Before 1955, a second was defined as a specific fraction of the time the Earth took to rotate once in relation to the stars. Then came the era of highly precise atomic clocks, which proved a much more stable way of defining a physical second.

From the late 1960s, the world started using coordinated universal time (UTC) to set time zones. UTC relies on atomic clocks but still keeps pace with the planet’s rotation. However as the rotation speed is not constant, the two timescales slowly diverge. This means a “leap second” must be added every now and then to bring them back into alignment.

Changes in Earth’s rotation over the long term have been influenced by the friction of the tides on the ocean floor, which has slowed down its rotation. Recently, the impacts of melting polar ice, driven by humans burning planet-heating fossil fuels, have become a significant factor.

As the ice melts into the ocean, meltwater moves from the poles toward the equator, which further slows the speed of the Earth’s rotation.

Polar ice melt “has been large enough to noticeably affect the rotation of the entire Earth in a way that is unprecedented,” Agnew said. “To me, the fact that human beings have caused the rotation of the Earth to change is kind of amazing.”

What is clear, according to the study, is that despite melting polar ice exerting a slowing influence, overall the Earth’s rotation is speeding up. That means the world will soon need to subtract a second for the first time.

“A second doesn’t sound like much, but computing systems set up for activities such as stock exchange transactions need to be accurate to a thousandth of a second.”

Many computer systems have software enabling them to add a second, but few have the capability to subtract one. Humans will need to reprogram computers, introducing the potential for error.

This may sound like more Y2K propaganda, but it’s a reminder that global warming will have effects way beyond eating away at seaside properties and disappearing polar bears. Only time will tell.

[source:cnn]