Scientific issues in the design of metrics for inclusion of oxides of nitrogen in global climate agreements

Abstract

The Kyoto Protocol seeks to limit emissions of various greenhouse gases but excludes short-lived species and their precursors even though they cause a significant climate forcing. We explore the difficulties that are faced when designing metrics to compare the climate impact of emissions of oxides of nitrogen (NO(x)) with other emissions. There are two dimensions to this difficulty. The first concerns the definition of a metric that satisfactorily accounts for its climate impact. NO(x) emissions increase tropospheric ozone, but this increase and the resulting climate forcing depend strongly on the location of the emissions, with low-latitude emissions having a larger impact. NO(x) emissions also decrease methane concentrations, causing a global-mean radiative forcing similar in size but opposite in sign to the ozone forcing. The second dimension of difficulty concerns the intermodel differences in the values of computed metrics. We explore the use of indicators that could lead to metrics that, instead of using global-mean inputs, are computed locally and then averaged globally. These local metrics may depend less on cancellation in the global mean; the possibilities presented here seem more robust to model uncertainty, although their applicability depends on the poorly known relationship between local climate change and its societal/ecological impact. If it becomes a political imperative to include NO(x) emissions in future climate agreements, policy makers will be faced with difficult choices in selecting an appropriate metric.

Fig. 1.

Schematic illustrating possible metrics for NO_x emissions that lead to perturbations of both ozone and methane. Shown are the cases of a discrete pulse emission of NO_x (a-c) and a sustained emission change (d-f). (a and d) The evolution of the concentrations of NO_x, ozone, and methane. (b and e) The net (ozone plus methane) RF (the individual ozone and methane RFs follow the curves for the burden in a and d) and the parameters that can be used for climate metrics. The absolute GWP (AGWP) is the time-integrated RF over some time horizon (H). The degree of compensation between the ozone and methane RFs depends on the value of H. The RF at some time H could also be used in a metric. (c and f) The global-mean surface-temperature change in response to the RF from b and e. The absolute global temperature potential (AGTP) at some time H is another possible metric.

Fig. 2.

Zonal and annual mean surface-temperature change (in milli-Kelvin) in the two GCMs (a and b, UREAD; c and d, ECHAM4) for the NO_x Europe (a and c) and the NO_x Asia (b and d) cases using the LMD CTM ozone. Note that the x axes are linear in the sine of latitude such that they are linear in surface area. The global-mean change (in milli-Kelvin) is shown.