The effect of a long-term acclimation to elevated temperature and pCO2 scenarios was investigated by determining whole-organism resting metabolic rates at acclimated temperatures in two intertidal gastropod species; Littorina littorea and Nucella lapillus. Temperatures in treatments followed annual seasonal temperature changes. Comparisons were made between two temperature treatments (ambient and +4°C) and three pCO2 treatments (380, 750 and 1000μatm) after 12 and 15 months exposure. Further investigation of the variations in thermal sensitivity (Q10) were made; at the 12 month time point to an increase of 5°C and at the 15 month to a simulated heat wave of 10°C. Variations in the whole-organism resting metabolic rates were observed after exposure to different treatments, suggesting possible knock-on effects on energetic demands. At the 12 month time point, L. littorea displayed an overall increase in whole-organism resting metabolic rate in response to an increase in temperature suggesting an increase in metabolic plasticity. The same effect was observed in response to a simulated heat wave in both species at the 15 month time point, suggesting that the predicted increased frequency of heat waves could result in biogeographic range shifts in both species, which could in turn affect wider ecosystem processes, as both L. littorea and N. lapillus have key roles in ecosystem function.