Initial Land Use/Cover Distribution Substantially Affects Global Carbon and Local Temperature Projections in the Integrated Earth System Model
Initial land cover uncertainty facilitates carbon and temperature uncertainties.
International modeling efforts aim to understand global change and its impacts on humans and the environment. Modeling how human activities change vegetation (e.g., forest to cropland), and subsequently the greater environment, is difficult and highly uncertain, yet crucial to understanding impacts of global change. Sources of uncertainty in this context that have received little attention include the land change data, how these data are translated for use by a model, and how a model represents and implements land change. These uncertainties generate larger challenges for scenario-driven modeling and multimodel assessment in that they culminate in each model simulating a unique Earth, which reduces comparability and the potential for scientific consensus on understanding model results.
Understanding how land data uncertainty affects Earth system projections in a single model provides insight to the potential range of related projection uncertainty across multiple models and will advance implementation and understanding of multimodel comparisons. Using a single model provides the opportunity to isolate and assess a specific uncertainty, which in this case is the initial land cover distribution and its associated initial Earth state. Presenting the substantial effects of a single source of uncertainty in a controlled environment will provide impetus for widespread quantification of land change uncertainty. Increased understanding of how specific land representation affects Earth projections will facilitate development of more flexible models that can accommodate multiple land data and boundary conditions both for exploring uncertainty and for applying standards in multimodel comparison exercises.
The team of scientists use the integrated Earth System Model (v1.0), which is a version of the Community Earth System Model that allows for variability in land conversion assumptions, to show that initial land cover uncertainty substantially affects future projections of carbon and temperature. Under a Representative Concentration Pathway 4.5 experiment, they estimate that an uncertainty range in year 2005 global forest of 3.9 M km2 (9% to 14% of the total) generates an uncertainty of 6 ppmv in atmospheric carbon dioxide concentration that increases to 9 ppmv by 2095. Similarly, the 2005 uncertainty in terrestrial carbon associated with land cover uncertainty is 26 petagrams of carbon (Pg C), and this uncertainty increases to 33 Pg C by 2095.
Furthermore, local surface temperature uncertainties range from –0.57 to 0.72°C and persist throughout the 21st century.
Alan Di Vittorio
Lawrence Berkeley National Laboratory
This work is supported by the Office of Biological and Environmental Research (BER), within the U.S. Department of Energy (DOE) Office of Science, under Award No. DEAC02-05CH11231 as part of the E3SM project in BER’s Earth and Environmental Systems Modeling program.
Di Vittorio, A.V., Shi, X., Bond-Lamberty, B., Calvin, K., Jones, A. “Initial land use/cover distribution substantially affects global carbon and local temperature projections in the integrated Earth system model.” Global Biogeochemical Cycles 34(5), ee2019GB006383 (2020). [DOI:10.1029/2019GB006383]