They were 6.7 per cent lower in 2015 than the highest point in the 1990s, in 1999.
The US population actually grew by closer to 76 million people over this period, going from 249 million in 1990 to 325 million in 2017 - an increase of 30.5 per cent. Its GDP nearly doubled from $8.9 trillion to $16.8 trillion.
So Tillerson's statement that US emissions were now at "1990 levels" despite large demographic and economic growth stacks up.
However, his claim that the US record on reducing emissions is "unparalleled" is not as accurate.
EU members states have cut emissions by 23 per cent over the same period.
How does NZ compare?
Over the same period, New Zealand greenhouse gas emissions have risen by 24.1 per cent - much higher than the 3.5 per cent rise in the US.
It is worth noting that New Zealand's population grew at a slightly higher rate (38 per cent) over the same period, and its GDP tripled over that time.
On a per capita basis, Americans (23.9 tonnes of CO2) are bigger polluters than Kiwis (18.8 tonnes).
The two countries made similar commitments under the Paris Agreement (before the US signalled it would withdraw last week), but over different timespans.
NZ has committed to cutting emissions by 30 per cent compared to 2005 levels by 2030, while the US committed to a 26-28 per cent cut by 2025.