A fundamental issue with changes in atmospheric chemistry is that there may be multiple, and potentially competing effects in terms of problems caused to ecosystems or human welfare. For instance, aerosol declines in developed nations since the middle of the century result in less health and pollution issues, but also lead to global brightening which makes the more of the greenhouse gas influence show up.
The Montreal Protocol put limits on ozone-depleting substances such as Clorofluorocarbon’s (CFC’s). HCFC’s and HFC’s usage have increased as a replacement for CFC’s, and the usage of HFC’s is expected to grow as the 21st century progresses. HFC’s do not deplete the ozone layer, however a tradeoff exists: they are greenhouse gases and thus significant usage of them will contribute to radiative forcing that will add to the influence of CO2, CH4, etc in the future. However, the future emission trajectories and impacts of increasing HFC’s are not well studied. The incremental radiative forcing change with similar concentration changes in HFC’s are much larger than that of carbon dioxide, owing to where they block outgoing radiation and the low background concentration. For instance, the radiative efficiency for a small change in CO2 from a background of 380 ppm is approximately .014 W m-2 ppm-1, whereas, say, HFC-134a is 160, HFC-23 is 190, and so forth (see IPCC AR4 Table 2.14). The Global Warming Potential (which is proportional to radiative efficiency and changes with lifetime in the atmosphere) of HFC 134a is about 1,430 times that of CO2 given a time horizon of 100 years.
A recent paper by Guus Velders et al have focused on this specific topic, presenting consumption/emission scenarios as an update to SRES. According to their results, in developing nations, use of HFC’s is expected to increase 800% more than in developed nations, and total GWP-weighted HFC emissions are about 6–9 Gt CO2-equivalent per year by 2050, several times larger than SRES projections. In 2050, the RF of global HFC’s is suggested to be the range of 0.25–0.40 Watts per Square meter, several times larger than SRES suggestions and a bit smaller than modern methane forcing. The authors express concern that HFC’s could become a very important contribution agent to climate forcing is unconstrained, and discuss mitigation options in the linked text.