<p>Most climate policies are designed under a deterministic Earth system and their climate implications evaluated ex-post. Approaches that incorporate uncertainty ex-ante to anticipate Earth system risks remain underexplored. Here, we derive global climate strategies with an ex-ante approach, employing an integrated assessment framework that embeds estimates of physical uncertainty obtained through Bayesian fusion of Earth system models’ and observations’ data. These ex-ante strategies mitigate risks in the Earth system through precautionary measures unseen with the ex-post approach, in cost-benefit analysis and cost-effective implementations of various Earth system targets. Net-zero CO₂ emissions must typically be reached a decade earlier, which can require up to a doubling of the near-term carbon price. Importantly, sustained and possibly century-long net-negative emissions must be planned for, albeit not to overshoot targets as in traditional scenarios but to mitigate long-term Earth system risks. This heightens the challenge faced by humanity to build a safe future within Earth system boundaries.</p>