaller than IEA since the EPA does not include the wastewater sector. The top axis and the numbers to the right of the bars show the resulting reductions in northern hemispheric surface ozone ultimately achieved if the available methane reductions are implemented. For more details, please refer to West and Fiore, Environ. Sci. & Technol., 39 (13) 2005. (larger image)

GFDL scientists have been working to quantify the potential climate and air quality benefits resulting from controls on methane emissions. A 20% decrease in anthropogenic methane emissions would decrease surface ozone concentrations globally (Figure 4). The 20% decrease in anthropogenic methane emissions would also lower radiative forcing from both methane and ozone. In contrast, while reductions in traditional ozone precursors (nitrogen oxides and non-methane hydrocarbons) improve air quality, they have little effect on climate.

In addition to advancing the scientific understanding of the relationship between methane and surface ozone, researchers at GFDL and Princeton are working to translate those findings into terms more relevant for decision makers. Figure 5 illustrates an analysis led by Jason West of the costs of reducing anthropogenic methane emissions. Cost-savings reductions, which enable the capture of methane for use as natural gas and are therefore profitable, correspond to roughly 10% of current global anthropogenic methane emissions. Approximately 20% of global anthropogenic methane emissions can be reduced at <$10 per ton CO2 equivalent, a cost for control measures that should be economically feasible. For example, GFDL researchers found that the monetized global health benefits resulting from the decrease in surface ozone (shown in Figure 4) outweigh the control costs of a 20% reduction in global anthropogenic methane emissions. Figure 5 further shows that implementing all identified control measures, regardless of cost, could achieve a ~30% reduction in global anthropogenic methane emissions and yield a maximum decrease in global surface ozone of < 2 ppbv. The reduction potentials in Figure 5 are conservatively estimated because they omit many abatement opportunities in the large agricultural sector (cattle and rice cultivation), for which costs are not well quantified. The existence of relatively low-cost options for controlling methane is attractive because it is increasingly expensive to reduce surface ozone in the United States through additional controls on the traditional ozone precursors (nitrogen oxide and non-methane hydrocarbons).

At GFDL, researchers Arlene Fiore, Jason West, and Larry Horowitz are continuing their efforts to quantify the double dividend of air quality and climate benefits from controlling methane emissions. They have developed several methane control scenarios projecting emissions to 2030, which will be used to identify the resulting benefits for ozone air quality and climate. An assessment of the costs of methane emission controls as well as the benefits to global health associated with lower global surface ozone concentrations will be conducted, with the aim of translating GFDL model results into information that may be useful to decision makers.