Using Cape Grim carbon dioxide (CO₂) and non-CO2 greenhouse gas data we are able to derive key Kyoto and Montreal Protocol policy-relevant parameters for Cape Grim such as CO₂ levels, equivalent-CO₂ (CO₂-e) levels and Effective Equivalent Stratospheric Chlorine (EESC). We are also able to do this globally using data from the CSIRO and AGAGE (Advanced Global Atmospheric Gases Experiment) global networks.

Global CO₂ levels exceeded 400 ppm in 2015 and Southern Hemispheric CO₂ in March 2016, and have remained above 400 ppm ever since. Just as global 400 ppm CO₂ was a significant psychological milestone that resonated in the public and policy domains, it is likely that this will also be the case for 500 ppm CO2-e. When will this occur? We had originally predicted that global CO2-e would occur around 2020, with Cape Grim level likely to follow within a year of the global value exceeding 500 ppm.

So what has changed? A 2016 update of radiative forcing formulas (Etminan et al.) suggested that inclusion of shortwave absorption would have a significant effect on the radiative forcing of methane. New research published recently in Science, which draws on the 2016 work of Etminan et al., reveals that inclusion of the shortwave component of methane’s absorption spectrum requires an upward revision of methane’s radiative forcing, by 15%. By applying the new formulas outlined in Etminan et al. 2016 for CO₂, CH₄ and N₂O for radiative forcing we have found that Cape Grim has already passed the anticipated milestone in July 2018, and that the Global CO2-e reached this milestone in 2017. We will show the CO2-e results based on these new formulations and compare them to those outlined in IPCC (2001), along with the effects of which minor non-CO2 greenhouse gases are also used in the calculations.