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Providing robust projections regarding loss of land as well as loss of valuable coastal freshwater resources requires a detailed understanding of the physics that will contribute to future sea-level rise (SLR). Unfortunately, determining the time until sea level rises to critical tipping points for loss of freshwater or land resources is not straightforward. While much of the physics controlling the rate of SLR—such as that of the thermal expansion of water—is well understood, there are two components that add considerable uncertainty to existing projections. The first of these is the incomplete knowledge regarding the physics of rapid ice sheet retreat. For example, the Antarctic ice sheet is a large potential contributor to future SLR, but the rate at which this ice sheet is expected to retreat cannot be adequately modeled. In place of models, evidence of SLR during past warm periods can be used to constrain potential rates of ice sheet collapse. The second component to the uncertainty in future rates of SLR is the shorter-term annual to decadal scale variability that contributes to extreme events. The processes driving this short-term variability include changes in winds and current strength, as well as storm surges. The timing of some of these events can only be explained in terms of the probability of event frequencies, such as that of hurricanes that may threaten our coastlines. Hence, the timing at which we reach critical tipping points of SLR is highly uncertain, yet there is certainty that sea level will continue to rise. For this reason, despite the apparent utility of sea-level projections that are used for regional planning and adaptation, I argue that it is dangerous to latch on to a particular projection for future SLR scenarios. |