Marine cloud brightening (MCB) is a proposed technique to limit global warming through injections of sea spray into the marine boundary layer. Using the Norwegian Earth System Model, the sensitivity of MCB to sea salt amount and particle size was studied by running a set of simulations in which Aitken (re = 0.04 µm), accumulation (re = 0.22 µm), or coarse (re = 2.46 µm) mode sea salt emissions were increased uniformly by 10−11 to 10−8 kg m−2 s−1. As desired, accumulation mode particles had a negative radiative effect of down to −3.3 W m−2. Conversely, for Aitken mode particles, injections of 10−10 kg m−2 s−1 or greater led to a positive forcing of up to 8.4 W m−2, caused by a strong competition effect combined with the high critical supersaturation of Aitken mode sea salt. The coarse mode particles gave a positive forcing of up to 1.2 W m−2 because of a decrease in activation of background aerosols. Sensitivity experiments show that the competition effect dominated our results. MCB may have a cooling effect, but if the wrong size or injection amount is used, our simulations show a warming effect on the climate system.