Let's not get stuck on CO2 - other gas emissions may be easier to reduce

There remain a number of uncertainties in calculating the climatic effects of non-CO2 gases. One of these is the accuracy of global warming potentials. Analysis has shown that the GWPs currently in use significantly underestimate the role of methane. This error is due in part to omitted interactions, such as the role of methane in tropospheric ozone formation. Any correction of this bias would amplify the importance of the non-CO2 greenhouse gases.

The GWPs also fail to adequately portray the timing of the climate effects of abatement efforts. Because of its relatively short lifetime in the atmosphere, abatement efforts directed at methane have benefits in slowing climate change over the next few decades, whereas the benefits of CO2 abatement are spread over a century or longer.

To the extent one is concerned about slowing climate change over the next 50 years methane and HFCs - that last a decade or so - have an importance that is obscured when 100-year GWPs are used to compare the contributions of the various gases. Economic formulations of the GWP indices have been proposed that would address these concerns, but calculations are bedeviled by uncertainties, such as how to monetize the damages associated with climate change.

A still more difficult issue is whether and how to compare efforts to control other substances that affect the radiative balance of the atmosphere, such as tropospheric ozone precursors, black carbon, and cooling aerosols. The main issue with these substances is that, even though their climatic effects are important, a more immediate concern is that they also cause local and regional air pollution affecting human health, crop productivity, and ecosystems. Moreover, their climatic effects are mainly regional, or even local, which creates difficulties in designing a single index to represent their effects across the globe. It is essential to consider these substances as part of climate policy, but more research and analysis is needed to quantitatively establish their climate influence and to design policies that take account of their local and regional pollution effects.

Putting aside the local and regional air pollutants, the quantitative importance of the other non-CO2 greenhouse gases has now been relatively well established. One of the major remaining concerns is accurate measurement and monitoring so compliance can be assured, whatever set of policies are in place. This has less to do with the type of greenhouse gas than with the nature of its source. It is far easier to measure and monitor emissions from large point sources, such as electric power plants, than from widely dispersed non-point sources, such as automobile and truck tailpipes or farmers' fields. Methane released from large landfills can be easily measured, and is in the United States, but it is impractical to measure the methane emitted from each head of livestock, or the N2O from every farmer's field. A different regulatory approach may be needed for different sources.

Scientists have long recognized the roles of non-CO2 greenhouse gases and other substances that contribute to climate change. Only in the past few years have that the pieces of this complex puzzle come together to demonstrate how critical the control of these gases is for a cost-effective strategy to slow climate change. Control of non-CO2 greenhouse gases is a critical component of a cost-effective climate policy, and particularly in the near term these reductions can complement early efforts to control carbon dioxide.