How Do I Change The Spacing Between Header and Text?

Whenever I write using ‘heading 2’, there is very little space between the heading and the text below. How can I increase the spacing using CSS?

This page shows an example of the lack of space – http://www.aftw.net/environment/we-cannot-eat-money/

I am looking to increase the spacing under headings such as ‘the problem’ and ‘the solution’.

Thanks in advance!