Trade-offs in aviation impacts on climate favour non-CO mitigation.

Climate assessments of civil aviation have consistently quantified the dominant climate-forcing components: (1) CO emissions, (2) NO (NO + NO) emissions and (3) persistent contrails. All three components exert a positive radiative forcing (RF) and lead to climate warming of similar magnitudes. The aviation community is actively seeking to reduce its climate footprint through advanced engine technologies, more sustainable aviation fuel and optimal routing plans. These approaches usually involve a trade-off of CO against NO or contrails (non-CO), such as burning 1% more fuel to decrease contrail RF by 4%. Here, we show that a climate-trade-off risk curve derived from uncertainties in the RF components can give the probability that a specified trade-off ratio will produce a climate benefit. For each component, we calculate the integrated effective RF resulting from 1 year of flights: global warming per activity (GWA). The complementary cumulative probability distribution of the GWA(non-CO) to GWA(CO) ratio results in a climate-trade-off risk curve giving the likelihood of a positive climate outcome as a function of the trade-off-CO to trade-off-non-CO ratio, because the product, GWA × trade-off, should be the same for both. We find a likely (67%) chance of climate mitigation on a 100-year time horizon for the above suggested ratio of 1:4, favouring proposed non-CO mitigation efforts with ratios smaller than this.