He had a change of heart after making billions on coal.
Check it out:

California billionaire Tom Steyer has spent the better part of the last two years crusading against climate change, but during that time he has been ridiculed as a hypocrite for making himself fabulously wealthy in part from dealing with companies that sell fossil fuels. At last he has spoken out about this essential disconnect in a message posted on his climate change website.

In a June 25 message to his NextGen Climate website, Steyer claimed that he has decided to “chart a different course” from his days as chief of Farallon Capital Management, a global fund that invested heavily in coal companies in Australia and Indonesia.