To make all the engines in the world work they need power.
This can be got by burning petrol, by using gas or coal, by using electricity which is generated by burning coal or gas.
These fuels are called fossil fuels and when they are burnt they give off a gas called carbon dioxide.
This goes up into the atmosphere and traps the heat of the sun.
The more carbon dioxide in the atmosphere the more heat that is trapped and the warmer the earth gets.