Over the past 10 years at least, sea levels have been rising relatively steadily. This is mostly due to melting glaciers and ice sheets, and is a natural — if detrimental — consequence of global warming. The rate of ocean level rise has been a little over 3 millimeters per year (about 1/8th of an inch per year)… until last year. The rate of increase suddenly reversed itself in 2010, and the sea levels actually dropped a bit, by about 6 mm. What happened?
La Niña happened. Equatorial ocean temperatures fluctuate on a cycle; when they are warmer it’s called an El Niño, and when they’re cooler it’s La Niña. As you might expect, this affects how water evaporates off the ocean surface, and therefore rainfall across the world as well. Right now we’re in a La Niña, characterized by drought conditions in the southern US (like in Texas), and heavier than usual rainfall in Australia, northern South America, and other locations:
That map is from the NASA/German Aerospace Center’s Gravity Recovery and Climate Experiment (GRACE) satellites, which map where water is on the Earth and how it moves around. This change in rainfall is the culprit for the lowering sea level:
So where does all that extra water in Brazil and Australia come from? You guessed it–the ocean. Each year, huge amounts of water are evaporated from the ocean. While most of it falls right back into the ocean as rain, some of it falls over land. "This year, the continents got an extra dose of rain, so much so that global sea levels actually fell over most of the last year," says Carmen Boening, a JPL oceanographer and climate scientist. Boening and colleagues presented these results recently at the annual Grace Science Team Meeting in Austin, Texas.
That’s pretty interesting! I didn’t realize it could rain so much that sea levels could be affected, but there you go. Doing the math, I find that a 6 mm drop is equivalent to a volume of very roughly 700 billion cubic meters of water, or 700 cubic kilometers (about 180 cubic miles). That is a lot of water! Read More
Like any scientist, I love a good mystery. Sometimes it’s fun when they are long, complicated, involve subtle and difficult layers, and require a vast effort to unravel.
And sometimes it’s cool when they are simply stated and simply solved. Like asking "Where does the water in Saturn’s upper atmosphere come from?" and finding out the answer is "It rains down from the moon Enceladus."
Water has been seen deep in Saturn’s atmosphere before, but a few years back it was detected in the upper atmosphere as well, and that’s a bit weird; there don’t appear to be any ways to get it from deep down in Saturn to the top parts of its clouds. So how did it get there?
Well, the tiny, icy moon Enceladus was discovered to have geysers at its south pole, actively spewing out quite a bit of water into space. Most of it goes into space and is gone forever. Some actually forms a ring around Saturn called the E-ring, and some no doubt hits other moons. Generally, when a moon blasts stuff into space (like Jupiter’s moon Io does with its sulfur volcanoes) the material forms a big donut-shaped region around the planet. It was figured that Enceladus was doing the same thing with water around Saturn, but even the Cassini spacecraft, which is right there, couldn’t detect it. It’s pretty hard to sample.
But astronomers used Herschel, an Earth-orbiting infrared observatory, to observe Saturn. They found a peculiar feature in the infrared spectrum of Saturn, and realized it’s from this Enceladusian water torus. Apparently, about 3-5% of the water from Enceladus’s geysers falls on Saturn, literally raining down in sufficient quantities to explain the presence of the water detected in the ringed planet’s upper atmosphere.