The planet Venus has an atmosphere containing 92% carbon dioxide and an atmospheric pressure nearly 100 times that of Earth. As a consequence, the greenhouse effect on Venus is enormous. Without that greenhouse effect the planet would be several hundred degrees less hot than it is at the surface. The Earth has very little CO2 in comparison, only 396 parts per million of the atmosphere by molecule or 0.04% by volume. How can such a seemingly feable amount of the gas be a concern for us here on Earth? Well for one thing, the total greenhouse effect on Earth amounts to 33C rather than 100s of degrees on Venus, and CO2 accounts for maybe 6C degrees on it own, while water vapor and clouds make up about 26C degrees. Not much compared to Venus, but quantity does not tell the whole story.
The absolute quantity of a greenhouse gas is less important than the marginal difference increase as a percentage of the total. For each doubling of CO2 3.7 watts per square meter (3.7W/m^2) of additional energy is retained within the troposphere (the lowest level of the atmosphere). We will double CO2 over pre-industrial levels ( 280 part per million / 560ppm) by mid 21st century. By that time 3.7W/m^2 of additional warming energy will be radiated to the surface by CO2. This will warm the surface by 1.2C after the surface temperature reaches thermal equilibrium with the new “forcing”. This is basic physics. In actuality there will be more warming than that, how much more we are not certain because of feedbacks within the climate system. One thing we do know, the last time it was that warm or warmer, sea levels were at least 10s of feet higher than today, and that is no joke.