The time scale of the silicate weathering negative feedback on atmospheric CO 2

Abstract

The ultimate fate of CO 2 added to the ocean‐atmosphere system is chemical reaction with silicate minerals and burial as marine carbonates. The time scale of this silicate weathering negative feedback on atmospheric p CO 2 will determine the duration of perturbations to the carbon cycle, be they geological release events or the current anthropogenic perturbation. However, there has been little previous work on quantifying the time scale of the silicate weathering feedback, with the primary estimate of 300–400 kyr being traceable to an early box model study by Sundquist (1991). Here we employ a representation of terrestrial rock weathering in conjunction with the “GENIE” (Grid ENabled Integrated Earth system) model to elucidate the different time scales of atmospheric CO 2 regulation while including the main climate feedbacks on CO 2 uptake by the ocean. In this coupled model, the main dependencies of weathering—runoff, temperature, and biological productivity—were driven from an energy‐moisture balance atmosphere model and parameterized plant productivity. Long‐term projections (1 Myr) were conducted for idealized scenarios of 1000 and 5000 PgC fossil fuel emissions and their sensitivity to different model parameters was tested. By fitting model output to a series of exponentials we determined the e ‐folding time scale for atmospheric CO 2 drawdown by silicate weathering to be ∼240 kyr (range 170–380 kyr), significantly less than existing quantifications. Although the time scales for reequilibration of global surface temperature and surface ocean pH are similar to that for CO 2 , a much greater proportion of the peak temperature anomaly persists on this longest time scale; ∼21% compared to ∼10% for CO 2 .