Compound Desertification from Land to Ocean under Multi-Centennial Climate Warming

Earth's biomes are undergoing fundamental reorganisation under anthropogenic warming, yet it is unclear how they may be changing on multi-centennial timescales. We examine the co-evolution of land and ocean biome distributions under a high-CO2 emission scenario through the 23rd century using the Community Earth System Model version-2 Large Ensemble (CESM2-LE). We apply the Köppen-Geiger climate classification for land (15 classes) and a chlorophyll-based classification for ocean (7 classes). Our findings exhibit sharply decoupled trajectories of biome reorganization between land and ocean.

The response of terrestrial biomes to rising global temperatures appears to be approximately linear in time and with global mean surface temperature. Driven by rising aridity thresholds and decreased precipitation, arid deserts and steppe regions gradually expand, eventually extending to about 35% of the world's land surface in the extended future. On the other hand, marine biome responses are strongly non-linear. Despite rising temperatures and enhanced stratification, the expansion of oligotrophic “ocean deserts” is initially buffered until about 2100. Phytoplankton’s adaptive strategies such as N2 fixation, enhanced organic nutrient recycling, and stoichiometric plasticity support this resilience. However, these adaptive mechanisms break down when a warming threshold of about 2-6°C is exceeded, leading to a sudden increase in extreme oligotrophic areas that eventually cover almost 25% of the world's ocean surface.

We refer to this degradation in the extended future as “compound desertification”, in which terrestrial desert expansion is followed by the sudden acceleration of marine oligotrophication. In subtropical regions including the Mediterranean, Central America, and Southern Africa, this phenomenon is particularly noticeable and poses serious cross-domain risks to biodiversity and food security. Additionally, we pinpoint important land-ocean feedbacks, such as increased dust-driven iron deposition from growing terrestrial deserts, which influences marine productivity in High-Nutrient Low-Chlorophyll (HNLC) regions to some extent. Our results emphasise the need to account for distinct response timescales of land and ocean biomes and highlight the latent vulnerability of marine ecosystems under sustained greenhouse gas emissions.