Although reducing emissions of carbon dioxide is essential if climate change is to be slowed and then halted in the long run, researchers are analyzing a number of alternative pathways for reducing human-related forcing on climate over the next few decades. These analyses focus on reductions in the emissions of other gases and particles. For example, a 2012 paper by Drew Shindell of the NASA Goddard Institute for Space Studies and a host of co-authors examines various means for slowing near-term changes in climate while also improving air quality by controlling emissions of methane and black carbon, or soot. The assumed technological improvements and regulations were estimated to reduce global climate warming by about 0.5 degrees Celsius by 2050 -- in large part through cuts in methane emissions, but with an important contribution from black-carbon emission reductions. But there is another category of emissions that should also be targeted: the compounds currently being heavily used as refrigerants, thermal-insulating foam production agents, and, in some applications, propellants.
The Montreal Protocol is a multilateral agreement that has proven to be highly successful in protecting stratospheric ozone -- which acts as a shield against ultraviolet light that is harmful to life -- by regulating the use and emissions of various chlorine- and bromine-containing halocarbon compounds. The protocol has already resulted in the phase-out of chlorofluorocarbons (CFCs), used largely as refrigerants and foam blowing agents, and halons, the bromine-containing compounds used largely as fire-fighting agents. The Montreal Protocol has been beneficial in two ways: It has helped stabilize the stratospheric ozone layer, and, because long-lived CFCs and other controlled halocarbons act as greenhouse gases, it has prevented a large effect on climate.
Hydrochlorofluorocarbons -- several of which served as the initial replacements for CFCs -- are in the process of being phased out because they too can destroy stratospheric ozone molecules, even though they have shorter atmospheric lifetimes than CFCs. As a result, replacements have been found for most uses of hydrochlorofluorocarbons, largely in the form of hydrofluorocarbons, which do not contain chlorine. But the increasing use and emissions have given rise to a new concern: Some of the key compounds in this class have long atmospheric lifetimes, and they are also greenhouse gases that can affect climate. HFCs have already become heavily used in home air conditioners, refrigeration applications, and the creation of insulating foam, and their production has been increasing by about 15 percent per year.
Through absorption of the Earth's radiation (mostly at infrared wavelengths), certain atmospheric gases contribute importantly to determining the average temperature of the planet. These so-called greenhouse gases include water vapor, carbon dioxide, methane, and CFCs and all of the halocarbons. As concentrations of greenhouse gases increase, more infrared radiation is trapped by the atmosphere, leading to a tendency for warming on the Earth's surface. The atmospheric lifetime of a heat-trapping gas is an important factor in determining its effect on climate; the longer the atmospheric lifetime the larger the potential climate concern.
HFCs are from 100 to 15,000 times more powerful than carbon dioxide in terms of radiative forcing integrated over a 100-year horizon per unit of mass emitted, according to the International Panel on Climate Change. Their use is expected to continue to increase, especially in developing countries in Asia. It is projected that the radiative forcing caused by HFCs could be 14 percent to 27 percent of the increase in carbon-dioxide forcing by 2050, should the trend of increasing usage of HFCs continue. There is much to be gained by limiting future production of HFCs.
HFCs are not currently included in the Montreal Protocol. Some argue that they should be added to that agreement, because HFCs are largely used as replacements for the compounds controlled under the protocol. Others argue that HFCs are not ozone-depleting substances and should therefore be controlled along with other compounds affecting climate under the UN Framework Convention on Climate Change.
The chemical industry is in the process of introducing a number of chemicals that have the potential of replacing hydrochlorofluorocarbons and HFCs in many uses. Each of these chemicals is likely to react rapidly with hydroxyl in the atmosphere, making their atmospheric lifetimes a month or less. As a result, their effects on climate should be appreciably smaller than the compounds they would replace. Depending on how rapidly the marketplace can convert to such new compounds, the effects on climate from HFCs can be greatly reduced -- perhaps equaling the positive climate effects from the cuts in black carbon emissions that Shindell contemplated. The possible use of these new compounds as replacements for the longer-lived HFCs will be an important consideration for policymakers over the next few years. The time has come to recognize the relationships between policies that protect the ozone layer and policies that minimize human influence on the Earth's climate.