Amazonian ecosystems range from rainforest to open dryland vegetation, with a following decrease in biomass along the moisture gradient. Biomass can vary greatly at the ecological transition zone between grass dominated savannahs and the forest. It is not well understood if the transition zone could expand under climate change, and thereby reduce ecosystem stability and carbon storage in biomass. Here, we quantify such changes by using a high-resolution regional Earth system model under RCP 8.5 climate scenario. We disentangle the effects of climate, CO₂, and land use by considering vegetation-climate feedbacks. Our results suggest that future climate change combined with elevated atmospheric CO₂ concentration tends to induce a larger spatial gradient of ecosystem states, increasing the transition area by ∼110% at the end of the century. Vegetation feedbacks generally amplify the climate effect by intensifying the climate-induced warming and drought, further enhancing spatial heterogeneity.