What scientific concept is the most important thing people know too little about?

This week's most important scientific news was enough that NASA and NOAA in the United States confirmed that 2016 was the warmest year.

The European Copernicus program recounted the same already for more than a half week ago, but since it's not exactly the same data set that underlies the two statements, there was still some uncertainty about whether 2016 was actually warmer than in 2015.