The Spectral Nature of Stratospheric Temperature Adjustment and its Application to Halocarbon Radiative Forcing.

Stratospheric temperature adjustment (STA) is often a significant component of greenhouse gas radiative forcing (RF), for both the most widely used definition of RF and effective radiative forcing. It is well established that the magnitude and sign of STA differs among greenhouse gases, being negative (at the tropopause) for CO2 increases (because it induces a cooling of the stratosphere) and positive for many halocarbons (because they induce a warming of the stratosphere); this effect is strongly related to the opacity (and hence the effective emitting temperature) of the troposphere at the wavelengths at which gases absorb. Here the spectral variation of STA is examined for the first time by systematically imposing a weak absorber in each of 300 bands of a narrow‐band radiation code. For this weak absorber, STA is negative for wavelengths greater than about 13 μm and positive in the 8‐ to 13‐μm "window" except in the vicinity of the 9.6‐μm ozone band. By combining narrow‐band and line‐by‐line model results, these findings are used to improve a widely used fast method to estimate radiative efficiency (RE, the RF per unit change in concentration) directly from laboratory measurements or theoretical calculations of halocarbon absorption cross sections; the new method reproduces detailed RE calculations to better than 1.4%. This is a significant improvement over a cruder method used to account for STA in RE tabulations in the Intergovernmental Panel on Climate Change's Fifth Assessment Report, which were used to calculate metrics such as the Global Warming Potential, for halocarbons and related substances. Plain Language Summary: Increased emissions and concentrations of greenhouse gases, due to human activity, are a major driver of climate change. Carbon dioxide is the most important such gas, but many other gases are emitted which, collectively, contribute significantly to climate change. This paper focuses on methods to calculate the climate impact of halocarbons which are used in refrigeration, insulation, air conditioning, etc. This includes chlorofluorocarbons (CFCs), whose usage has been phased out because they deplete the ozone layer; hydrochlorofluorocarbons (HCFCs) which were regarded as transitional substances to replace the CFCs; and hydrofluorocarbons, which are widely used replacements for CFCs and HCFCs. It is important to quantify the climate impacts of halocarbons and to compare the relative impact of different halocarbons; this information is used in international agreements aimed at reducing greenhouse gas emissions. This paper presents an important refinement of a widely used simple method for calculating and comparing the climate impact of different halocarbons. This refinement considers the impact of these gases on temperatures in the stratosphere (i.e., altitudes of about 10–50 km); to achieve this, the paper first develops a general understanding of the way such gases influence stratospheric temperatures. The new technique is a significant improvement over previous methods. Key Points: Stratospheric temperature adjustment is an important component of the effective radiative forcing for many greenhouse gasesThe variation of the magnitude and sign of this adjustment with the wavelength at which greenhouse gases absorb is investigatedThe results are used to enhance a widely used simple method for estimating halocarbon radiative efficiency [ABSTRACT FROM AUTHOR]