CO2 stabilization, climate change and the terrestrial carbon sink

A nonequilibrium, dynamic, global vegetation model, Hybrid v4.1, with a sub daily timestep, was driven by increasing CO2 and transient climate output f rom the UK Hadley Centre GCM (HadCM2) with simulated daily and interannual variability. Three IPCC emission scenarios were used: (i) IS92a, giving 790 ppm CO2 by 2100, (ii) CO2 stabilization at 750 ppm by 2225, and (iii) CO2 stabilization at 550 ppm by 2150. Land use and future N deposition were not included. In the IS92a scenario, boreal and tropical lands warmed 4.5 degr ees C by 2100 with rainfall decreased in parts of the tropics, where temper atures increased over 6 degrees C in some years and vapour pressure deficit s (VPD) doubled. Stabilization at 750 ppm CO2 delayed these changes by abou t 100 years while stabilization at 550 ppm limited the rise in global land surface temperature to 2.5 degrees C and lessened the appearance of relativ ely hot, dry areas in the tropics. Present-day global predictions were 645 PgC in vegetation, 1190 PgC in soil s, a mean carbon residence time of 40 years, NPP 47 PgC y(-1) and NEP (the terrestrial sink) about 1 PgC y(-1), distributed at both high and tropical latitudes. With IS92a emissions, the high latitude sink increased to the ye ar 2100, as forest NPP accelerated and forest vegetation carbon stocks incr eased. The tropics became a source of CO2 as forest dieback occurred in rel atively hot, dry areas in 2060-2080. High VPDs and temperatures reduced NPP in tropical forests, primarily by reducing stomatal conductance and increa sing maintenance respiration. Global NEP peaked at 3-4 PgC y(-1) in 2020-20 50 and then decreased abruptly to near zero by 2100 as the tropical source offset the high-latitude sink. The pattern of change in NEP was similar wit h CO2 stabilization at 750 ppm, but was delayed by about 100 years and with a less abrupt collapse in global NEP. CO2 stabilization at 550 ppm prevent ed sustained tropical forest dieback and enabled recovery to occur in favou rable years, while maintaining a similar time course of global NEP as occur red with 750 ppm stabilization. By lessening dieback, stabilization increas ed the fraction of carbon emissions taken up by the land. Comparable studie s and other evidence are discussed: climate-induced tropical forest dieback is considered a plausible risk of following an unmitigated emissions scena rio.