An increase in plastic on the sea surface may affect the absorption depth of solar radiation energy, resulting in changes in the sea surface temperature (SST), stratification, and mixed layer depth. This study evaluated the relationship between the rise of SST according to the concentration of plastic on the sea surface, as well as the effects of these temperature increases on phytoplankton concentrations and climate change. Low-density polyethylene (LDPE) has lower specific heat than water. Therefore, adding LPDE to distilled water increased the heating rate of the water, and significant differences were observed between the temperatures of LPDE-treated and untreated distilled water. Furthermore, our findings suggested that the water temperature was affected by the total amount of added LDPE regardless of the size of the LDPE. The concentration of Chl-a in seawater tended to increase as the amount of added LDPE increased. Therefore, ocean plastic will increase the SST, which, in turn, increases phytoplankton, and, again, the increased phytoplankton densities are likely to increase SST. This chain effect could exacerbate climate change.