Detection of a climate-induced increase in the lunar recession rate

Gravitational interplay between the Earth and Moon is believed to steadily increase the length of day (ΔLOD) and cause a secular drift of the Moon away from the Earth (lunar recession). When attempting to close the budget of the astronomically observed secular ΔLOD trend based on underlying geophysical processes, recent studies have noted an enigmatic bias (i.e., a discrepancy) of ∼0.1 milliseconds per century (ms/cy). Here we show that this bias might not arise from modeling deficiencies or uncertainties in the geophysical/astronomical data, but could be explained by plate tectonics (∼0.04 ms/cy; specifically mantle density heterogeneities and super plumes) and climate-induced variations in the rate of tidal friction (∼0.06 ms/cy). We demonstrate that the latter is consistent with independent model estimates of secular changes to the semidiurnal lunar ocean tide under climate change, induced mainly by the enhanced ocean stratification (∼0.042 ms/cy) and the mass-driven sea level rise (∼0.018 ms/cy); this in turn is caused by the melting of global glaciers and polar ice sheets (Greenland and Antarctica) and variations in land hydrology. Hence, we propose that climate change has increased the rate of tidal friction by ∼0.06 ms/cy–primarily in the post-industrial era and specifically since 1900–and has thus induced an additional lunar recession of ∼1 millimeter per year (mm/yr) on top of the background trend of ∼38.3 mm/yr. Our results bridge astronomy and climate science and have considerable implications, including for navigation in space and models of lunar and planetary ephemerides.