Nature, 459, 829-832 (11 June 2009); doi: 10.1038/nature08047; received 4 December 2008; accepted 14 April 2009.

The proportionality of global warming to cumulative carbon emissions

H. Damon Matthews* (Department of Geography, Planning and Environment, Concordia University, 1455 de Maisonneuve Blvd W., Montreal, Quebec H3G 1M8, Canada), Nathan P. Gillett (Canadian Centre for Climate Modelling and Analysis, Environment Canada, 3800 Finnerty Road, Victoria, BC V8P 5C2, Canada), Peter A. Stott (Met Office Hadley Centre, FitzRoy Road, Exeter, Devon EX1 3PB, U.K.) and Kirsten Zickfeld (Canadian Centre for Climate Modelling and Analysis, Environment Canada, 3800 Finnerty Road, Victoria, BC V8P 5C2, Canada) 

Abstract

The global temperature response to increasing atmospheric CO₂ is often quantified by metrics such as equilibrium climate sensitivity and transient climate response¹. These approaches, however, do not account for carbon cycle feedbacks and therefore do not fully represent the net response of the Earth system to anthropogenic CO₂ emissions. Climate–carbon modelling experiments have shown that: (1) the warming per unit CO₂ emitted does not depend on the background CO₂ concentration²; (2) the total allowable emissions for climate stabilization do not depend on the timing of those emissions^{3, 4, 5}; and (3) the temperature response to a pulse of CO₂ is approximately constant on timescales of decades to centuries^{3, 6, 7, 8}. Here we generalize these results and show that the carbon–climate response (CCR), defined as the ratio of temperature change to cumulative carbon emissions, is approximately independent of both the atmospheric CO₂ concentration and its rate of change on these timescales. From observational constraints, we estimate CCR to be in the range 1.0–2.1 °C per trillion tonnes of carbon (Tt C) emitted (5th to 95th percentiles), consistent with twenty-first-century CCR values simulated by climate–carbon models. Uncertainty in land-use CO₂ emissions and aerosol forcing, however, means that higher observationally constrained values cannot be excluded. The CCR, when evaluated from climate–carbon models under idealized conditions, represents a simple yet robust metric for comparing models, which aggregates both climate feedbacks and carbon cycle feedbacks. CCR is also likely to be a useful concept for climate change mitigation and policy; by combining the uncertainties associated with climate sensitivity, carbon sinks and climate–carbon feedbacks into a single quantity, the CCR allows CO₂-induced global mean temperature change to be inferred directly from cumulative carbon emissions.

*Correspondence e-mail: dmatthew@alcor.concordia.ca

Link to abstract:  http://www.nature.com/nature/journal/v459/n7248/abs/nature08047.html