>>8215325CO2 absorbs a pretty wide range of infrared light, preventing Earth from radiating more heat.
Looking at data from ice cores, we see that CO2 has always been a significant driver of climate change throughout Earth's past.
The climate has been warming over the last 100 years despite steady solar output.
People say "likely" because science doesn't deal with absolutes. Scientists go by the best working model, like how Newton's theories were likely to be true, we have found them to be false (though they are still useful in most practical circumstances) and they were replaced by Einstein's theories.