Scientists can currently measure the length of the day to an accuracy of about 10 microseconds (1/100,000 of a second). It fluctuates slightly, based primarily on changes in atmospheric winds and ocean currents, which affect Earth's angular momentum as it spins on its axis. Angular momentum measures the rotation of a non-rigid body, such as a planet, including its ability to continue to spin. As angular momentum is conserved, the solid Earth's rate of rotation is affected by the movement of its nonrigid components, the ocean and atmosphere.
The Belgian scientists estimated the effect of the ocean and atmosphere, caused by increasing the amount of atmospheric carbon dioxide by one percent per year and its effect on global warming, on Earth's angular momentum. This rate of increase, they note, is a common scenario, based on current human activity. They used 14 different computer models, obtained from the Coupled Model Intercomparison Project, which showed reasonable agreement, they say.
They find that the length of day would increase as a result of angular momentum changes associated with global warming, including variations in surface pressure over land masses, average surface pressure over the ocean, and zonal winds and currents, that is, those moving in an east-west or west-east direction. The amount of lengthening would be small, on the order of microseconds (millionths of a second) per year, and would be difficult to distinguish in any given year from naturally occurring variations.
On a scale of decades or longer periods, though, the effect of global warming on the length of the day would be measurable, the researchers say. They anticipate an increase of around 11 microseconds (11/1,000,000 of a second) per decade in the 21st century, resulting in a total lengthening of the day by 0.11 (11/100,000 of a second) for the century as a whole.
The study was funded in part by the Belgian Fonds National de la Recherche Scientifique.