True Polar Wander (TPW) describes the reorientation of a planet's rotational axis relative to its surface and arises from the conservation of angular momentum between the planetary components, including mantle, core, and climate system (atmosphere, ocean, cryosphere, and terrestrial hydrology). TPW has long been hypothesized to influence planetary climates and, consequently, to play a critical role in assessments of planetary habitability. Recently, several studies have developed simplified frameworks to examine the climatic and habitability implications of TPW. Owing to their schematic nature and their primary focus on Earth, these approaches may be inadequate for capturing the full range of TPW dynamics, particularly for planets with more complex geometries and physical characteristics. Here we present a general theoretical framework for the coupled TPW-climate system of an arbitrary planet, incorporating solid and fluid cores, mantle, and surface climate dynamics. We present a few simple examples to show that TPW can lead to modification of planetary snowball states, substantially increasing the global mean temperature in regions confined to certain, obliquity-dependent latitudes, and thus enabling the survival of life. Furthermore, we demonstrate that TPW can affect the initiation of snowballs by modifying the planetary radiative forcing. This theory provides a unified basis for investigating the long-term climate evolution and habitability of planets undergoing TPW and offers a foundation for identifying promising observational targets in the search for life.

We assess the role of Terrestrial Water Storage (TWS; that is, land hydrology in canopies, snow, soil, groundwater, lakes, wetlands, reservoirs, and rivers) and confirm its significant impact in driving polar motion on a wide range of timescales. For this purpose, we use the comprehensive hydrological model Water-GAP v2.2e in the range 1901-2019 with daily resolution and under climate forcing and direct human influence. TWS-induced polar motion excitation exhibits a prominent long-term trend of ∼4.80 milliarcseconds per year (mas/yr) towards ∼139.25 • E, which we contrast with satellite observations (GRACE/SLR) and reveal inconsistencies of different geophysical origin. The mentioned trend is mainly caused by snow water storage (∼4.87 mas/yr towards ∼140.82 • E; driven by changes in snowfall patterns and melting in Greenland), groundwater (∼0.16 mas/yr towards ∼21.16 • E; due to anthropogenic groundwater withdrawal), and reservoir storage (∼0.06 mas/yr towards ∼103.40 • W; resulting mainly from impoundment of artificial reservoirs by humans). Furthermore, there are fluctuations on seasonal and longer timescales (with our focus up to a period of 6 years), in both prograde and retrograde frequency bands and with amplitudes as large as ∼8.95 mas. We verify these fluctuations against the geodetically observed polar motion excitation, as well as the independent TWS excitation series of GFZ German Research Center for Geosciences, demonstrating significant Pearson correlations (as large as ∼0.81) and coherency (up to ∼0.91). These 1 results improve our understanding of the Earth's rotational dynamics and have considerable implications in the fields of geodesy and geophysics.

We use the hydrological model WaterGAP v2.2e in the range 1901-2019 and with climate forcing and including the direct human impacts to calculate the hydrologically-induced perturbations in the Earth's axial moment of inertia and rotation rate of the mantle, thereby deriving deviations in Length of Day (∆LOD) from the nominal 86,400 seconds. We present 10 individual components of this Terrestrial Water Storage (TWS), namely, water storage in canopies, snow, soil, groundwater, local lakes, global lakes, local wetlands, global wetlands, reservoirs, and rivers. We show that changes in snowfall patterns and melting of glaciers under climate change, groundwater usage due to direct human influence, as well as impoundment of reservoirs, are the three most important long-term contributors, exhibiting trends as large as 0.19 milliseconds per century. However, we argue that there are discrepancies between this trend and that derived from satellite gravimetry, thus revealing the unaccounted contribution of polar ice sheets. We demonstrate the consistency of our estimates with independent series generated by GFZ German Research Center for Geosciences in the range 1976-2019. In addition, by comparing our results with the observed ∆LOD from space geodesy, we explain a considerable portion of this observed signal on seasonal to interannual timescales. Our results provide daily hydrological excitations of ∆LOD, extending the temporal range of the currently available excitation series to the early 20 th century and including individual contributors to TWS under climate change and direct human influence. This proves useful in better constraining other important contributors to ∆LOD, particularly the core dynamics.

Climate, and more specifically atmospheric dynamics, dominates the variations  in Length of Day (ΔLOD) up to interannual timescales (periods of  approximately 2 to 5 years), through exchange of angular momentum with the solid mantle. Specifically, modern atmospheric observations have suggested  the significant influence of El Ni˜no–Southern Oscillation (ENSO) on interannual ΔLOD. However, these estimates are limited to the past few decades (mainly since the second half of the 20th century), which hinders  the analysis of mechanisms responsible for ΔLOD over longer time periods. Here, for the first time, we use tree ring records in the range 900–2020 CE to infer ENSO-driven ΔLOD, with a temporal resolution of 1 year. We demonstrate the consistency of these estimates with ΔLOD inferred from modern ENSO observations and interannual atmospheric angular momentum (for the range 1979–2020). In addition, we reconcile our estimates with those inferred from the output of a paleoclimate model (specifically, the ICON model) in the entire range 900–2020 CE. Finally, by comparing our derived ΔLOD with the astronomically-observed ΔLOD (from lunar occultation and eclipse records and focusing on the range 1800–2020), we can explain the interannual part of this observed signal within one standard deviation of the associated uncertainties, although there are considerable discrepancies that could be attributed to the fluid motion at the top of the Earth’s core (thus providing constraints for core dynamics). Our findings improve understanding of the causes and mechanisms responsible for the observed ΔLOD on interannual timescales, which is a fundamental problem in global geophysics and integral in constraining many dynamical properties of the Earth, including for climate, mantle, and core dynamics.

Changes in the Earth’s global parameters—particularly variations in the  length of day, ΔLOD—are caused by a multitude of geophysical processes, including that of core dynamics. New signals have emerged in the recent space-geodetic record of ΔLOD that are not explained solely based upon dynamics of the Earth’s core. On the other hand, recent studies emphasize the increasing impact of climatic processes on ΔLOD, yet even these fail to account for the mentioned emerging signals. Here we propose that these signals are explained once we account for ’core-climate coupling’ (3C). Using the observations of ΔLOD, global climate change, and core flow models, we unravel a 3C with a coupling strength of 4±2%. This dynamic link elucidates nonlinear interactions between the climate and core processes. To explain the origin of this coupling, we propose a mechanism that accounts for 79 ± 18% of the coupling strength and is based upon the pole tide caused by the deviations of the Earth’s rotational pole induced by climatic processes. This newly discovered 3C phenomenon demonstrates the interplay between internal and external geodynamics, which is fundamental for a better understanding of global geophysics.