There is a consensus among scientists that stark dangers await in a world where the global mean temperature rises by more than about 2 degrees Celsius. That threshold corresponds to a collective human carbon emissions “budget” of around a trillion tons, of which half has been spent. This paper uses a new simulation model to look at strategies to stay within that budget, specifically assessing the impact of improvements in energy efficiency, aggressive deployment of renewables, and energy technology innovation. The simulations examine the timing of investments, turnover of capital stock, and the effect of learning on costs, among other factors. The results indicate that efficiency, renewables, and technology innovation are all required to keep humanity within the trillion-ton budget. Even so, these measures are not by themselves sufficient: changes in land use and a price on carbon emissions are also needed.