It could be positive if a warmer ocean means less biological productivity. But even if all species increase DMS production when temperature or sunlight increases, the total production of DMS might still decrease if warmer SST and more sunlight favored diatoms over coccolithophorids and phaeocystis in species competition, because diatoms produce less DMS per capita. The highest concentrations of DMS anywhere in the world are near the coast of Antarctica. Their habitat is the sea-ice zone in both the Arctic and Antarctic. Perhaps they produce so much DMS to shield themselves from the high salinity of the brine pockets in sea ice, where they spend the winter.
Different answers result on different time scales.
Warren says we were lucky to avoid a surprise here, lucky that clouds are so insensitive to increases in sulfur. I on Cloud Radiation Feedbacks and the Credibility of Atmospheric Models . So if we clean the sulfur out of the fuels, we might have no compensating effect and we might get major warming.