All over the world, the oceans are changing in profound ways.
They’re storing up more heat as the world warms. Marine ecosystems are shifting in response to climate change. Sea levels are rising as the world’s melting glaciers crumble into the ocean.
And now, scientists have identified another transformation: The water itself has been moving faster over the past few decades.
A study published yesterday in Science Advances finds that global ocean circulation has been accelerating since the 1990s. The study examined decades of observations from instruments that monitor the seas’ kinetic energy—a measurement of how fast the water is moving.
A closer look, region by region, suggests that much of the acceleration is happening in the tropics. And circulation also appears to be speeding up in other swaths of the Atlantic, Indian and Pacific oceans.
There are still some parts of the ocean that either aren’t speeding up or are actually slowing down, the study notes. But on the whole, as a global average, the waters are moving faster and faster.
The reason is accelerating winds.
Multiple studies have suggested recently that winds over the oceans are speeding up. And because winds are some of the primary forces that drive ocean circulation, they’re also causing the seas to move faster.
There are other consequences, as well, previous studies have suggested. One paper, published last spring, found that faster winds are creating bigger ocean waves in some parts of the world (Climatewire, April 26).
It’s unclear exactly what’s driving the faster wind speeds. For now, scientists believe it may be a combination of both global warming and natural climate variations. Temperature changes in the atmosphere can alter the way air flows around the world, causing shifts in global winds.
But models suggest that continued global warming may have a growing effect in the future. More climate change may mean a greater strengthening of ocean winds, and potentially even faster ocean circulation.
The implications of faster oceans are still unclear, according to lead study author Shijian Hu, a scientist with the Chinese Academy of Sciences. In an email to E&E News, he noted that faster circulation could change the way heat and nutrients are transported around the world. That could potentially alter global weather patterns and affect marine ecosystems.
“It is expected that the acceleration of global mean ocean circulation has a wider implications, but further studies are needed,” he said.
That said, there’s one notable exception to the global speedup of oceans.
Recent studies have found that a major Atlantic Ocean current—which ferries water back and forth from the equator to the Arctic, like a kind of gigantic conveyor belt—is slowing down. Known as the Atlantic Meridional Overturning Circulation, or AMOC, the huge current helps distribute heat throughout the ocean and strongly affects weather patterns in the Northern Hemisphere.
The new study didn’t look specifically at the AMOC. But it did identify a decrease in kinetic energy in parts of the northern Atlantic, according to Hu, which likely corresponds to the AMOC’s slowdown.
Studies disagree about exactly how long the AMOC has been slowing and what factors are responsible. Some researchers suggest that the AMOC goes through long-term, natural cycles of slowing and accelerating, and the recent slowdown is part of the cycle.
But many scientists believe that climate change is playing a strong role. They suggest that cold, fresh meltwater from the Greenland ice sheet is disrupting the current’s flow and causing it to slow down. Modeling studies suggest this effect may worsen in the future if the ice sheet melts at faster rates.
A slower AMOC could have major effects on climate and weather patterns in the Northern Hemisphere, particularly in places like the eastern United States and Europe. Models generally suggest that a weak AMOC could cause a substantial cooling effect in Western Europe, as less and less heat is transported north from the equator.
The new research is another example of the often competing effects of climate change. In this case, warming drives stronger winds, which result in faster ocean circulation across much of the globe. But it also drives faster ice melt, which may simultaneously be causing one of the world’s most important ocean currents to weaken. Both can be true at once, Hu noted.
“This study is quite [a] surprise indeed but consistent with the past studies,” he said.
Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.