These key ideas illustrate the differences between weather and climate and unravel some of the processes that cause natural climate variability, and abrupt climate change, and human-caused climate change. Understanding climate variability such as the El Niño/Southern Oscillation is critically important in helping scientists tease apart natural variation from human-induced climate change. In this principle, the human impact on the climate through burning of fossil fuels is clearly differentiated from naturally occurring climate processes.