Discrepancies in historical emissions point to a wider 2020 gap between 2 °C benchmarks and aggregated national mitigation pledges

Aggregations of greenhouse gas mitigation pledges by countries are frequently used to indicate whether resulting global emissions in 2020 will be 'on track' to limit global temperature increase to below specific warming levels such as 1.5 or 2 degrees C. We find that historical emission levels aggregated from data that are officially reported by countries to the UNFCCC are lower than independent global emission estimates, such as the IPCC SRES scenarios. This discrepancy in historical emissions could substantially widen the gap between 2020 pledges and 2020 benchmarks, as the latter tend to be derived from scenarios that share similar historical emission levels to IPCC SRES scenarios. Three methods for resolving this discrepancy, here called 'harmonization', are presented and their influence on 'gap' estimates is discussed. Instead of a 3.4-9.2 GtCO(2)eq shortfall in emission reductions by 2020 compared with the 44 GtCO(2)eq benchmark, the actual gap might be as high as 5.4-12.5 GtCO(2)eq (a 22-88% increase of the gap) if this historical discrepancy is accounted for. Not applying this harmonization step when using 2020 emission benchmarks could lead to an underestimation of the insufficiency of current mitigation pledges.