Divergent Land and Ocean Biome Trajectories in a Warming World

Abstract. Global biomes, shaped by climate, have long provided a unifying framework for understanding climate change and its impacts on ecosystems and biogeochemical cycles. Using Earth system model projections under continued greenhouse gas emissions, we evaluate global biome redistributions from the present to the year 2300. On land, biomes generally migrate poleward and eastward, whereas in the ocean they exhibit pronounced hemispheric asymmetry, with increasing oligotrophication in the Northern Hemisphere and eutrophication in the Southern Ocean. Terrestrial desert areas are projected to expand moderately, increasing from ~15% to ~20% of the global land surface between 2000 and 2300, with the rate of expansion scaling approximately linearly with global mean warming. In contrast, the simulated extent of oligotrophic ocean regions remains nearly unchanged during the 21st century, after which expansion accelerates, reaching up to 18% of the global ocean by 2300. This nonlinearity arises from phytoplankton's adaptive strategies under nutrient stress – such as atmospheric nitrogen fixation and flexible nutrient uptake ratios – which delay the onset of widespread phosphate limitation. The expansion of terrestrial deserts, and the associated increase in atmospheric iron deposition, further delays the emergence of oligotrophication in some downwind regions. Whereas projections of terrestrial biome changes are broadly consistent across Earth system models, ocean biome projections remain highly divergent, underscoring the critical role of poorly constrained biological processes in shaping marine biogeography and global food security.