I understand "and fast" as "and act quickly." Am I on the right track?
The phrase "fossil fuels, and fast" appears to be a typical alliteration, which explains why "fast" rather than "quick" is used. I am not absolutely sure.
The New York Times tweeted:
The world’s leading energy agency said nations must drop fossil fuels, and fast, if they want to avert catastrophic climate change.
Source: Twitter