The Earth is getting warmer because people are putting greenhouse gases into the atmosphere. This is called the greenhouse effect because a greenhouse stops some of the sun's heat from escaping, and these gases stop some heat escaping from Earth. The greenhouse gas which makes the most warming is carbon dioxide (short name CO2), 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 CO2 is released into the atmosphere. Usually it means how much warmer the Earth would be if the atmosphere had twice as much CO2 as the year 1750, before people started burning lots of coal. Scientists think that when CO2 in the atmosphere doubles an extra 4 joules of heat gets trapped every second on every square metre on Earth, and that after a long time the Earth will be about 3 °C hotter.