Some Opinions on Rock-Weathering-Related Carbon Sinks from the IPCC Fifth Assessment Report

Many recent researches show that rock-weathering-related carbon sink, especially carbonate-weathering-related carbon sink, actively takes part in the modern global carbon cycle, which might greatly contribute to balancing global carbon budget. Some new opinions on flux, time scale and effect of rock-weathering-related carbon sink were released in IPCC fifth Assessment Report (AR5). The flux of global rock-weathering-related carbon sink is about 0.4 Pg C/a in AR5 report, which accounts for about 1/2~1/3 of unbalanced global carbon budget. New time scale of global rock-weathering-related carbon sink was released. Time scale of silicate-weathering-related carbon sink is 10⁴~ 10⁶ year, while carbonate-weathering-related carbon sink (karst processes) is 10³~10⁴ year. A highlight is that rock-weathering-related carbon sink is listed as one of four carbon dioxide removal methods in AR5 report, whose time scale is 10²~ 10³ year. Although AR5 report released these new opinions, it is still thought that the rate of global rock-weathering-related carbon sink is currently too small to offset the rate at which fossil fuel CO₂ is being emitted. According to many researches in the field of rock weathering science in the past decades, this review discussed the shortage in carbon flux, time scale and effect of rock-weathering-related carbon sink in AR5 report. Many recent researches indicated that the carbon sink originated from carbonate rock weathering was a fast and sensitive geochemical process, which showed multiple time scales (diel, seasonal, annual or storm event). A new model based on H₂O carbonate-CO₂-aquatic phototroph interaction was established, which coupled geological, land surface water, and submerged phototroph carbon cycle process. The global carbon sink from carbonate rock weathering in new model is 0.477 Pg C/a,which accounts for about 39%~59% of unbalanced global carbon budget. Some key scientific issues on rate, stability, controlling mechanisms and scaling of rock-weathering-related carbon sink should be vitally concerned in the future, which could greatly contribute to balancing the global carbon budget.