CO2 and the other greenhouse gases that produce global warming are not like regular air pollution such as sulfur dioxide (SO2), oxides of nitrogen (NOx), smog and fine particles (PM). These conventional air pollutants only remain in the atmosphere for a few hours or days. If the emissions that produce these conventional air pollutants are stabilized, their concentration in the atmosphere is also quickly stabilized:
That is not true of most other greenhouse gasses produced by human activities. For example, much of the CO2 that enters the atmosphere stays there for 100 years or more. If we only stabilized emissions, the atmospheric concentrations that cause warming would continue to grow:
If we want to reduce atmospheric concentrations, we must reduce emissions by something like 80%:
A useful analogy is a bathtub (the atmosphere) with a very large faucet (human emissions) and a much smaller drain (natural processes that remove other greenhouse gasses from the atmosphere). Unless the faucet is turned down very low, the bathtub continues to fill up.
Sometimes experts argue about exactly how much reduction is needed. The amount of reduction depends on what people think is a "safe level" of CO2 in the atmosphere. The exact amount is far less important than the basic insight that we need very deep reductions in emissions.