On 10 3 -to 10 6 -year timescales, global sea level is determined largely by the volume of ice stored on land, which in turn largely reflects the thermal state of the Earth system. Here we use observations from five well-studied time slices covering the last 40 My to identify a well-defined and clearly sigmoidal relationship between atmospheric CO 2 and sea level on geological (near-equilibrium) timescales. This strongly supports the dominant role of CO 2 in determining Earth's climate on these timescales and suggests that other variables that influence long-term global climate (e.g., topography, ocean circulation) play a secondary role. The relationship between CO 2 and sea level we describe portrays the "likely" (68% probability) long-term sea-level response after Earth system adjustment over many centuries. Because it appears largely independent of other boundary condition changes, it also may provide useful long-range predictions of future sea level. For instance, with CO 2 stabilized at 400-450 ppm (as required for the frequently quoted "acceptable warming" of 2°C), or even at AD 2011 levels of 392 ppm, we infer a likely (68% confidence) long-term sea-level rise of more than 9 m above the present. Therefore, our results imply that to avoid significantly elevated sea level in the long term, atmospheric CO 2 should be reduced to levels similar to those of preindustrial times.S ea-level change is one of the most significant and long-lasting consequences of anthropogenic climate change (1). However, accurate forecasting of the future magnitude of sea-level change is difficult because current numerical climate models lack the capacity to accurately resolve the dynamical processes that govern size changes of continental ice sheets [e.g., total disappearance of the current continental ice sheets would raise mean sea level by about 70 m (1)]. This complicates long-range sea-level projections because the retreat of continental ice sheets will increasingly contribute to sea-level rise as the 21st century progresses (2), and because this rise will continue long into the future, even if temperatures were stabilized, according to different mitigation scenarios for greenhouse gas emissions (1). Because of the absence of adequate ice-dynamical processes in models, even the most recent estimates have to rely on assumed (linear) relationships between ice-volume reduction and global mean temperature increase (1), which as yet remain largely untested. Therefore, here we provide a natural context to projections of future long-term (multicentury) sea-level rise, by assessing key relationships in the Earth's climate system using recent high-quality data from the geological past. Because global mean temperature is hard to measure in the geological past without applying (often problematic) assumptions about polar amplification or deep-sea temperature relationships (3, 4), we instead concentrate on quantifying the "likely" [68% probability (5)] long-term relationship between two entities that can be measured more directly, namely i...