It was previously shown that the so-called barystatic processes (continental ocean mass redistribution due to melting of polar ice sheets and mountain glaciers and shifts in the terrestrial hydrology) slow down the Earth’s rotation. This is due to the fact that as sea levels in the equatorial regions rise, the Earth gets slightly more oblate, causing the Earth’s moment of inertia to increase, thereby slowing down its rotation due to the law of conservation of angular momentum. A slowdown in the Earth’s rotation implies a prolongation of completion of one revolution, thus an increase in the length of day. The research mentioned shows the variable rates of length of day variations under 20^th climate change, which has accelerated at the turn of the 21^st century. The rates are imperceptible to humans, only at around 1 millisecond per century. Yet they are important for precise timekeeping and navigation in space. If the ongoing climate change is not curbed, the mentioned rate would more than double at the end of the century.

The increase in the length of day is not the only large-scale, planetary effect caused by the ongoing climate change. In a new study, it is argued that the rate of increase in the Earth-Moon distance is also altered by the aforenoted barystatic processes. The paper presents an astonishing value of 1 millimeter per year (mm/yr), implying that the Earth-Moon distance is increasing by an additional 1 mm/yr due to the ongoing climate change.

The increase in the Earth-Moon distance, often termed lunar recession, is the gradual increase in the semi-major axis of the Moon’s orbit around the Earth. The current background value (without the consideration of climate change) is around 38.3 mm/yr. This implies that each year the distance between the Earth and Moon, with a current value of around 385,000 kilometers, is increased by 38.3 millimeters. But this value is sensitive to the amount of water in the Earth’s oceans and the geometry of the coastlines and ocean basins. Furthermore, how the water is distributed vertically in the oceans also affects the rate of lunar recession, that is, the influence of ocean stratification. With the ongoing climate change, the amount of ocean water is altered; in addition, the ocean density is rather impacted because the melting of polar ice sheets results in the addition of fresh water to the oceans, which has a different density than the salty ocean water. Hence, it is possible that the ongoing climate change is in fact influencing the rate of lunar recession.

In order to quantify the impact of ongoing climate change on lunar recession, the mentioned research first quantified the so-called tidal friction. Tidal friction is caused by the dissipation of the Earth’s rotational energy in the oceans and solid Earth, with the latter being rather negligible and thus ignored in the analyses. The Moon exerts an additional gravitational torque on the Earth’s equatorial bulge in an attempt to align it to the semi-major axis of Moon’s orbit around the Earth. This clockwise torque generates friction with the counterclockwise sense of Earth’s rotation. Over geological timescales, this results in a slowdown in the Earth rotation and by the conservation of angular momentum, an increase in the Earth-Moon distance. Tidal friction is derived by solving the Laplace tidal equations, which are modified to incorporate the effect of changes in sea level and ocean stratification on tidal dynamics. By deriving the tidal friction, the lunar recession can be calculated based on astronomical relations. For this purpose, the mentioned study used the barystatic data since 1900 up to the end of 2018 and with yearly temporal resolution. The study showed that around 30% of the changes in the rate of lunar recession are caused by sea level change, while around 70% by changes in ocean stratification. These presented the first estimates of the effect of ongoing climate change on lunar recession.

To verify the mentioned estimates, the study also analyzed the historical measurements of length of day variations generated from astronomical observations of lunar occultation and eclipses during the past 27 centuries. The study argues for a rather enigmatic secular trend in these data that is not explained by the sum of glacial isostatic adjustment and background tidal friction. The study goes into some depths about the possible role of other geophysical processes, such as the fluid motion at the top of the Earth’s core, or the movement of air in the atmosphere. The study argues that none of these other processes can generate a secular trend large enough to close the budget of observed secular trend in the length of day. As a result, the  study argues that this enigmatic residual signal is caused by the changes in the rate of tidal friction.

This study provides the first estimates on yet another planetary-scale impact of ongoing climate change. Although small in magnitude, it has profound implications, particularly for the navigation in space and precise timekeeping. In order to mitigate these effects, the ongoing climate change needs to be abated. As the study argues, if the ongoing climate change continues to exacerbate, then the rate of change in the lunar recession might be significantly larger at the end of the century.