Setting cumulative emissions targets to reduce the risk of dangerous climate change

Abstract

Avoiding "dangerous anthropogenic interference with the climate system" requires stabilization of atmospheric greenhouse gas concentrations and substantial reductions in anthropogenic emissions. Here, we present an inverse approach to coupled climate-carbon cycle modeling, which allows us to estimate the probability that any given level of carbon dioxide (CO2) emissions will exceed specified long-term global mean temperature targets for "dangerous anthropogenic interference," taking into consideration uncertainties in climate sensitivity and the carbon cycle response to climate change. We show that to stabilize global mean temperature increase at 2 degrees C above preindustrial levels with a probability of at least 0.66, cumulative CO2 emissions from 2000 to 2500 must not exceed a median estimate of 590 petagrams of carbon (PgC) (range, 200 to 950 PgC). If the 2 degrees C temperature stabilization target is to be met with a probability of at least 0.9, median total allowable CO2 emissions are 170 PgC (range, -220 to 700 PgC). Furthermore, these estimates of cumulative CO2 emissions, compatible with a specified temperature stabilization target, are independent of the path taken to stabilization. Our analysis therefore supports an international policy framework aimed at avoiding dangerous anthropogenic interference formulated on the basis of total allowable greenhouse gas emissions.

Fig. 1.

Path independency of cumulative CO₂ emissions. (A) Cumulative CO₂ emissions and (B) CO₂ concentrations compatible with a global mean temperature increase of 2 °C relative to preindustrial times. The different curves refer to experiments with different prescribed temperature change trajectories (C). The red-dashed trajectory is the standard trajectory used throughout the analysis. Cumulative emissions are computed from the year 2001 onwards.

Fig. 2.

Dependence of cumulative CO₂ emissions on climate sensitivity. Cumulative CO₂ emissions (A–C) and CO₂ concentrations (D–F) compatible with a long-term global mean temperature increase, (ΔT), of 2 °C (A and D), 3 °C (B and E) and 4 °C (C and F) relative to preindustrial times (the prescribed temperature trajectories are displayed in Fig. S3). The different curves refer to experiments with climate sensitivities (cs) in the range 2–9 °C (the curves for cs = 1 °C are not shown for clarity). The standard climate sensitivity of the UVic ESCM is 3.6 °C (red-dashed curves). (A–C Right) Cumulative emissions in 2500 under assumption of weak (W) and strong (S) climate-carbon cycle feedbacks are shown.

Fig. 3.

Probability of exceeding specified global mean temperature targets for different CO₂ emissions levels. (A) 2 °C target. (B) 3 °C target. (C) 4 °C target. The brighter colors denote the range in emissions spanned by the different climate sensitivity PDFs, and the weaker colors denote the range spanned by additional uncertainty in the strength of the climate-carbon cycle feedback. Using the nomenclature suggested by the IPCC (37), we have divided the probability range into “very unlikely” (0.01 < P < 0.1), “unlikely” (0.1 < P < 0.33), “medium likelihood” (0.33 < P < 0.66), “likely” (0.66 < P < 0.9), and “very likely” (0.9 < P < 0.99).