Satellites, coral reefs, ancient Roman fishponds and sinking cities help us understand how humans are changing sea level

Visit the beach on a hot afternoon and you may not realize it, but someone — or rather something — is watching from above. If you stand in the right place, the silent watcher’s invisible spotlight will pass right over you, like the spotlight of a police helicopter flitting overhead.

That aerial observer zooming over your head is the Jason-2 satellite. It flies 1,340 kilometers (832 miles) high — as far above the ground as New York City is from Chicago. It travels 25,000 kilometers per hour, 27 times as fast as a commercial jet. And it circles Earth a little over 12 times a day.

Two thousand times per second, Jason-2’s spotlight — pointed down at Earth — flashes on for an instant. It isn’t a flash that you could see even if you were looking. The spotlight is throwing off radio waves, which are invisible to the eyes of humans and other animals. Those waves ripple down to Earth and bounce off of its surface, back into space. A computer aboard the satellite times exactly how long those reflected radio waves take to return — usually, about nine-thousandths of a second.

The Jason-2 satellite before it was launched into orbit. Circling Earth at an exact height, the satellite measures sea surface by beaming radio waves down to the planet and timing how long they take to bounce back. Credit: NASA Jet Propulsion Laboratory

By measuring how long the signal takes to bounce back, Jason-2 can measure the distance between itself and Earth’s surface. The satellite was launched into space to measure sea-surface heights. Or, more to the point, Jason-2 is measuring how quickly the planet’s seas are rising.

Scientists these days are worried about sea level. As Earth warms, the surface of the ocean is creeping upward. This creep is happening partly because saltwater expands a tiny bit as it warms. “Warmer water literally is taller,” explains Josh Willis. He’s a climate scientist at the NASA Jet Propulsion Laboratory in Pasadena, Calif.

Sea level also is rising because warm temperatures have prompted glaciers in Antarctica, Greenland and other usually cold places to melt more quickly. Glaciers are essentially rivers of ice, and their melting adds freshwater to the ocean. Antarctica and Greenland are together losing about 350 cubic kilometers of ice per year — enough meltwater to fill up 80,000 Yankee baseball stadiums. Spread over the world’s oceans, that meltwater alone raises sea level about 1 millimeter (1/25^th of an inch) or so each year.

Jason-2 has shown that overall, sea level is currently rising about 2.4 millimeters per year — a little more than the thickness of a quarter.

That may not sound like much — but those quarters stack up year after year. This slow rise is expected to cause flooding in many of the world’s coastal cities in the next 50 to 100 years. Worse yet, the speed of sea level rise is also expected to grow. Seas may eventually rise four to eight times faster than they are today.