Sensitivity of climate mitigation signals to climate engineering choice and implementation

Unlike historical carbon emissions, which have been driven by economics and politics, climate engineering methods must be scientifically assessed, with consideration as to the type, rate, and total amount implemented. Temperatures reductions from carbon dioxide removal have been found to be proportional to the cumulative amount of carbon removed.  Climate engineering “co-benefits”, such as reduced ocean acidification, may also occur and should be considered when optimising an engineered climate solution. In this study we examine the sensitivities of climate engineering to its implementation, focussing on the effects of its time of onset and rate of carbon capture or enhanced weathering, as well as  background emissions and ocean physics.

We use two simple coupled models– a Gnanadesikan-style coupled atmosphere-ocean model and the intermediate-complexity Earth system model GENIE – with idealised setups for negative emissions through either carbon capture and sequestration, enhanced weathering, or a combination. The inclusion of enhanced weathering provides insight as to how changes in ocean carbonate chemistry may impact climate, both in terms of temperature and pH changes. We have created ensembles in which the timing, rate, background emissions scenario, and model physics of the model vary and use these ensembles to understand how these decisions may impact the efficacy of climate engineering.

We find that the effectiveness of climate engineering is dependent upon the background carbon emissions and the choice of climate engineering. Carbon capture reduces surface average temperature more per PgC captured than enhanced weathering, and both are more effective under low emissions scenarios. Additionally, background emissions determine how the impact of climate engineering is realised: under high emissions, earlier implementation of climate engineering results in faster temperature mitigation, although the end state is independent of the onset. When considering reductions in ocean acidification, we find that the alkalinity flux in our enhanced weathering experiments leads to a higher pH than for carbon capture, as well as the pH signals being less dependent on the timing. Thus, the timing and pathway of the climate engineering is important in terms of the resulting averted warming and acidification, though the final equilibrium is still effectively determined by the cumulative carbon budget.