A simple equation for the surface-elevation feedback of ice sheets
Abstract. In recent decades, the Greenland Ice Sheet is been losing mass and thereby contributed to global sea-level rise. The ice loss is likely to increase under future warming. Beyond a critical temperature threshold, a meltdown of the Greenland Ice Sheet is induced by the self-enforcing feedback between its lowering surface elevation and its increasing surface mass loss: The more ice is lost, the lower the ice surface reaches into the atmosphere and the warmer the surrounding air becomes which fosters melting and further ice loss. The rate of ice loss is highly relevant for coastal protection worldwide. The computation of this rate so far relies on complex numerical models as it should be. In order to contribute a little to the conceptual understanding, we derive here a simple equation for the self-enforcing feedback and use it to estimate the melt time for different levels of warming using three observable characteristics of the ice sheet itself and its surroundings. When the volume loss is dominated by the feedback, the resulting logarithmic equation unifies existing numerical simulations and shows that the melt time depends critically on the level of warming with a critical slowing-down near the threshold: The median time to lose 10 % of the present-day ice volume varies between about 3500 years for a temperature level of 0.5 °C above the threshold and 500 years for 5 °C. Unless future observations show a significantly higher melting sensitivity than currently observed, a complete melt down is unlikely within the next 2000 years without significant ice-dynamical contributions.