Global Groundwater–Carbon Cycling Shaped by Tropical Basins and Extra-Tropical Sinks

Abstract

Groundwater’s role in the global carbon cycle is poorly quantified compared to other carbon reservoirs, particularly on a global spatial scale. Here we show that the carbon stock of terrestrial groundwater is approximately equal to that stored in the modern atmosphere, and substantially exceeds that held in vegetation. Globally, groundwater represents a natural net sink equivalent in magnitude to the natural net ocean flux (circa −0.4 versus +0.6 PgC/yr, respectively), but this is unevenly distributed. Furthermore, the regions with large groundwater–carbon fluxes─either as CO2 emissions or sinks─are in regions projected to undergo major climate-driven hydrological change. The largest natural groundwater CO2 emissions occur in circum-Atlantic tropical basins (i.e., Amazon and Congo), which account for more than half of global terrestrial groundwater CO2 gas emissions; however, high recharge here largely offsets these emissions, rendering them currently near-CO2-neutral. In contrast, the greatest terrestrial groundwater–carbon sinks occur in the extra-tropical regions of Southern Africa, Europe, and the Mediterranean, along with tropical Madagascar, all of which are projected to experience substantial future changes in rainfall and evapotranspiration. Understanding how such hydroclimatic shifts will alter groundwater–atmosphere carbon exchanges therefore represents a key opportunity for future research.