2020 emissions levels required to limit warming to below 2 °C

This paper presents a systematic scenario analysis of how different levels of short-term 2020 emissions would impact the technological and economic feasibility of achieving the 2 degrees C target in the long term. We find that although a relatively wide range of emissions in 2020-from 41 to 55 billion tons of carbon dioxide equivalent (Gt CO(2)e yr(-1))-may preserve the option of meeting a 2 degrees C target, the size of this 'feasibility window' strongly depends on the prospects of key energy technologies, and in particular on the effectiveness of efficiency measures to limit the growth of energy demand. A shortfall of critical technologies-either for technological or socio-political reasons-would narrow the feasibility window, if not close it entirely. Targeting lower 2020 emissions levels of 41-47 Gt CO(2)e yr(-1) would allow the 2 degrees C target to be achieved under a wide range of assumptions, and thus help to hedge against the risks of long-term uncertainties.