We all know that humans have quite a knack for changing the natural environment.
In spite of this, you probably think some things are simply too fundamental for human activity to affect. For instance, most people would imagine that the Earth's twenty-four hour rotation would be immune to our tampering. As it turns out, this isn't the case. We've sped our planet up.
How fast anything spins is partly determined by how its mass is distributed. The closer an object's mass is to its axis of rotation, the faster it spins. To picture this, imagine an ice skater doing a pirouette. When her arms are extended, she spins slowly and gracefully. But when she draws her arms inward, she spins much faster. It took more energy to start spinning with her arms extended, so as she draws them in, that extra energy is converted into faster spinning.
What's true for skaters is also true for planets. Our Earth doesn't have arms, but it does have a lot of oceans, which tend to bulge out at the equator. This weight distribution is like a skater with arms out, spinning slowly.
In the past century, however, we've changed the way Earth's water is distributed. By making reservoirs, we've effectively moved some ten trillion tons of water closer to the poles in the last forty years. Closer to the poles means closer to the axis of rotation-like a skater drawing her arms in, this has increased our Earth's rotation. It's an extremely small--but still measurable-amount. Reservoirs have made each day eight millionths of a second faster than it was forty years ago!