Transgression of the climate change planetary boundary critically affects the status of other boundaries

To date, statues and trajectories of planetary boundaries have mostly been investigated separately, without fully quantifying if and to what extent transgression of one or more boundaries affects the status of respective others. To address this research gap, we have configured the state-of-the-art LPJmL Dynamic Global Vegetation Model so as to represent the terrestrial planetary boundaries (for land-system change, biosphere integrity, freshwater change, and biogeochemical/nitrogen flows) in an internally consistent, process-based framework. As the model simulates these boundaries underlying processes and control variables in a spatially explicit and dynamic manner, and as it also accounts for effects of climate change (a fifth planetary boundary considered through external forcing), it enables systematic studies of interactive effects among any of the five boundaries considered. In a scenario study focused on here, we employed the model to systematically quantify the effects of different transgression levels of the climate change boundary (using gridded climate output from ten CMIP6 models for distinct atmospheric CO2 levels from 350 ppm to 1000 ppm) upon the land-system change boundary (areal extent of temperate, boreal and tropical forest biomes). Changes are analysed both by the end of this century and, to account for long-term legacy effects, by the end of the millennium, respectively. The simulations indicate that staying within the 350 ppm climate change boundary would stabilize the land-system change boundary, not inducing notable expansions or contractions of forest biome extent (on top of the historical shifts that have been brought about by anthropogenic deforestation). However, transgressing the climate change boundary beyond its zone of increasing risk (>450 ppm) is simulated to lead to increasingly substantial forest biome shifts, the higher the ppm level rises and the more time passes. Specifically, this involves a poleward tree-line shift, boreal forest dieback, expansion of temperate forest into todays boreal zone, and a slight tropical forest expansion. We furthermore find that these one-way interactions imply changes of the status of other planetary boundaries as well, as shifts in their control variables (e.g. large soil moisture and runoff anomalies) are simulated for the very areas where the forest biome shifts occur. Moreover, the vegetation changes are likely to provide feedback to the climate change boundary itself. In additional simulations (making use of a planetary boundary simulation package linked to the LPJmL model), we investigate the historical evolution of the terrestrial planetary boundaries statuses during the past century. This examination suggests that the timing and spatial location of transgressions differs strongly among boundaries, with multiple boundaries crossed in the late 20th century, and transgression of the climate change boundary gaining increasing impact. Possible cascading and compound effects of these simultaneous transgressions, and particularly their likely aggravation in the future, require comprehensive analyses in further studies.