A central challenge for sustaining international cooperation to cut global greenhouse gas emissions is confidence that national policy efforts are leading to a meaningful impact on the climate. Here, we apply a detection protocol to determine when the measurable signal of atmospheric CO2 can be distinguished from the noise of the carbon cycle and uncertainties in emission trends. We test that protocol with a database of 226 emission mitigation scenarios—the universe of scenarios vetted by the Intergovernmental Panel on Climate Change. These scenarios are descriptive of ‘baseline’ trajectories of emissions trends in the absence of new policies along with trajectories that reflect substantial policy efforts to stop warming at 1.5 °C–2 °C above pre-industrial levels, as embodied in the Paris Agreement. The most aggressive mitigation scenarios (i.e. 1.5 °C) require 11–16 years to detect a signal of demonstrable progress from the noise; 2 °C scenarios lengthen detection by at least a decade. As more climate policy analysts face the reality that goals of 1.5 °C–2 °C seem infeasible, they have developed ‘overshoot’ scenarios with emissions that rise above the agreed goal and then, later on, fall aggressively to achieve it. These pathways come at the political cost of a 1–2 decade delay in detection, even for the 1.5 °C scenarios. The Paris Agreement requires a global ‘stocktake’ that interrogates national mitigation efforts; our results suggest that this effort must grapple with the question of when the world can gain confidence that the diplomacy on climate is demonstrably making an impact.
The increase in atmospheric CO 2 since the turn of the 19th century has been driven by anthropogenic emissions from fossil fuel burning and industry (FF, hereafter "fossil emissions") and emissions and removals from land use change (LU, hereafter "land use flux"), including land management and related land cover changes (Friedlingstein et al., 2020). These emissions are offset by natural uptake of CO 2 by the terrestrial biosphere (B, hereafter referred to as the "terrestrial sink," for which "natural" is implied) and the ocean (O) (Broecker et al., 1979;Siegenthaler & Sarmiento, 1993). The balance of these sources and sinks determines the magnitude of the atmospheric growth rate (AGR): where all fluxes are in units of PgC year −1 , and positive values for each term indicate increasing strength of their source (FF, LU) or sink (O, B). The atmospheric CO 2 growth rate is well known from contemporary observations and historical reconstructions (Conway & (1) AGR = FF + LU − O − B
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