True, the rate is only half a millisecond every century. But because the culprit appears to be that usual suspect of modern planetary phenomena, global warming, the slowing could become more noticeable as climate change accelerates.
Bodies such as the Earth have a fixed amount of rotational energy, which is shared between the solid planet and the atmosphere. If winds in the atmosphere make the air move faster, the atmosphere gains rotational energy - which it must "take" from the planet below, under the principle called the "conservation of angular momentum". As global warming heats the atmosphere, that effect will increase.
According to a team at the Toulouse Space Centre in France, heightened winds in the upper atmosphere caused by el Nino, the Pacific ocean current, made the days longer by 0.4 milliseconds in 1998.
"There has been a net loss in angular momentum by the solid Earth," Rodrigo Abarca del Rio told New Scientist magazine. "The sets of data suggest that global warming has caused a slowdown of the Earth at a rate of 0.56 milliseconds a century."
Such changes are measured using atomic clocks, which can measure time to an accuracy of two nanoseconds per day, or one second in 1.4 million years.
The Earth's gradual slowing has occurred as the planet's temperature has risen by 0.79C over the past 50 years. The French team believes every 0.1C degree rise in temperature would produce a slowdown in the Earth's rotation of 0.07 milliseconds.Reuse content