The 21st century population-energy-climate nexus

World population is projected to reach 10.9 billion by 2100, yet nearly one-fifth of the world's current 7.2 billion live without access to electricity. Though universal energy access is desirable, a significant reduction in fossil fuel usage is required before mid-century if global warming is to be limited to <2 °C. Here we quantify the changes in the global energy mix necessary to address population and climate change under two energy-use scenarios, finding that renewable energy production (9% in 2014) must comprise 87-94% of global energy consumption by 2100. Our study suggests >50% renewable energy needs to occur by 2028 in a <2 °C warming scenario, but not until 2054 in an unconstrained energy use scenario. Given the required rate and magnitude of this transition to renewable energy, it is unlikely that the <2 °C goal can be met. Focus should be placed on expanding renewable energy as quickly as possible in order to limit warming to 2.5-3 °C.