The role of artificial atmospheric CO2 removal in stabilizing Earth’s climate

Abstract:

The current CO₂ emission trend entails a risk that the 2°C target will be missed, potentially causing “dangerous” changes in Earth’s climate system. This research explores the role of artificial atmospheric CO₂ removal (also referred to as “negative emissions”) in stabilizing Earth’s climate after overshoot. We designed a range of plausible CO₂ emission scenarios, which follow a gradual transition from a fossil fuel driven economy to a zero-emission energy system, followed by a period of negative emissions. The scenarios differ in peak emissions rate and, accordingly, the amount of negative emissions, to reach the same cumulative emissions compatible with the 2°C temperature stabilization target. The climate system components’ responses are computed using the University of Victoria Earth System Climate Model of intermediate complexity.

Results suggest that negative emissions are effective in reversing the global mean temperature and stabilizing it at a desired level (2°C above pre-industrial) after overshoot. Also, changes in the meridional overturning circulation and sea ice are reversible with the artificial removal of CO₂ from the atmosphere. However, sea level continues to rise and is not reversible for several centuries, even under assumption of large amounts of negative emissions. For sea level to decline, atmospheric CO₂ needs to be reduced to pre-industrial levels in our simulations.

During the negative emission phase, outgassing of CO₂ from terrestrial and marine carbon sinks offsets the artificial removal of atmospheric CO₂, thereby reducing its effectiveness. On land, the largest CO₂ outgassing occurs in the Tropics and is partially compensated by CO₂ uptake at northern high latitudes. In the ocean, outgassing occurs mostly in the Southern Ocean, North Atlantic and tropical Pacific. The strongest outgassing occurs for pathways entailing greatest amounts of negative emissions, such that the efficiency of CO₂ removal – here defined as the change in atmospheric CO₂ per unit negative emission - decreases with increasing amounts of negative emissions.