By: Ahmed Mukhtar
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.
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.
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 baseball stadiums. Spread over the world’s oceans, that meltwater alone raises sea level about 1 millimeter (1/25th 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.
High and dry
Scientists have long known that sea level changes over time. A scientist has found boulders covered in barnacle shells some 30 meters (100 feet) above sea level. Those high and dry barnacles are several million years old. They serve as evidence that sea level was once much higher. Scientists have also found dead coral reefs buried 150 meters beneath the sea. When that coral was alive, it grew just below the water’s surface. Today, those coral skeletons provide evidence that sea level was once much lower, too.
Sea level has risen and fallen in sync with the ice ages, over hundreds of thousands of years. During past ice ages, oceans were lower because more water was tied up in glaciers on land. But between ice ages, sea level sometimes rose higher than it is today, as melting glaciers sweated their water into the ocean. Some 125,000 years ago, just before the last ice age began, sea level was a whopping 5 to 8 meters (16 to 26 feet) higher than it is today.
Some 2.2 million people live just north of the city of Manila in the Philippines. This area is known as KAMANAVA (an abbreviation combining the names of four towns). Already, high tides flood some streets in KAMANAVA several times each year — and these floods can last weeks.
The sea around the Philippines is rising 7 to 8 millimeters per year, three times the worldwide average. But this is only part of the problem. Even as the sea around Manila and KAMANAVA rises, the land they sit on is sinking. This sinking is called subsidence.
The land is sinking because people have drilled so many wells and pumped so much water out of the ground for drinking and for watering crops. They have pumped out 7 cubic kilometers of water — almost enough water to build a mountain. The depleted land is deflating, like an apple shrinking as it dries. It is sinking 90 millimeters (3.5 inches) per year in some spots.
Humans have caused coastal land to sink in many other places, too — including China, the Netherlands, southeast England and parts of the United States. Some areas of New Orleans have sunk 5 meters (16 feet) in the last 150 years. And that partly explains why the city was devastated in 2005 by storm surges from Hurricane Katrina.
Many people have referred to sea level rise as a future problem. But KAMANAVA and New Orleans offer two examples of how in some parts of the world sea level rise is already a problem. A problem that scientists need to watch closely.
The Jason-2 satellite that is tracking sea level will have to be replaced in several years. Heavy radiation from the sun is slowly cooking the satellite’s computers. And in order to make exact measurements of the sea’s height, Jason-2 has to travel a precise orbit around Earth. But as it zooms around Earth 4,562 times each year, this satellite is slowly wandering out of that orbital path.
The NASA scientists are getting Jason-2’s replacement ready. Jason-3 is almost a carbon copy of Jason-2 which will be launched in to space in 2014.