That's not entirely accurate. Short lived climate pollutants (SLCPs) that are being continuously emitted (e.g. SO4, NOx, contrail-cirrus) have cumulative effects. The heat trapped on earth is cumulative with a long equilibration time constant, and SO4 (not just from ships) has wiped out a huge fraction of the warming up to now.
(disclaimer that I manage a climate research group)
There is heat being continually trapped by the SLCP, which analogous to a one-time (large) release of CO2. The earth will warm up in response, and will keep absorbing heat from the SLCP as long as the pollutant is being emitted.
As we heat up, though, we'll radiate more heat back into space (blackbody) leading to a new equilibrium temperature (the trapped flux and outgoing flux are now both larger, but balanced).
If you think about it as heat, it's cumulative. If you think about it as temperature, it'll effectively lead to a ~fixed bump (not quite a fixed one-time bump, as the effects are not linear, but one can think of it that way).