Envisaging the global carbon cycle as a component of the climate system was recognised already in the 1970s, but its full integration into Earth System models was achieved only after the year 2000. Landmark contributions from Hamburg in the 1980s and 1990s significantly fostered this endeavour. This chapter describes the Hamburg efforts embedded in the history of global carbon cycle research in the twentieth century: Representing the key ocean and land carbon cycle processes in a mechanistic, dynamic and geographically explicit form to be linked to quasi-realistic climate models and developing a methodology to quantify, constrain and optimise these models from global observations.