Here I(gas) is the instantaneous radiative forcing by the gas at time t. I(gas) depends on the basic molecular properties of the gas and on general atmospheric composition (distribution of gases, clouds, aerosols). M(gas) is the mass of added gas still remaining at time t [initially M(gas) = M(CO2)]. It depends on the lifetime of the gas, which in turn depends on both the amounts of the gas itself and of other gases. Finally, Tis the time horizon for integration (gases with lifetimes longer or shorter than CO2 have GWPs increasing or decreasing, respectively, with increasing T ).
At least three major difficulties are inherent in the definition of GWPs, and these difficulties limit their usefulness. The first is that the lifetime of a gas (and hence M(gas)) may not simply be a function of its own concentration. For example, the lifetime of CH4 depends on OH, which depends not only on CH4, but also on the levels of NOx, CO, and other hydrocarbons. The OH levels also depend on ultraviolet fluxes, which in turn depend on levels of gases that deplete stratospheric ozone like N2O and the CFCs. 2
The second is that the time horizon T is arbitrary. It should logically be long enough to correspond to long-term climatic (as opposed to weather) variations, but short enough to have relevance to the development of human activity. There is also the matter of the timing of emission reductions of various gases relative to the particular target year chosen for assessing the climate changes avoided by these reductions (the year 2100 is often used as this target year). The Kyoto Protocol chooses T = 100 years. But the effects of changing this assumption can be profound. For example, the GWP of CH4 is 23 for T = 100 years but increases to 62 for T = 20 years and decreases to 7 for T = 500 years (the lifetime of CH4 is about 10 times less than CO2). Conversely the GWP of SF6 rises from 15,100 to 22,200 to 32,400 for T = 20,100, and 500 years, respectively (the lifetime of SF6 is about 30 times greater than CO2). As an example of the combined effects of these first two shortcomings, Reilly et al. (1999) have shown that the use of 100-year GWPs to convert non-CO2 emissions into equivalent CO2 emissions leads to much greater model-projected warming between 1990 and 2100 than a similar model run that uses actual non-CO2 gas emissions and includes all of their chemical and radiative interactions. Specifically, CH4 emissions reductions are significantly undervalued using T = 100 years.
Finally, the GWP definition depends upon M(CO2) and hence upon all the assumptions regarding oceanic and terrestrial CO2 sinks. The many uncertainties in the carbon cycle discussed elsewhere in this volume therefore carry over into all of the GWP calculations.
Here is a partial list of outstanding issues regarding the budgets of non-CO2 gases and their climatic effects relative to CO2:
1. For methane, the magnitudes of its wide range of surface sources as functions of position, time, and process need better quantification. What drives the lowering of the trend in CH4? Is it driven by steady increases in OH (Krol et al. 1998; Karls-dottir and Isaksen 2000) or by near-constant long-term OH and emissions, leading to an approach to a (zero-trend) steady state (Dlugokencky et al. 1998; Prinn et al. 2001).
2. For nitrous oxide, similar questions remain concerning its surface sources. What causes the steady increase in N2O since 1978 (Prinn et al. 2000)?
3. For halocarbons and SF6, there is a continued need for improved measurements and industrial emission estimates to reconcile the global budgets of many species.
4. For nonmethane hydrocarbons, CO, NOx, and tropospheric O3, there is a need to better understand the roles of these species in controlling OH levels and hence the lifetimes of CH4, HFCs, and HCFCs (Prinn 2003). For this purpose, improved emission and /or trend measurements of hydrocarbons, NOx, and O3 over the globe are essential. For stratospheric O3, there is a need to improve knowledge of its role in controlling the lifetimes of trace gases through modulation of ultraviolet fluxes to the troposphere.
5. For both methane and nitrous oxide, the combined roles of climate change and ecosystem (soil, wetland) changes on the surface fluxes of these gases need better definition. We expect their emissions to increase with increased warming, soil wetting, N deposition, and soil carbon (Hall and Matson 1999; Prinn et al. 1999). Is this occurring?
6. Methods for lowering emissions of these non-CO2 gases need to be carefully evaluated and included along with CO2 in climate policy (Robertson, Chapter 29, this volume).
7. For ranking the climatic effects of non-CO2 gases relative to CO2, there is a need to improve the GWP concept (Manne and Richels, Chapter 25, this volume). It is probable that a single number will not be adequate and that the use of models that explicitly include the interactions missed by the GWPs will be necessary.
This work was supported by the National Science Foundation (Grant ATM-0120468)
and by the federal and industry sponsors of the MIT Joint Program on the Science and
Policy of Global Change.
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