The reversibility of CO₂ induced climate change

Abstract

This paper investigates the reversibility of CO₂ induced climate change and in particular the potential impacts of different rates of CO₂ reduction using a coupled climate model. Atmospheric CO₂ concentration is ramped up by 0.5 %/year from the preindustrial value to 4×CO₂ and then ramped down from 2×CO₂ to 4×CO₂ with different rates. How the response of the climate system is affected by the peak atmospheric CO₂ concentration and the rate of long term decline is vital information for those considering hypothetical geoengineering options to remove CO₂. Major components of the climate system including global mean surface air temperature and precipitation, contribution of thermal expansion to global sea level rise, loss of the Arctic sea ice, weakening of the Atlantic meridional overturning circulation (AMOC) and the South Asia monsoon are analyzed. We have found no ‘tipping points’ or thresholds beyond which CO₂ induced climate change in these components become irreversible within this model under the specific scenarios. However, there are strong inertias and path-dependent hysteresis in the climate system linked through oceanic memory. Initially the strengthened global hydrological cycle accelerates further in response to a CO₂ ramp-down before weakening. Thermal expansion of the oceans continues for many decades after CO₂ concentration starts to decrease. A 0.5 %/year reduction from 4×CO₂ could see a further 25 % sea level rise. The weakening of the AMOC is reversible, but the build-up of highly saline subtropical waters during global warming drives an overshoot of the AMOC after the CO₂ ramp-down and extends the warming of the northern high latitudes by many decades. The South Asia monsoon strengthens in response to a CO₂ ramp-up marked by an increase in summer monsoon rainfall. This increase reverses rapidly following a CO₂ ramp-down, displaying an undershoot in monsoon rainfall for rapid CO₂ reductions.