Cooperative Institute for Research in Environmental Sciences

Shorter-lived trace-gases: opportunities for mitigating ozone depletion and climate change

S. Montzka (1,2), G. Dutton (2), B. Miller (2), C. Siso (2), D. Mondeel (2), T. Conway (1), E. Dlugokencky (1), B. Hall (1), D. Nance (2), J. Elkins (1), and J. Butler (1)

(1) National Oceanic and Atmospheric Administration, Boulder, CO (2) Cooperative Institute for Research in Environmental Sciences

The depletion of stratospheric ozone is caused primarily by chemicals having lifetimes of 30 to 100 years. It might come as a surprise, then, that only a few years after nations across the globe limited production of these chemicals via the Montreal Protocol on Substances that Deplete the Ozone Layer the atmospheric concentration of ozone-depleting halogen began decreasing. The key to this rapid turnaround was restrictions on the production of shorter-lived, ozone-depleting chemicals, specifically methyl chloroform and methyl bromide. Controls on shorter-lived gases, however, have limits; the decline in ozone-depleting halogen concentration has been sustained over nearly 2 decades because long-lived gases were also controlled in the Montreal Protocol.

A parallel situation exists for greenhouse gases. Most radiative forcing is contributed by human-induced changes in carbon dioxide concentration. Furthermore, CO2 behaves as a long-lived gas: the global atmospheric concentration of CO2 is elevated for decades to millennia after an emission input. The trace gas with the next-largest contribution to radiative forcing, however, is methane. Methane is thought to have an atmospheric lifetime of ~10 years, and this timescale is set by the global mean hydroxyl radical concentration. Other non-CO2 greenhouse gases (N2O, CFCs, HCFCs, HFCs) also contribute to atmospheric radiative forcing and many have lifetimes shorter than CO2. Given this, reductions in emissions of these non-CO2 gases have the potential to slow the increase in radiative forcing on shorter timescales than could be achieved by reductions in CO2 emissions alone. In this presentation the factors influencing these timescales will be discussed, including variability in the global mean hydroxyl radical concentration, as will the limits of reducing emissions of non-CO2 greenhouse gases compared to those of CO2 as a means to slow the ongoing increase in global radiative forcing.