Climate models simulate winds, pressure, ocean circulation, ice and other changing parts of the Earth system. Researchers can use those outputs to estimate how moving mass and momentum affect Earth’s spin—but many climate simulations do not let changes in rotation feed back into the model itself. Instead, the rotation response is calculated separately using geophysical measurements and angular-momentum conservation.
How changing mass and motion affect Earth’s rotation
Two related mechanisms connect climate processes to Earth’s rotation, but they describe different effects.
Mass redistribution can move the spin axis
When ice, groundwater, ocean water or other mass shifts location, it changes how mass is distributed around the planet and therefore changes Earth’s inertia tensor. That redistribution can move the rotation axis relative to the solid crust, a motion called polar motion. NASA’s polar-motion visualization illustrates contributions from surface mass changes, such as ice melt and sea-level change, as well as internal processes including glacial rebound and mantle convection. The visualization exaggerates the displayed signals for clarity.
Angular-momentum exchange changes the length of day
The atmosphere carries angular momentum (AAM) through both its winds and its mass distribution. NASA’s AAM service distinguishes the motion contribution, associated with winds relative to Earth, from the mass contribution, associated with atmospheric mass distribution. When total angular momentum is conserved to a close approximation, changes in atmospheric angular momentum correspond to opposite changes in the solid Earth’s angular momentum. Changes in spin rate are commonly expressed as changes in the length of day (LOD).
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A rotation calculation can apply angular-momentum conservation to climate-model winds and pressure fields to estimate the corresponding LOD response. The result depends on which reservoirs, time scales and external torques the calculation includes.
What climate models simulate—and what they may hold fixed
Climate models represent interacting parts of the Earth system and enforce physical conservation laws. NASA’s climate-model explainer describes simulations of ocean–atmosphere interactions and land changes that follow conservation of mass, momentum and energy. Its GEOS-5 description gives a concrete example: atmosphere, ocean, land and sea ice exchange momentum, heat and freshwater in a coupled framework.
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That does not mean every model dynamically updates Earth’s rotation. A model may simulate circulation in enough detail to provide winds and surface pressure for an AAM analysis, while holding its rotation rate fixed during the run. In a 2022 HadGEM3 study, the authors wrote: “This model knows nothing of the fluctuations in Earth’s rotation rate.” They used the model’s predicted AAM fluctuations to examine observed LOD variability and its predictability; the statement describes that study’s setup, not every climate model.
One common workflow is therefore to simulate climate fields first, then diagnose their implications for rotation in a separate calculation. That is different from coupling a changing rotation rate back into the climate simulation.
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How researchers estimate rotation effects from model output
A 2026 study in npj Climate and Atmospheric Science illustrates the post-processing approach. It analyzed large ensembles under the SSP3-7.0 scenario, calculated atmospheric angular momentum from climate-model output, and translated changes in total AAM into LOD anomalies using angular-momentum conservation.
| Model ensemble | Simulations used | Scenario |
|---|---|---|
| CESM2-LE | 100 | SSP3-7.0 |
| ACCESS-ESM-1.5 | 40 | SSP3-7.0 |
| MIROC6 | 50 | SSP3-7.0 |
For this analysis, the researchers derived motion AAM from zonal winds and mass AAM from surface pressure. They also used an inverted-barometer approximation for the ocean’s response to atmospheric pressure loading. These methodological choices matter when comparing its results with studies that include different reservoirs or use observations rather than climate-model output.
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How observations help attribute mass changes
Climate-model outputs are not the only evidence. NASA JPL’s polar-motion visualization combines different data and modeled estimates for two periods:
- 2003–2015: GRACE satellite measurements of continental mass changes, paired with model-based solutions for associated ocean redistribution.
- 1900–2000: modeled mass transport that includes ice sheets, glaciers, reservoir impoundment, groundwater depletion, glacial rebound and mantle convection.
NASA’s 2024 summary of studies on climate and Earth’s rotation reports that the spin axis meandered about 30 feet (10 meters) over the preceding 120 years. It also reports a study finding that groundwater, ice sheets, glaciers and sea level accounted for 90% of recurring polar-motion fluctuations from 1900 to 2018; most of the remainder was attributed to Earth-interior dynamics. That is an attribution for the period and fluctuations studied, not a universal breakdown of all polar motion.
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What reported LOD figures do—and do not—mean
NASA’s 2024 account reports several estimates connecting climate-related ice and water movement to LOD. They refer to different periods or a conditional future scenario, so they should not be read as interchangeable measurements.
| Reported figure | What it describes |
|---|---|
| 1.33 milliseconds per century | LOD increase attributed to ice and groundwater movement during 2000–2018, as reported by NASA in 2024. |
| 0.3 to 1.0 milliseconds per century | Range of LOD increase attributed to ice and groundwater movement during earlier periods of the prior century, as reported by NASA in 2024. |
| Up to 2.62 milliseconds per century | Climate-related LOD increase under continued high emissions, a scenario-dependent estimate summarized by NASA in 2024—not an observation or a forecast for every emissions pathway. |
| Average 2.4 milliseconds per century | LOD increase from lunar tidal friction, cited by NASA in 2024 as a comparison for the high-emissions estimate. |
The atmospheric-angular-momentum study is a separate line of analysis: it uses CESM2-LE, ACCESS-ESM-1.5 and MIROC6 ensembles under SSP3-7.0. Its setup and estimates should not be conflated with the ice-and-groundwater figures summarized by NASA.
How to compare studies without mixing unlike results
Before comparing two reported effects, check what each one measures and how it was calculated:
- Process: Does the analysis focus on atmospheric angular momentum, surface mass redistribution such as ice and groundwater, or both?
- Quantity: Is the result polar motion (axis movement relative to the crust), LOD (spin-rate variation), or both?
- Time window: Is it seasonal or interannual variability, a historical attribution, or a future projection?
- Inputs: Does it use satellite observations, reanalysis, modeled mass estimates or climate-model output?
- Assumptions: Which reservoirs, ocean responses, external torques and model couplings are included?
- Scenario and ensemble: For a projection, which emissions scenario and simulations underpin the result?
The IPCC’s Sixth Assessment Report, Chapter 9 also describes changes in gravity, rotation and solid-Earth deformation associated with redistribution among terrestrial ice, water reservoirs and the ocean.
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