The Earth is getting warmer because people are putting greenhouse gases into the atmosphere. This is called the greenhouse effect and that article explains more about it. The greenhouse gas which makes the most warming is carbon dioxide (short name CO), and burning coal is the biggest thing which puts it in the atmosphere. Climate sensitivity means how much the Earth will warm when a certain amount of CO is released into the atmosphere. Usually it means how much warmer the Earth would be if the atmosphere had twice as much CO as the year 1750, before people started burning lots of coal. Scientists think that after CO in the atmosphere doubles an extra 4 W/m energy stays at the Earth's surface and this eventually heats the Earth by about 3 °C.