Leonardo Dicaprio's interview with Sunita Narain in Before the flood is a very good explanation of why this happens. Here it is: https://www.youtube.com/watch?v

Leonardo Dicaprio's interview with Sunita Narain in Before the flood is a very good explanation of why this happens. Here it is: https://www.youtube.com/watch?v=90CkXVF-Q8M https://www.youtube.com/watch?v=90CkXVF-Q8M (starting from 33:20 for 5 mins).