The heat capacity of the global ocean is very large compared to the rest of the spheres in the climate system of the Earth. It is therefore reasonable to assume that ocean heat uptake or release would manifest itself in corresponding changes in the atmospheric energy content. Since there is no convincing evidence that this is the case, an investigation of this was carried out in this thesis. The archive of IPCC’s model runs for the 20th century experiment (20C3M) were used to study the Earth’s energy budget by categorizing the main time scales involved and performing separate case studies for each timescale. Four models were evaluated: CCSM3 and Gfdl CM2.0 with variations in both natural and anthropogenic forcing and HadCM3 and Echam5 with only variations in anthropogenic forcing. The results from this thesis showed that the models were relatively realistic regarding the ocean heat content (0-300 m) and global surface air temperature. The ocean and the atmosphere were closely connected, i.e. changes in the ocean usually occurred simultaneously with the atmosphere. Due to major volcanic eruptions occurring in the late 19th and the 20th century, the ocean was rapidly cooled until a recovery was onset over a longer period of time. After the signiﬁcant increase in greenhouse gases in the middle of the 20th century, there was a clear warming in the ocean in the 1960s, as well as in the atmosphere, in HadCM3 and Echam5. The warming was also evident in CCSM3 and Gfdl CM2.0, but started much earlier due to an increase in incoming solar variability. This warming was anyhow delayed for a couple of decades due to the volcanic eruptions in 1963 and 1982 aswell as the increase in anthropogenic aerosols. During the modeled 20th century, cooling events in the climate system were mainly controlled by less energy in the atmosphere, hence in the ocean. Nevertheless, there were occurrences where the ocean provided its own timescale and imposed a cooling signal in the atmosphere, suggesting that the ocean has the ability to independently cool and heat the climate system.