Young People's Burden: Requirement of Negative CO2 Emissions
  Global temperature is a fundamental climate metric highly correlated with sea level, which implies that keeping shorelines near their present location requires keeping global temperature within or close to its preindustrial Holocene range. However, global temperature excluding short-term variability now exceeds +1degC relative to the 1880-1920 mean and annual 2016 global temperature was almost +1.3degC. We show that global temperature has risen well out of the Holocene range and Earth is now as warm as during the prior interglacial, when sea level reached 6-9 meters higher than today. Further, Earth is out of energy balance with present atmospheric composition, implying more warming is in the pipeline, and we show that the growth rate of greenhouse gas climate forcing has accelerated markedly in the past decade. The rapidity of ice sheet and sea level response to global temperature is difficult to predict but is dependent on the magnitude of warming. Targets for limiting global warming should aim to avoid leaving global temperature at Eemian or higher levels for centuries. Such targets require "negative emissions", extraction of CO2 from the air. If phasedown of fossil fuel emissions begins soon, improved agricultural and forestry practices may provide much of the extraction, and the magnitude and duration of global temperature excursion above the natural range of the current interglacial could be limited and irreversible impacts minimized. In contrast, continued high emissions place a burden on young people to undertake massive technological CO2 extraction to limit climate change and its consequences. Proposed methods of extraction have minimal estimated costs of 89-535 trillion dollars this century and have large risks and uncertain feasibility. Continued high emissions unarguably sentences young people to a massive, implausible cleanup, growing deleterious climate impacts or both. 
James Hansen, Makiko Sato, Pushker Kharecha, Karina von Schuckmann,   David J Beerling, Junji Cao, Shaun Marcott, Valerie Ma...
physics.ao-ph