What's the difference between 1.5°C and 2°C of global warming?

World leaders and scientists have stressed the need to limit global warming to 1.5 degrees Celsius. The 2015 Paris Agreement commits countries to limit the global average temperature rise to well below 2°C above pre-industrial levels, and to aim for 1.5°C. But what is the difference between 1.5°C and 2°C of warming? Francesca Lynagh has more.