Enhanced weathering strategies for cooling the planet and saving coral reefs

Abstract:

Acceleration of the chemical weathering sink for atmospheric CO₂ via distribution of pulverized silicate rocks across terrestrial landscapes has been proposed as a macro-engineering Carbon Dioxide Removal (CDR) scheme, but its effectiveness and response to ongoing global change is poorly understood. We employ a detailed spatially resolved weathering model driven by two ensemble Representative Concentration Pathway (RCP) projections of 21^st Century climate (RCP8.5 and RCP4.5) to assess enhanced weathering and examine feedbacks on atmospheric CO2 and ocean carbonate biogeochemistry. Atmospheric CO₂ reduction of ~100-260 ppm by year 2100, the range depending mainly on rock composition, is obtained by spreading 5 kg m^-2 yr^-1 over 20 Mkm² tropical weathering ‘hotspots’. Ocean acidification is neutralized in RCP4.5 and ameliorated in RCP8.5 due to enhanced land-ocean export of weathered alkalinity products and reduced CO₂ forcings, and the aragonite saturation state of surface oceans is raised to >3.5, thus avoiding likely extinction of coral reef ecosystems. We suggest that accelerated weathering has substantial potential to help limit global warming and benefits to marine life not obtained from other CDR approaches, but major issues of cost, social acceptability, and potential unanticipated consequences should encourage urgent efforts to phase down fossil fuel emissions.