Equivalence of greenhouse-gas emissions for peak temperature limits

Climate policies address emissions of many greenhouse gases including carbon dioxide, methane, nitrous oxide and various halogen-containing compounds. These are aggregated and traded on a CO2-equivalent basis using the 100-year global warming potential (GWP100); however, the GWP100 has received scientific and economic criticism as a tool for policy. In particular, given international agreement to limit global average warming to 2 °C, the GWP100 does not measure temperature and does not clearly signal the need to limit cumulative CO2 emissions. Here, we show that future peak temperature is constrained by cumulative emissions of several long-lived gases (including CO2 and N2O) and emission rates of a separate basket of shorter-lived species (including CH4). For each basket we develop an emissions-equivalence metric allowing peak temperature to be estimated directly for any emissions scenario. Today’s emissions of shorter-lived species have a lesser impact on ultimate peak temperature than those nearer the time of peaking. The 2 °C limit could therefore be met by setting a limit to cumulative long-lived CO2-equivalent emissions while setting a maximum future rate for shorter-lived emissions.