Top 5 Ways to Mess With Earth's Day

The internet is abuzz today with the news that Saturday's massive magnitude 8.8 earthquake in Chile shortened Earth's day by about 1.26 microseconds (one microsecond is a millionth of a second) and nudged the planet's axis of balanced mass a couple of inches. Those aren't big numbers, but it takes a lot to move the entire planet.

As it turns out, it's not as rare as you may think. You don't need a destructive earthquake to change the length of Earth's day, though that happens plenty often. Everything from wind patterns to trees can fiddle with rotation on a small scale, and over a long period of time, our spin is on a slow, steady decline that will add up to 25-hour days and more in the distant future.

Here are the top five ways Earth's day is getting messed with:

1. Earthquakes: the 2004 Sumatra-Andaman earthquake spawned a horrific tsunami throughout the Indian Ocean. It also shortened the day by about 2.7 microseconds.

Not all large quakes shorten the day, but both the Chile and Indonesian temblors were "megathrusts"; faults where the ocean floor is diving down underneath a piece of lighter continental crust on a grand scale. Usually the two tectonic plates are locked, storing up strain and seismic energy. When they pop, the ocean floor suddenly dives into the mantle, sometimes close to 100 feet in a few seconds.

Earth reacts by speeding up, like a spinning figure skater pulling her arms in.

2. Global Warming: Yep, you heard right. It's one more way our civilization is changing the planet -- by making our day longer. Warm El Nino conditions that occur every few years in the tropical Pacific Ocean can lengthen the day up to a millisecond (1/1000th of a second) before subsiding again. According to recent research, more warmth in the tropics could mean many more El Nino events. With more energy getting pumped into the atmosphere, high-level winds will speed up.

To conserve momentum in the Earth-atmosphere system, the planet will be forced to slow down. The cumulative effect could be a half millisecond added to our day by the year 2100.

3. Glaciers (or lack thereof): Glaciers are heavy, and their absence since the last ice age is speeding up the planet about 0.6 milliseconds per day per century. For insight into why this is happening, we go back to our spinning figure skater. In this case, Earth's mantle is moving away from the equator and towards the poles (red areas in image above) where the heavy glaciers used to be. The pattern is akin to a skater puling her arms in during a spin, much like the earthquake, though it happens slower and more steadily.

Next to the action of the tides, this "post-glacial rebound" is the biggest force influencing the length of Earth's day.

4. Trees: Once while on an interview one day with a scientist, we got to talking about strange, un-expected things that trees can do. Like influence the rate at which the Earth spins.

Every spring in the northern hemisphere, deciduous trees take nutrients out of the soil, carbon dioxide from the air, and build leaves. Those leaves spin round and round high on tree limbs through the warm months, and then die off in the fall, dropping a huge amount of biomass down to the ground. That mass imperceptibly speeds up Earth's rotation (there is much less temperate land mass in the southern hemisphere, so the inverted seasons don't fully balance out the effect).

I don't remember the scientist's name, and it didn't make it into the story I was writing. But I was gobsmacked, and definitely remember tweeting about it.

Luckily, other folks with blogs have had a crack at this problem, and one fellow even came up with a respectable estimate for the magnitude of this effect: 1/2,000,000,000,000,000th of a day. Or about .0432 billionths of a second, somewhere around the limits of detection of today's atomic clocks. That's way less than I remember my mystery guest telling me, but either way it's a measurable change!

5. Tides: This is the granddaddy of all rotational influences. Even accounting for all other forces the Moon's gravitational tug on our oceans causes the water to act like a brake on the solid earth, slowing rotation down by an average of 1.7 milliseconds per day per century. For an explanation of how that's calculated, see here and here. That figure has been steady for at least the last 2,700 years, but it can vary a lot over longer periods of time -- the slow-down can even stop for a while. Overall, it piles up though: Earth has packed on several hours to its day since the time of the dinosaurs.

Image: NASA