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 (GWP(100)); however, the GWP(100) has received scientific and economic criticism as a tool for policy(1-4). In particular, given international agreement to limit global average warming to 2 degrees C, the GWP(100) does not measure temperature and does not clearly signal the need to limit cumulative CO2 emissions(5-7). 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 degrees 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.