Global rules for translating land-use change (LUH2) to land-cover change for CMIP6 using GLM2

摘要

Anthropogenic land-use and land-cover change activities play a critical rolein Earth system dynamics through significant alterations to biogeophysicaland biogeochemical properties at local to global scales. To quantify themagnitude of these impacts, climate models need consistent land-cover changetime series at a global scale, based on land-use information fromobservations or dedicated land-use change models. However, a specificland-use change cannot be unambiguously mapped to a specific land-coverchange. Here, nine translation rules are evaluated based on assumptionsabout the way land-use change could potentially impact land cover. Utilizingthe Global Land-use Model 2 (GLM2), the model underlying the latest Land-UseHarmonization dataset (LUH2), the land-cover dynamics resulting fromland-use change were simulated based on multiple alternative translationrules from 850 to 2015 globally. For each rule, the resulting forest cover,carbon density and carbon emissions were compared with independentestimates from remote sensing observations, U.N. Food and AgriculturalOrganization reports, and other studies. The translation rule previouslysuggested by the authors of the HYDE 3.2 dataset, that underlies LUH2, isconsistent with the results of our examinations at global, country and gridscales. This rule recommends that for CMIP6 simulations, models should (1) completely clear vegetation in land-use changes from primary and secondaryland (including both forested and non-forested) to cropland, urban land andmanaged pasture; (2) completely clear vegetation in land-use changes fromprimary forest and/or secondary forest to rangeland; (3) keep vegetation inland-use changes from primary non-forest and/or secondary non-forest torangeland. Our analysis shows that this rule is one of three (out of nine)rules that produce comparable estimates of forest cover, vegetation carbonand emissions to independent estimates and also mitigate the anomalouslyhigh carbon emissions from land-use change observed in previous studies inthe 1950s. According to the three translation rules, contemporary globalforest area is estimated to be 37.42×106 km2, within the rangederived from remote sensing products. Likewise, the estimated carbon stockis in close agreement with reference biomass datasets, particularly overregions with more than 50 % forest cover.