Long-term Terrestrial Carbon and Water Cycle Responses to Projected Climate Change Beyond 2100

Understanding the long-term responses of ecosystems to climate change is important for quantifying the strength of carbon-climate feedbacks and informing carbon management and energy policies. Using simulations from the Community Earth System Model version 1.0 (CESM1-BGC), we quantified the effects of increasing atmospheric CO2 and the resulting increases in temperature and changes in precipitation from 1850 to 2300 on terrestrial ecosystems. We analyzed the evolution of the global carbon and water cycles in four sets of Historical (1850-2005), Representative Concentration Pathway 8.5 (2006-2100), and Extended Concentration Pathway 8.5 (2101-2300) simulations with active land and ocean biogeochemical cycles. In the first set of simulations, the increasing atmospheric CO2 and other greenhouse gases and aerosols were radiatively coupled. In the second set, the CO2 was radiatively uncoupled; while in the third, CO2 and the other atmospheric forcing agents were radiatively uncoupled. The fourth set was radiatively coupled, but the ocean and land biogeochemistry experienced only the pre-industrial CO2 forcing. We found that carbon-climate feedbacks intensify as a consequence of weakening CO2 uptake by oceans and the terrestrial biosphere through the end of the 23rd century. We also investigated the effects of changing water availability on land uptake and biosphere productivity, and found an increase in terrestrial water storage globally as a result of intensification of the water cycle. Water cycle changes were driven by the balance between increasing temperatures, changing precipitation, melting snow and ice, thawing permafrost, and increasing leaf area and water use efficiency. Regional changes in precipitation and runoff were not consistent within latitude zones, resulting in differential effects on ecosystem productivity and fire frequency, particularly in the tropics. While tropical and mid-latitude productivity increases largely slowed or became negative at or before 2250, when atmospheric CO2 was stabilized, productivity in high latitudes continued to rise as temperatures and soil moisture continued to increase.