The 3‐Machines Energy Transition Model: Exploring the Energy Frontiers for Restoring a Habitable Climate.

To stabilize the climate, we need to urgently decarbonize our society and remove excess CO2 from the atmosphere. Ambitions for the transformation are ultimately limited by bio‐physical constraints, which cannot be transgressed even if all economic and societal obstacles could be overcome. Even though it is essential to know what transformation pathways are still feasible, there is a lack of studies and models exploring bio‐physical frontiers for climate action. In this paper, we take a first step to explore the energy frontier by introducing the "3‐machines energy transition model." This simplified representation of the global energy system includes energy feedbacks and is constrained by the maximum renewable energy potential. Simulation experiments with the model show that with most ambitious actions global peak heating may exceed 1.5°C with a chance of 14%. Simultaneously, it is still energetically possible to return to 350 ppm this century, which is considered to be a safe level for atmospheric CO2 concentration. While these energy‐constrained transformation pathways show that the climate can still be stabilized with a fair chance, they also illustrate the urgency and far‐reaching change required in society. Plain Language Summary: The rapidly depleting carbon budget for limiting peak heating to 1.5°C makes it necessary to design more ambitious and faster transformation pathways. Additionally, Earth's atmospheric CO2 concentration is rising faster than ever and far above the range of the past 1 million years. Stabilizing the climate in the long run will therefore require to return to a safe CO2 concentration. On our finite planet, there are, however, bio‐physical limits for accelerating climate action—yet, these frontiers are not well studied. This paper contributes a first step to explore the energy frontier for a fast and complete transition to renewable energy as well as removing excess CO2 from the atmosphere, identifying promising strategies to investigate further with more detailed modeling frameworks. Key Points: Given the urgency of the climate crisis, actions that are ultimately limited by the availability of energy have to be most ambitiousLowest climate risks can be achieved by a limited and short‐term increase of fossil emissions to kick‐start the fastest possible transitionReturning to safe climate conditions is energetically possible this century by using solar overcapacity on the built environment [ABSTRACT FROM AUTHOR]