Global warming refers to the upward temperature trend across the entire Earth since the early 20th century, and the increase in fossil fuel emissions since the industrial revolution. Global warming, also referred to as climate change, is the observed century-scale rise in the average temperature of the Earth's climate system, and its related effects. Multiple lines of scientific evidence show that the climate system is warming. Possible societal responses to global warming include Mitigation by emissions reduction, Adaption to its effects, building systems resilient to its effects, and possible future climate engineering. Global warming refers to global averages. Because it is not a uniform phenomenon, effects can vary by region.