Abstract. During the Quaternary period, spanning the last 2.6 million years, the characteristic frequency and amplitude of glacial-interglacial cycles evolved from low-amplitude 41,000-year cycles to high-amplitude 100,000-year cycles. This transition occurred around 1.2 to 0.8 million years ago and is referred to as the Mid-Pleistocene Transition (MPT). While the 41 kyr cycles are driven by changes in Earth’s obliquity, which largely affect the incoming solar insolation, no apparent change in external orbital forcing during this period could explain the shift towards 100 kyr cycles. Several theories have been put forward to explain this shift, including scenarios of both gradual and abrupt changes in the internal climate system throughout the Pleistocene. In order to test which theory best matches the observations, we have constructed a conceptual model capable of simulating changes in the global ice volume over the past 2.6 Ma and accurately reconstructing the MPT and its associated change in amplitude and frequency. Four different forcing scenarios are implemented, ranging from a purely orbitally driven model to a ramp-like change in internal forcing. The model is in favour of a ramp-like forcing scenario, where the gradual change in internal forcing is limited in time and started around 2 Ma. These findings imply that the climate system had already undergone major changes in the early Pleistocene and support the idea of a long-term climatic shift as a cause of the MPT. For the best-performing model, we included an ice volume dependency in the state thresholds, demonstrating that glacial terminations during the past 900 ka are mainly driven by precession rather than obliquity.