Reduction of CO2 emissions will require a transition from fossil fuels to alternative energy sources. Expansion of Brazilian sugarcane ethanol1, 2 provides one near-term scalable solution to reduce CO2 emissions from the global transport sector. In contrast to corn ethanol, the Brazilian sugarcane ethanol system may offset 86% of CO2 emissions compared to oil use, and emissions resulting from land-use change to sugarcane are paid back in just 2–8 years3, 4. But, it has been uncertain how much further expansion is possible given increasing demand for food and animal feed, climate change impacts and protection of natural ecosystems. We show that Brazilian sugarcane ethanol can provide the equivalent of 3.63–12.77 Mb d−1 of crude oil by 2045 under projected climate change while protecting forests under conservation5 and accounting for future land demand for food and animal feed production. The corresponding range of CO2 offsets is 0.55–2.0 Gigatons yr−1. This would displace 3.8–13.7% of crude oil consumption and 1.5–5.6% of net CO2 emission globally relative to data for 20146, 7.