Why do we need to limit cumulative emissions of carbon dioxide?
A certain fraction of the carbon dioxide we release accumulates in the atmosphere, in effect, indefinitely because of the way it interacts with ocean chemistry. So it is the total amount that we emit that matters in the long term, not the rate we emit it in any given year or the average rate of emission over a limited period.
Both peak temperatures and peak carbon dioxide concentrations, which matter for impacts like ocean acidification, are primarily determined by cumulative carbon dioxide emissions.