Parties to the 2015 Paris Agreement pledged to limit global warming to well below 2 °C and to pursue efforts to limit the temperature increase to 1.5 °C relative to pre-industrial times 1 . However, fossil fuels continue to dominate the global energy system and a sharp decline in their use must be realized to keep the temperature increase below 1.5 °C (refs. [2][3][4][5][6][7] ). Here we use a global energy systems model 8 to assess the amount of fossil fuels that would need to be left in the ground, regionally and globally, to allow for a 50 per cent probability of limiting warming to 1.5 °C. By 2050, we find that nearly 60 per cent of oil and fossil methane gas, and 90 per cent of coal must remain unextracted to keep within a 1.5 °C carbon budget. This is a large increase in the unextractable estimates for a 2 °C carbon budget 9 , particularly for oil, for which an additional 25 per cent of reserves must remain unextracted. Furthermore, we estimate that oil and gas production must decline globally by 3 per cent each year until 2050. This implies that most regions must reach peak production now or during the next decade, rendering many operational and planned fossil fuel projects unviable. We probably present an underestimate of the production changes required, because a greater than 50 per cent probability of limiting warming to 1.5 °C requires more carbon to stay in the ground and because of uncertainties around the timely deployment of negative emission technologies at scale.

The design of cost effective power systems with high shares of variable renewable energy technologies (VREs) requires a modelling approach that simultaneously represents the whole energy system combined with the spatiotemporal and inter-annual variability of VREs. Here we soft-link a long term energy system model, that explores new energy systems configurations from years to decades, with a high spatial and temporal resolution power system model that captures VRE variability from hours to years. Applying this methodology to Great Britain for 2050, we find that VRE focused power system design is highly sensitive to the inter-annual variability of weather and that planning based on a single year can lead to operational inadequacy and failure to meet long term decarbonisation objectives. However, some insights do emerge that are relatively stable to weather year. Reinforcement of the transmission system consistently leads to a decrease in system costs while electricity storage and flexible generation, needed to integrate VREs into the system, are generally deployed close to demand centres.

Global ambition to limit anthropogenic warming to 2°C requires a radical transformation of the energy system to one that produces 'net-zero' GHG emissions before 2100 1 . For a 1.5°C limit, action has to be even more rapid, with net-zero emissions achieved much earlier 2 . The goal of net-zero GHG emissions is expressed in the Paris Agreement as a system that achieves 'a balance between anthropogenic emissions by sources and removals by sinks' 3 . In