The Earth’s surface temperature is determined by the balance between incoming solar radiation and outgoing infrared radiation. Radiative Forcing (RF) is the measurement of the capacity of a gas or other forcing agents to affect that energy balance, thereby contributing to climate change. Put more simply, RF expresses the change in energy in the atmosphere due to GHG emissions. The RF of a gas is defined as the difference between incoming solar radiation and outgoing infrared radiation caused by the increased concentration of that gas . Radiative forcing is expressed in Watts per square meter (W/m2) or the rate of energy change per unit area of the globe as measured at the top of the atmosphere.
- Positive radiative forcing results in an increase in Earth’s energy budget and ultimately leads to warming. Because GHGs absorb infrared radiation and re-emit it back to the Earth’s surface, thus increasing the Earth’s energy balance, they have positive RF values.
- Negative radiative forcing results in a decrease in the energy budget and ultimately leads to cooling. Aerosol particles reflect solar radiation, leading to a net cooling, and therefore have negative RF values.
The radiative forcing of a GHG is determined by its atmospheric concentration, warming capacity, residence time, and spatial distribution:
Amount/Atmospheric Concentration is determined by the emitted quantity of a GHG and by how much of it stays in the atmosphere. The greater the concentration of a GHG in the atmosphere, the larger its impact will be.
Warming or Cooling Capacity refers to the “strength” or potency of an emitted gas to act as a GHG. Not all GHGs have the same warming/cooling capacity; some gases are more effective than others at trapping heat . For example, over a 100-year time frame (see Global Warming Potential), a molecule of methane is approximately 25 times more potent (effective at trapping radiation and inducing warming) than a molecule of CO2.
Duration/Residence Time in the Atmosphere refers to the time a GHG stays in the atmosphere. Some GHGs are short-lived while others remain in the atmosphere for hundreds or thousands of years. To properly asses the climate impacts of a combination of gases, the lifetime of each gas has to be taken into account. For example, the warming impacts of CO2 persist for hundreds of years, whereas the warming impacts of ozone or contrails last only days or months.
Spatial Distribution refers to how far GHGs spread geographically. Long-lived greenhouse gases spread across the entire global atmosphere (e.g. CO2 and methane); their warming impact is therefore global in scale. Other gases are short-lived and their warming effects are local or regional. Residence time of GHGs is therefore related to spatial distribution. Globally-averaged radiative forcing calculations (see, for example, Figure 2) do not take into account these differences in spatial distributions.
Radiative forcing has been used as a proxy to express the climate response of different GHGs. Figure 2 illustrates the RFs from aircraft emissions in 1992 and 2000 as reported by Sausen et al. (2005) . These RF calculations are based on atmospheric concentrations of GHGs in 2000 due to aviation emissions starting in the 1940s.
Figure 2: Radiative Forcing of Aircraft Emissions in 1992 and 2000 (emissions from 1940s to 2000) (Source: Sausen et al., 2005)
Scientific Uncertainty: Figure 2 reflects scientific uncertainties of specific emissions both with error bars and terminology along the x-axis. For example, little is known about the warming impacts of aircraft-induced cirrus clouds, as indicated by the rating of “poor” at the bottom and by the different graphical representation (lines instead of bars for the cirrus RF). Because of these uncertainties, total RF on the right does not include effects of cirrus clouds.
Future Impacts Not Counted: Figure 2 does not represent future warming impacts from any of the emissions. This underestimates the total impacts of long-lived gases such as CO2 when compared to short-lived gases like ozone.
Past Cumulative Impacts: Figure 2 shows the RF of long-lived GHGs from air travel which have accumulated over approximately 60 years. The RF for short-lived gases, on the other hand, does not include past emissions, but only current emission levels, because short-lived gases decay quickly and past emissions are therefore no longer present in the atmosphere.
In order to evaluate if RF can be used as a metric for determining the climate footprint of an air travel passenger, it is important to understand the following characteristics of RF:
RF is an instantaneous measure: it expresses the climate forcing of a greenhouse gas at a particular point in time.
Yet it also has a temporal component: it is a backward-looking metric because it measures the RF of a GHG that has accumulated in the atmosphere over a certain period of time (e.g. aviation emissions over approximately 60 years in Figure 2).
Short-lived GHGs, such as ozone, do not accumulate over time because they decay rapidly. The RF given in Figure 2 therefore shows only the RF of current concentrations of short-lived gases, whereas it shows the RF of accumulated concentrations for long-lived GHGs. In other words: if emissions stay constant, the RFs for short-lived GHGs stay constant over time. Long-lived gases such as methane and CO2, on the other hand, accumulate over time, even if emissions stay constant, and thus their RF will increase over time.
Because RF is an instantaneous, backward-looking metric, it does not account for future impacts of GHGs. Long-lived GHGs will continue to warm the atmosphere for the duration of their residence time. Consequently, RF values do not express the total climate response of long-lived gases.
Globally-averaged RF values, such as the ones in Figure 2, do not account for regional variability in forcings and their climate responses. This is pertinent, since the potential damage of local warming due to locally-occurring GHGs (and their potential positive feedbacks) might be more intense than if the warming impact of those GHGs is spread out globally (RF values in Figure 2 apply only to total global annual emissions). If RFs are globally averaged, it might seem that cooling effects and warming effects can neutralize each other, yet this is not necessarily the case:
Importantly, global cancellations between the responses of different forcings do not necessarily represent regional cancellation between their responses. […]The net effect, given the regional pattern of airline flights, is therefore a Northern Hemisphere warming and Southern Hemisphere cooling (Forster and Rogers 2008).
It is, for example, plausible that a small global-mean temperature response could occur from large temperature changes of opposing signs in the two hemispheres; it is unlikely that the global-mean response would adequately reflect the impact (e.g. the damage) associated with such a response. However, we are unaware of any simple models that have, as yet, adequately addressed this generic weakness.