The large spread among Earth system model terrestrial carbon cycle projections for the 21st century highlights divergent understanding of biogeochemical processes in the Earth system and is taken as an indicator of large uncertainty. Climate scientists have analyzed uncertainty in climate projections in terms of model structure, forcing scenario, and internal variability. The same framework can be applied to the carbon cycle. In this seminar, I review sources of uncertainty in the carbon cycle (model structure, scenario, internal variability). Emphasis is placed on the atmosphere and its internal variability in driving the carbon cycle, and especially the importance of internal variability as a limit to reducing uncertainty. Analyses show that model structure (including both climate and biogeochemistry) is the dominant source of uncertainty among CMIP5 carbon cycle projections for the 21st century. Additional analyses find that climate uncertainty, not biogeochemical models per se, is a large contributor to total uncertainty. Analyses of initial condition large ensembles with the Community Earth System Model (CESM2) further highlight the importance of internal variability in the atmosphere as a source of carbon cycle uncertainty.
Dr. Gordon Bonan is a senior scientist with the Climate and Global Dynamics group at the National Center for Atmospheric Research (NCAR). He received his Ph.D. degree in Environmental Sciences from the University of Virginia in 1988 and joined NCAR in 1991. His primary research focus is development of and experimentation with coupled models of Earth's biosphere, atmosphere, hydrosphere, and geosphere system. Bonan's research integrates ecological, biogeochemical, hydrological, and atmospheric sciences to study terrestrial ecosystems, their responses to climate change, feedbacks that amplify or mitigate climate change, and human perturbations in land cover, land use, and ecosystem functions that alter climate. He is the lead developer of the NCAR Community Earth System Model (CESM) and has published several books on climate and ecosystems and on terrestrial biosphere models, including the most popular books Ecological Climatology and Climate Change and Terrestrial Ecosystem Modeling. Dr. Bonan is a Fellow of AGU, ESA, and AMS and has published 154 academic papers. He has been named a Clarivate Web of Science highly cited researcher from 2014 through 2020.