About the actual radiative impact of greenhouse gas emission over time
There is no doubt that the sooner the mitigation effort happens, the greater will be its impact.

In other words, the impact on year 2100 climate of the sequestration of 20 billion tons $\ce{CO2}$-eq right now, is much bigger than the impact that the same action will have in 30 years from now. Therefore, a mitigation action today is much cheaper than one done in the future but with equal impact on year 2100 climate (assuming no dramatic changes in the sequestration technology).

This is due to the long lifetime of $\ce{CO2}$ in the atmosphere, that is usually estimated to be longer than a few centuries. Although, some sources place the lower limit at around 30 years, its lifetime is most likely longer than 50 years.

This means that any $\ce{CO2}$ that is not captured today (or emitted), will remain in the atmosphere trapping heat and rising Earth's temperature until year 2100 and beyond.