Global warming, climate change, GWPs and RFI explained
C. Shepherd Burton
Long-term average global surface temperature increases – global warming – result from changes in the Earth’s energy balance: more of the Sun’s incoming energy – radiation – is being retained (trapped) in the atmosphere than is being returned to space. Scientists have demonstrated that the relatively recently observed warming is, at least in part, a result of man’s release of certain “agents” into the atmosphere. Man’s activities, therefore, are changing the composition of the atmosphere in amounts sufficient to “force” changes to Earth’s climate – climate change. Weather patterns change in response to the change in energy balance. The most important agent is carbon dioxide, but others also contribute.
For any particular forcing, a change in the equilibrium global mean surface air temperature can be calculated. The formula for calculating the change is simple: ΔTs = λ RF, where ΔTs is the change in temperature (in degrees K), RF is the radiative forcing (W/m-2) and λ is proportionality constant with units of K/Wm-2, obtained from climate models. So, if one knows the radiative forcing (RF) for an agent and the proportionality constant (λ) one can use the formula for calculating the expected equilibrium global mean surface air temperature. If multiple agents are involved, the one calculates RF for each agent and sums up all the changes to get the combined temperature change of all agents. The proportionality in this formula forms one of the bases for the Kyoto Protocol and for emissions trading of the “agents” to be managed.
Some (forcing) agents make a positive (warming) contribution to the calculated change in equilibrium air temperature while others make a negative (cooling) contribution. So, care must be given to the sign of the change. The gaseous agents that make a positive contribution are called greenhouse gases. Further, equal amounts of greenhouse gases emitted to the atmosphere do not contribute equally to warming. No name has been given to those agents that cause cooling, either directly or indirectly; small suspended particles (aerosols) of sulfate cause direct cooling and emissions of NOx can reduce atmospheric levels of methane to cause a cooling but in doing so produce ozone which is a warming agent. The atmosphere we live in is complex.
Because the calculations required to estimate the change in equilibrium global surface temperature are costly and time consuming, scientists have devised two methods for analysts to compare, without calculation, the relative ability of (radiative) forcing agents to trap heat in the atmosphere. One (the global warming potential) applies to specific gaseous agents (greenhouse gases); the other (radiative forcing index) applies, collectively, to the agents released from aviation. Both are defined here even though our specific interest is aviation.
For specific gaseous agents, scientists have developed global warming potentials (GWPs). The GWP is used to compare the abilities of different greenhouse gases to trap heat (long-wave radiation emitted from the Earth’s surface into space) in the atmosphere. GWPs take into account the heat-absorbing ability of each gas relative to carbon dioxide as well as the removal of each gas from the atmosphere relative to carbon dioxide. So, a gas that is more effective in retaining heat than carbon dioxide and remains longer in the atmosphere than carbon dioxide has a GWP greater than 1. Some emitted gases have GWPs that are 20,000-fold more effective than carbon dioxide in trapping the Earth’s emitted heat. The GWP provides a way of converting emissions of various gases into a common unit, CO2-equivalents. In this way, analysts can add-up the radiative forcing of various greenhouse gases into carbon dioxide equivalents. The formula for calculating GWPs is correctly presented here.
An alternative to GWP is needed to measure the role of aviation in climate change. There are several forcing agents released during aircraft operations: some contributing directly to global warming and some making indirect contributions, each of which may contribute positively or negatively to warming. To account for all of them, scientists developed the radiative forcing index (RFI). The RFI is the ratio of the total radiative forcing – the sum of positive and negative contributions to warming – relative to carbon dioxide. A formula for RFI is RFI = Σ RFi/RFCO2, where RFi is the radiative forcing, RF, for the ith agent and RFCO2 is the RF for CO2. RFI, then, accounts for more than the release of carbon dioxide emissions from burning aviation fuel. It accounts for the release of the positive, direct contributors to warming – CO2, H2O (vapor) and soot from the combustion of fossil fuel – and to the indirect agents – NOx, sulfate and contrails, which also come from fossil-fuel combustion and which can contribute positively or negatively to global warming.
The most recent RFI for aviation was developed under the IPCC TRADEOFF project, with results published by Sausen et al. (2005) for aircraft operating during 2000. The components of the RFI for CO2, O3, CH4, H2O, sulfate, soot and contrails were estimates as 25.3, 21.9, -10.4, 2.0 -3.5, 2.5 and 10.0 W/m-2, respectively. The total across these estimates is 47.8 W/m-2, which when divided by 25.3 W/m-2 for the RF for CO2, gives 1.89 W/m-2 (= 47.8/25.3) for the RFI.
Reference.
Sausen, Robert, et al. (2005). “Aviation radiative forcing in 2000: An update on IPCC (1999)”, Meteorologische Festschrift (14) 555-561, 2005.
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