Anthropogenic carbon dioxide (CO2) emissions are responsible for most of the Earth’s recent global surface warming. The land and the oceans currently take up about half of the emissions through complex carbon cycle processes. Climate forcing by anthropogenic CO2 emissions only stops when achieving a balance between CO2 sources and sinks.
Since it is unfeasible to eliminate all CO2 emissions by mid-21st century, all plausible future emission scenarios aiming for temperature stabilisation consistent with the Paris Agreement include a portfolio of human activities, which combine emission cuts with of so-called carbon dioxide removal (CDR) measures that should compensate for remaining positive emissions.
However, CDR measures like most other human activities will cause additional climate forcing through emissions of non-CO2 greenhouse gases (GHGs) (e.g. methane or nitrous oxide), aerosols or by land-use changes (LUCs). At present, GHGs other than CO2 make up more than 40% of the global surface warming while aerosols compensate for some of the warming (Mengis and Matthews, 2020). Moreover, these additional forcing agents affect the carbon cycle, which in turn feeds back on the atmospheric CO2 concentration and hence surface temperature. This interaction influences the amount of CO2 removal required through CDR measures to achieve temperature stabilisation.
It is thus of high priority to assess the complete climate response to specific anthropogenic activities, including CDR measures, in order to guide well-informed actions towards temperature stabilisation. In particular, investigating the Earth system’s response to realistic portfolios of future anthropogenic activities requires including all related climate forcing – CO2, non-CO2 GHGs, aerosols and LUCs – to obtain best estimates of temperature stabilisation pathways.