evidence: Even under the most optimistic scenarios of the future – in which humans stop burning fossil fuels and reduce other greenhouse gas emissions – average global temperature will very likely remain at least 1 C above preindustrial temperatures, and possibly much higher, for multiple centuries. This new climate state, characterized by a multi-century global warming level of 1 C and higher, can be reliably compared with temperature reconstructions from the very distant past. [...] Without rapid and sustained reductions in greenhouse gas emissions, the Earth is currently on course to reach temperatures of roughly 3 C (5.4 F) above preindustrial levels by the end of the century, and possibly quite a bit higher.