Abstract
Abstract. The concept of Global Warming Potentials (GWPs) has been extensively used in policy consideration as a relative index for comparing the climate impact of an emitted greenhouse gas (GHG), relative to carbon dioxide with equal mass emissions. Ozone depletion due to emission of chlorinated or brominated halocarbons leads to cooling of the climate system in the opposite direction to the direct warming contribution by halocarbons as GHGs. This cooling is a key indirect effect of the halocarbons on climatic radiative forcing, which is accounted for by indirect GWPs. With respect to climate, it is critical to understand net influences considering direct warming and indirect cooling effects especially for Halons due to the greater ozone-depleting efficiency of bromine over chlorine. Until now, the indirect GWPs have been calculated using a parameterized approach based on the concept of Equivalent Effective Stratospheric Chlorine (EESC) and the observed ozone depletion over the last few decades. As a step towards obtaining indirect GWPs through a more robust approach, we use atmospheric models to explicitly calculate the indirect GWPs of Halon-1211 and Halon-1301 for a 100-year time horizon. State-of-the-art global chemistry-transport models (CTMs) were used as the computational tools to derive more realistic ozone depletion changes caused by an added pulse emission of the two major Halons at the surface. The radiative forcings on climate from the ozone changes have been calculated for indirect GWPs using an atmospheric radiative transfer model (RTM). The simulated temporal variations of global average total column Halons after a pulse perturbation follow an exponential decay with an e-folding time which is consistent with the expected chemical lifetimes of the Halons. Our calculated indirect GWPs for the two Halons are much smaller than those from past studies but are within a single standard deviation of WMO (2007) values and the direct GWP values derived agree with the published values. Our model-based assessment of the Halon indirect GWPs thus confirms the significant importance of indirect effects on climate.
Highlights
As a family of atmospheric gases, chlorinated and brominated halocarbons, including the human-produced chlorofluorocarbons (CFCs) and bromofluorocarbons, are all greenhouse gases (GHGs) in that they trap outgoing longwave radiation in the troposphere that would otherwise escape to space
For the given concentration of a gas, the radiative forcing depends primarily on the infrared absorption capacity of the gas. Since it was developed for the first scientific assessment of the Intergovernmental Panel on Climate Change (IPCC) in 1990, the concept of Global Warming Potentials (GWPs) has been used as a relative index for comparing the potential climate impact of different GHGs to a reference gas such as CO2 (IPCC, 1990, 2001, 2007; WMO, 2003, 2007)
The concept of GWPs has well recognized uncertainties and limitations of its own, GWPs are a better measure of the relative climate impacts than radiative forcing alone as they account for the atmospheric lifetime of the gases and the temporal change in concentration for a given emission
Summary
As a family of atmospheric gases, chlorinated and brominated halocarbons, including the human-produced chlorofluorocarbons (CFCs) and bromofluorocarbons (often refered to as Halons), are all greenhouse gases (GHGs) in that they trap outgoing longwave radiation in the troposphere that would otherwise escape to space. This trapped radiation warms the atmosphere, creating a positive forcing on the climate system, which in turn warms the Earth’s surface.
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