<p>Marine ice sheets are tightly coupled to the Earth system, exchanging mass and energy with its ocean, atmosphere and lithosphere. Yet, their behaviour is often framed solely through the marine ice-sheet instability hypothesis, which assumes that stability depends on only the bed slope at the grounding line. Although this view is appealing for its simplicity, it overlooks the complexities of real ice sheets and their interactions with other Earth system components. Here we propose an alternative perspective that is based on the mass and energy exchange principles and is consistent with observations. In this view, ice sheets are complex physical systems with intrinsic variability that cannot be separated from the Earth system. Their numerous nonlinear and spatially and temporally heterogeneous internal processes, together with interactions and feedbacks operating on a variety of timescales, generate diverse modes of variability that start to emerge in recent observations. The results of these observations and advances in understanding of the Earth system indicate a broad range of possible behaviours of ice sheets. Improving knowledge of internal processes and marine ice-sheet interactions with the Earth system components are necessary to confidently predict their future evolution.</p>