Climate change brews ocean trouble
The year 1972 was a banner one for wheat farmers in the United States, a bad one for Peru's anchovy fishermen, and therein lies a cautionary tale about coastal fisheries, climate change, and their impact on America's pocketbook.
That was the year that the Nixon administration brokered a $750 million deal to export grain to its cold-war rival, the then-Soviet Union. This extra demand for wheat sent US food prices soaring. The increases, followed the next year by an Arab oil embargo and skyrocketing oil prices, contributed to the worst bout of inflation in the US since the end of World War II.
Today, marine scientist Andrew Bakun adds an under-appreciated twist. That year, he explains, a strong El Niño shut down the oceanic "dumbwaiter" off the Peruvian coast that brings cold, nutrient-rich water from great depths up to the surface. The anchovy population collapsed, depriving American farmers and ranchers of a key source of protein-rich feed for their livestock. That sent them to the grain markets as well, adding to the demand that drove up grain prices. The fishery collapse was temporary. But its economic effect lingered.
Evidence is starting to accumulate that global warming may contribute to – or even trigger – troubling ecological changes taking place in these key regions of coastal upwelling, where some of the world's richest fisheries exist.
These coastal upwelling regions – for example, off Peru, northern California, Oregon, and the west coast of Africa – collectively cover less than 1 percent of the ocean. But they But they account for 20 percent of the world's fish catch. Some of these areas have shown remarkable resilience, notes Dr. Bakun, with the Pew Institute for Ocean Science at the University of Miami in Florida. Others have not. The concern, he continues, "is when you might push the ecosystem just a little too far."
This concern has prompted an increased interest in exploring how these ecosystems respond to change.
Coastal upwelling along the eastern margins of the Atlantic and Pacific gathers steam in the spring and summer, when the land heats up and the temperature differences between ocean and land grow. The difference sets off winds that flow along the coastline. As the winds interact with the ocean, they trigger surface currents that head offshore. The outbound water is replaced with cold, nutrient-laden water that rises from depths, in effect "fertilizing" plantlike plankton that form the foundation for the marine food chain.
In 1990, Bakun published a paper in the journal Science that suggested a way global warming could accelerate upwelling, with uncertain consequences. He posited that by increasing the temperature difference between the already warmer land and the coastal ocean during the day, and reducing the rate at which that difference narrows at night, global warming could lead to stronger winds – accelerating the upwelling.
But until last month, this effect appeared largely in the output of computer simulations of global warming – and in measurements taken over too short a period to be of much use in testing the idea. On Feb. 2, however, the broad theme in Bakun's hypothesis got a boost from a team led by Helen McGregor, a marine scientist currently at the University of Wollongong in Australia, which also published its results in Science.
Using chemical stand-ins as thermometers, her team analyzed two columns of sediment pulled from the ocean bottom off Morocco – the site of one major coastal upwelling zone. They reconstructed sea-surface temperatures there during the past 2,500 years – and found the pattern Bakun had theorized.
During the Medieval Warm Period (AD 1000-1270), water temperatures were cooler, indicating more intense upwelling. During the Little Ice Age (about AD 1300-1850), water temperatures were warmer, pointing to a slowdown in upwelling.
As for today: "Over the last 50 to 100 years we see a strong trend to global warming, and we see a corresponding trend to intensified upwelling," Dr. McGregor writes in an e-mail. Looking back across the 2,500-year span, sea-surface cooling off Morocco during the 20th century is unprecedented – consistent with increased upwelling, the team says.
At this point, the story would seem to be bright for fish: faster upwelling, more food, more fish. But researchers looking at upwelling regions off the west coast of southern Africa and the Pacific Northwest are finding potentially disturbing signs – the emergence of oxygen-starved "hypoxic" zones, and more recently, a delay in the onset of upwelling that risks reducing the number of juvenile organisms that survive to adulthood.
Researchers at Oregon State University have been tracking Oregon's unexpected hypoxic zone since it first appeared in 2002. Last year's event was the worst so far; winds driving the upwelling were twice their usual strength. No one has found a "smoking gun" to connect the hypoxic zone to global warming. But the scientists note that changes in wind patterns driving the plankton blooms that lead to hypoxia are consistent with projected effects of warming. Meanwhile, a team led by John Barth, another Oregon State marine scientist, published a study last month showing that early-season upwelling in 2005 came a month late due to an unusual shift in the jet stream. The delay triggered a serious shortage of nutrients at that led to a dramatic drop in shellfish larvae and juveniles at their usual near-shore "nurseries."
These events show that rising global average temperatures are not the only concern when it comes to global warming's effect on coastal upwelling regions, adds Dr. Barth. "It's also more variance, more variability" in extreme events, he says. If 2005 was extreme in the delayed onset of upwelling, 2006 was extreme in the upwelling's intensity, he says.
But for the University of Miami's Bakun, the poster child for the joint effects overfishing and climate change sits off the Namibian coast. Increased upwelling and overfishing have turned a section of ocean off Luderitz, Namibia, from a habitat once rich in sardines to a malodorous region fit only for jellyfish and other undesirable "weed" species.
Global efforts to move to aquaculture as a path toward sustainable fisheries rely so far on large quantities of fish meal. This comes largely from anchovies off Peru. The region has shown a remarkable ability to reset itself and return to its fecund state as El Niños wax and wane, he observes. But global warming and complex processes unique to marine ecosystems could lead to a tipping point that could turn the region into another Namibia.
"These are complex systems," Bakun says. "And when you change their basic functions, they can run away from you in ways you don't expect."
