In order to meet stringent temperature targets, active removal of CO 2 from the atmosphere may be required in the long run. Such negative emissions can be materialized when well-performing bioenergy systems are combined with carbon capture and storage (BECCS). Here, we develop an integrated global energy system and climate model to evaluate the role of BECCS in reaching ambitious temperature targets. We present emission, concentration and temperature pathways towards 1.5 and 2 • C targets. Our model results demonstrate that BECCS makes it feasible to reach temperature targets that are otherwise out of reach, provided that a temporary overshoot of the target is accepted. Additionally, stringent temperature targets can be met at considerably lower cost if BECCS is available. However, the economic benefit of BECCS nearly vanishes if an overshoot of the temperature target is not allowed. Finally, the least-cost emission pathway over the next 50 years towards a 1.5 • C overshoot target with BECCS is almost identical to a pathway leading to a 2 • C ceiling target.

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Most energy systems models treat reductions of technology investment costs exogenously. In these models, investments may be postponed until the costs become low. This model behaviour is unreasonable, since early investments are necessary to realize the cost reductions, a phenomenon known as the experience effect. We have developed a global energy systems model with endogenized experience curves, and have conducted a pilot study to demonstrate the new insights which can be obtained with the model. In this initial application, we study the emergence of new energy technologies such as photovoltaics and fuel cells, competition between technologies and technology lock-in effects.