FORTUNE: The world’s richest man says he’ll spend $1 billion to fight global warming and has persuaded other billionaires to join him. Here’s how he can get the most bang for his buck: On Monday, just in time for the start of the Paris climate talks, Bill Gates and other wealthy entrepreneurs announced the creation of the Breakthrough Energy Coalition, a global group of private investors committed to “early-stage investing in potentially transformative energy systems” with “n