Last time the Earth experienced a comparable concentration of CO2 was 3-5 million years ago and back then...

... the temperature was 2-3°C warmer, sea level was 10-20 meters higher than now,  World Meteorological Organization Secretary-General Petteri Taalas stated in a news release.

Since 1990, there has been a 43% increase in total radiative forcing by long-lived greenhouse gases (LLGHGs) – the warming effect on the climate - by long-lived greenhouse gases. CO2 accounts for about 80% of this, according to figures from the US National Oceanic and Atmospheric Administration quoted in the WMO Bulletin.