Tropical cyclones, or hurricanes as they are known in the regions bordering the Atlantic Ocean, are among nature's fiercest manifestations, capable of releasing as much energy as 10,000 nuclear bombs. Hurricane Katrina leveled New Orleans and the Mississippi Gulf Coast leaving more than 1,800 people dead; Typhoon Morakot killed more people and did more damage to Taiwan than any other storm there in recorded history; and Cyclone Nargis devastated Myanmar (Burma) and resulted in at least 146,000 fatalities.
Could the formation of these storm systems be tempered or even arrested by technical means?
This past June, a plan to reduce the severity and frequency of hurricanes leaked to the public in the form of a patent application under Bill Gates's name (along with many others), resuscitating speculation about a scheme that has been proposed off and on since the 1960s. The core of the idea remains the same: mixing the warm surface waters that fuel tropical cyclones with cooler waters below to drain storms of their energy. But now Stephen Salter an emeritus professor of engineering design at the University of Edinburgh proposes a new—and possibly more realistic—method of mixing.
Salter has outlined in an engineering paper the design for a floating structure 100 meters in diameter—basically a circular raft of lashed-together used tires (to reduce cost). It would support a thin plastic tube 100 meters in diameter and 200 meters in length. When deployed in the open ocean, the tube would hang vertically, descending through the warm, well-mixed upper reaches of the ocean and terminating in a deeper part of the water column known as the thermocline, where water temperatures drop precipitously. The point of this design is to transfer warm surface water into the deeper, cooler reaches of the ocean, mixing the two together and, hopefully, cooling the sea surface. Salter's design is relatively simple, using a minimum of material in order to make the construction of each of his devices cheap (millions of used tires are thrown away each year, worldwide); his scheme would also require the deployment of hundreds of these devices.
Using horizontal wave action at the ocean surface, passive no-return valves would capture energy by closing after a wave has passed through them, allowing the circular interior of each device to raise the level of the seawater within the device by, on average, 20 centimeters. The weight of the gathered warm water would thereby create downward pressure, pushing it down the tube.
The idea is that hundreds of these floating wave-powered seawater pumps would be deployed year-round in areas, such as the eastern tropical Atlantic and the Gulf of Mexico, where hurricanes typically spawn or grow in intensity. (The devices would not, as widely speculated, be deployed only in the path of a hurricane that already formed.)
Salter says he was inspired to invent his device after seeing the damage wrought by Katrina. "I was called to a meeting at [intellectual property firm] Intellectual Ventures where they wanted to talk abut hurricanes, and they were very enthusiastic about it," he says.
The pumps have been named the Salter Sink by the firm, which patented them. Bill Gates was in the session at which Salter proposed the pumps, according to Intellectual Ventures CEO Nathan Myhrvold, and it is the company's policy to list as authors everyone present at a brainstorming session on the patents that are filed as a result of it.
Biological productivity could be side benefit
By mixing warm sea-surface water with the colder water beneath year-round, Salter thinks these pumps could keep the surface temperature below the 26.5 degrees Celsius threshold, beyond which the frequency and severity of hurricanes increase markedly. Salter and some of his co-authors on the original patent think the pump might even increase the biological productivity of the seas in which it's deployed, because it would mix nutrient-rich, deep water with warm, relatively nutrient-poor surface water. Nutrients from deeper parts of the ocean would be brought to within 100 meters of the surface, the deepest that sunlight can penetrate and power the photosynthetic plankton that are the base of the ocean food chain. This would be a boon to fish populations in the ecologically unproductive "biological deserts" of tropical seas where hurricanes spawn and the devices would be deployed. In these areas, little mixing occurs and populations of plankton—and therefore fish—are limited by available nutrients.
Ricardo Letelier, a microbial oceanographer at Oregon State University, however, points out that the effects of increasing available nutrients in the ocean can be unpredictable. "If you were to keep the pumps running continuously…you may allow phytoplankton to bloom," he says. "If you do it for too long, you get a successional pattern where grazers take over and recycle nutrients. And that's one of the problems we've had with iron fertilization experiments—the response of biological systems are not linear."
And Letelier warns that deep ocean waters contain a great deal more dissolved CO2 than surface waters do, because expiring plankton sink in the water column, almost like the rotting leaves on a forest floor. In addition, the solubility of CO2 in water increases with depth and decreasing temperature. As a result, mixing the two layers of the ocean would inevitably lead to significant transfer of CO2 from the biggest carbon sink on Earth—the ocean—to the atmosphere. The process is similar to what happens when you open a carbonated beverage—the drop in pressure causes dissolved CO2 to come out of solution and enter the air.
Is the thermal effect sufficient to abort fledgling storms?
Even if the pump were to succeed, bringing surface water to the depths of the ocean would have little effect on sea-surface temperatures, says Bill Smyth, a physical oceanographer and member of the Ocean Mixing Group at Oregon State University's College of Oceanic and Atmospheric Sciences. That's because the first 20 to 100 meters of the ocean above the thermocline are already so well mixed.
"If you take 20 gigawatts of heat away from surface, you think that has to cool it, but that is not necessarily true," Smyth says. "What it's actually going to do is raise the base of the mixed layer. If the base is at 50 meters, and you pump away the upper meter of the ocean, the mixed layer will then extend down to 49 meters. It's not that the 20 gigawatts disappear into thin air. It's just that it's not doing anything useful in terms of changing sea-surface temperature."
Salter counters that many of the areas where his pumps would be deployed, such as the Caribbean, have thermoclines that start at depths as shallow as 10 to 15 meters below the surface, thereby requiring significantly less pumping in order to strip the warm water from the top layer.
"It would just mean you'd need to pump for longer, but then [the] effect would last a lot longer," Salter says.
Are ocean thermal systems just too big for this technique?
Letelier thinks that, based on Salter's current plan, the scale of any deployment that would have sufficient effect on ocean temperatures to alter hurricanes would be impractically large. And he may know whereof he speaks, because he collaborated on a different ocean-pumping scheme involving long, meter-wide plastic tubes designed to suck water from the depths. That project failed after only 48 hours in 2006 in Hawaii but nonetheless is still being pursued by a company called Atmocean.
"I wouldn't be surprised if in the work of Salter you'd need at least one of these pumps per square kilometer just to make a dent," Letelier says. "That is a huge endeavor. You cannot do it, basically."
But Salter estimates that the mean annual transfer rate of a Salter Sink from the ocean surface to the depths would be 150 cubic meters of seawater per second, or 9.5 gigawatts of power—the equivalent of 10 large nuclear or coal-fired power plants (although this thermal energy has proved difficult to harvest). So, at this rate he calculates that for hurricane alteration his plan would require hundreds of sinks, and not the millions that have been proposed in other oceanic pumping schemes, including one in which Letelier was involved.
Then, there are the shearing currents
Another concern is shearing currents. Jonathan Nash, an oceanographer at Oregon State who is also part of the Ocean Mixing Group, points out that at the interface of the ocean's warm surface water and the cooler water beneath a wide array of competing and reinforcing undersea streams cause powerful shearing currents in which different layers of the ocean move opposite one another at speeds ranging from 20 to 50 centimeters per second.
"Those currents will take this tube and push it sideways. It will get flattened where the shear layer exists," Nash says.
Salter avers that it would be possible to create a downward tube constructed of stiffer materials, but even an extremely strong structure might not be enough, Letelier says.
"In the open ocean the amount of shear that goes on in the upper 50 meters of the water column—the mixed layer—they are incredible forces," Letelier says. "A 100-meter diameter and 100-meter-deep system in the ocean is a big wall against those currents."
The next step: Money
Despite their concerns about the plan, both Smyth and Letelier think it is worth doing more work to address the issues in the existing proposal. Unfortunately, funds to actually build one of Salter's devices have yet to materialize. "We are doing early prototypes to test the idea but our business model is invention," says Intellectual Ventures spokesperson Shelby Barnes. "We are simply not funded to do the next level of in-depth research needed, but our inventors would be interested in collaborating if there were additional resources."
To wit, Intellectual Ventures does not actually build any of the ideas it patents, despite the substantial fortunes of Myhrvold and others who are involved, which means a willing licensee would have to take on the project.
One use of future versions of the Salter Sink could be experiments that examine the responsiveness of the microbial communities of the ocean to mixing. If the Salter Sink were to be evolved into a practical geoengineering scheme, preliminary experiments of this kind would be absolutely necessary, Letelier says.
"There are some pretty big holes in the thinking that need to be patched up," Smyth says. "In science we try not to say anything's impossible or step on anybody's bright idea, but stripping away the entire wind–mix layer of the ocean—that is a huge task."