Ignoring the notion that Trump, in his first full month in office, had anything whatsoever to do with this, I was wondering how it happened.
The most obvious factor is temperature. Indeed, for the continental US, Feb17 was 1.7°F warmer than Feb16, according to NOAA's data. Is that enough to account for an 8% drop?
I don't know. Here are US CO2 monthly emissions as a function of the average US continental temperature:
This graph makes general sense -- there is some baseline below which emissions don't drop, due to vehicle transportation and industrial use, and upward wings on both sides due to home air conditioning (on the right) and household heating (on the left).
That's all I know so far. I suspect that US CO2 emissions are going to keep falling during Trump's administration, as coal keeps going out of style and utilities switch to cheaper natural gas, and state- and city-wide efforts like this one in Portland, Oregon.
These individual efforts won't be enough -- I don't see how they can be without supporting federal regulations -- but it may well be that US CO2 emissions drop during Trump's administration.
Of course, he'll take full credit for it, even though he had nothing -- in fact, less than nothing -- to do with it.