Giant kelp farms to save the world!

[Eclipse]

Tim Flannery talks about using 9% of the world's oceans to farm kelp to sequester 40 Gigatons of carbon emissions annually, or roughly 2ppm CO2 per year. goo.gl/n6iFdG

Seaweed farming is already an established industry in many countries, but this project would be expanding it by 20,000 times.
https://en.wikipedia.org/wiki/Seaweed_farming

Thoughts:-

* SYNGAS FROM SEAWEED TO BACKUP ALL RENEWABLES WORLDWIDE! A ton of CO2 concentrated into biomass is about a ton of wood. 40 Gigatons would 40 cubic kilometres of woody waste to dispose of each year. We already know how to biochar any dried biomass waste. 40 cubic kilometres into a biochar unit could produce maybe 20 cubic km of biochar and 20 cubic km of synthetic gas to replace petroleum and natural gas? Wow that's a lot. That's vastly more than the 'cubic mile of oil' we use a year (or 1.6 cubic km). That kind of syngas is ... truly unimaginable. An energy baron's dream. Surely that makes backing up renewables possible. Solar & wind during the day, seaweed syngas at night. Done!

* CURRENTLY we farm nearly 2.5 million tons of seaweed. http://www.un.org/depts/los/global_reporting/WOA_RPROC/Chapter_14.pdf

* COWS: Seaweed can be fed to cows, just to supplement their diet a little, which has been shown to reduce their methane burps close to zero https://goo.gl/J27gw0

* SEQUESTERING LONG TERM: First, once we've switched to a CO2 neutral energy system like renewables + seaweed syngas as backup, we could use just a fraction of the 20 cubic km as biochar for soil remediation. Biochar in the soil is great, but tends to break down in a half life cycle of about 80 years. But there's a lot of soil that needs repair. But we're producing a LOT of biochar ! So once we've thoroughly rehabilitated ALL our farmland soils with biochar, and maybe some pasturelands as well (35% of the non-ice surface of the earth), what do we do with the rest to sequester it? Use industrial presses to crush it into bricks, maybe with a biomimicry agent to cement it, and then start rebuilding those coal-topped mountains with it? We've got to get rid of 20 cubic km's a year! Crush it into bricks and drop in the deep ocean? How does biochar interact with sea microorganisms: would it be better to powder it into the ocean to stimulate other systems?

 

[Moontanman]

Interesting, can we eat sea weed too?

[Danijel Gorupec]

Producing more sea weed is probably a good idea... Although using 9% of world oceans seems quite unrealistic to me. (In fact, I think and I hope that we will not choose any one single megalomaniac idea to pursue, but we will purse hundred of them in parallel in order to 'save the world').

 

But the very first step, I would say, must be to un-shrink that cubic mile back to its 4.1 cubic km value

[Essay]

 

Tim Flannery talks about using 9% of the world's oceans to farm kelp to sequester 40 Gigatons of carbon emissions annually, or roughly 2ppm CO2 per year. goo.gl/n6iFdG

Seaweed farming is already an established industry in many countries, but this project would be expanding it by 20,000 times.

https://en.wikipedia.org/wiki/Seaweed_farming

Thoughts:-

* SYNGAS FROM SEAWEED TO BACKUP ALL RENEWABLES WORLDWIDE! A ton of CO2 concentrated into biomass is about a ton of wood. 40 Gigatons would 40 cubic kilometres of woody waste to dispose of each year. We already know how to biochar any dried biomass waste. 40 cubic kilometres into a biochar unit could produce maybe 20 cubic km of biochar and 20 cubic km of synthetic gas to replace petroleum and natural gas? Wow that's a lot. That's vastly more than the 'cubic mile of oil' we use a year (or 1.6 cubic km). That kind of syngas is ... truly unimaginable. An energy baron's dream. Surely that makes backing up renewables possible. Solar & wind during the day, seaweed syngas at night. Done!

* CURRENTLY we farm nearly 2.5 million tons of seaweed. http://www.un.org/depts/los/global_reporting/WOA_RPROC/Chapter_14.pdf

* COWS: Seaweed can be fed to cows, just to supplement their diet a little, which has been shown to reduce their methane burps close to zero https://goo.gl/J27gw0

* SEQUESTERING LONG TERM: First, once we've switched to a CO2 neutral energy system like renewables + seaweed syngas as backup, we could use just a fraction of the 20 cubic km as biochar for soil remediation. Biochar in the soil is great, but tends to break down in a half life cycle of about 80 years. But there's a lot of soil that needs repair. But we're producing a LOT of biochar ! So once we've thoroughly rehabilitated ALL our farmland soils with biochar, and maybe some pasturelands as well (35% of the non-ice surface of the earth), what do we do with the rest to sequester it? Use industrial presses to crush it into bricks, maybe with a biomimicry agent to cement it, and then start rebuilding those coal-topped mountains with it? We've got to get rid of 20 cubic km's a year! Crush it into bricks and drop in the deep ocean? How does biochar interact with sea microorganisms: would it be better to powder it into the ocean to stimulate other systems?

 

 

...hopefully, by the time we need to think about excess biochar, our carbon emissions will be much lower and the critical balances restored,

so the program could be scaled back to some "maintenance" level. But....

 

 

This is the sort of "geoengineering" we need to be doing!

 

It can help undo several of the hazardous geo-changes that civilization has already engineered, increasingly, over the past several millennia*, centuries, and decades.

 

Restoring our croplands and rangelands, as well as restoring the CO2 balance, is key to sustainably feeding billions of people as this century unfolds.

It is like biomimicry, or maybe that should be called "ecomimicry," in that this sounds like geoengineering that helps restore the base of the food chain.

~

 

*per Ruddiman hypothesis

[Ken Fabian]

Unless sequestering CO2 is a low cost consequence of profitable kelp farming for other reasons it's no more than one more thought bubble. There may be sound reasons for activities that increase biosequestration - increased soil carbon, re-forestation - but I can't see how they can put enough Carbon back into sinks and stores to replace what clearing and agriculture took out let alone all the extra from excessive fossil fuel burning. That we are failing to do so in a prompt and adequate manner doesn't alter the clear imperative to drastically reduce that rate of burning and if we continue to fail no amount of kelp farming or biochar will prevent serious and irreversible climate change.

Edited January 10, 2017 by Ken Fabian

[EdEarl]

Unless sequestering CO2 is a low cost consequence of profitable kelp farming for other reasons it's no more than one more thought bubble. There may be sound reasons for activities that increase biosequestration - increased soil carbon, re-forestation - but I can't see how they can put enough Carbon back into sinks and stores to replace what clearing and agriculture took out let alone all the extra from excessive fossil fuel burning. That we are failing to do so in a prompt and adequate manner doesn't alter the clear imperative to drastically reduce that rate of burning and if we continue to fail no amount of kelp farming or biochar will prevent serious and irreversible climate change.

As robots take our jobs, they can recycle and restore the environment. It's just a mater of robots making robots until there are enough to restore the environment. It may take 100 years or more, and maybe it will be a continuing effort forever. Our current ideas about economics will be obsolete soon.

[Ken Fabian]

EdEarl, I think that without the low emissions energy technologies underpinning their production and use, robots will just add to rather than reduce the emissions problems. Automation/robotics will undoubtedly play a role in making those improved energy techologies at the scales needed but the emissions problem will only be solved by addressing the emissions problem, not by cleaning up the mess it makes afterwards.

[EdEarl]

I believe batteries will be powerful enough to make electric airplanes, cars, trucks, and heavy equipment in little over five years, which means solar PV, now the least expensive power, and batteries are beginning to replace oil and coal. It will take more than 20 years for the replacement to be complete, and there will be lots of clean up to do. Low emissions technology is assured. Oil and coal are on their last legs as major power sources.

[Ken Fabian]

I believe batteries will be powerful enough to make electric airplanes, cars, trucks, and heavy equipment in little over five years, which means solar PV, now the least expensive power, and batteries are beginning to replace oil and coal. It will take more than 20 years for the replacement to be complete, and there will be lots of clean up to do. Low emissions technology is assured. Oil and coal are on their last legs as major power sources.

 

I would like to believe we have that kind of battery technology in the near future and I can see it's heading in the right direction. I'm not convinced it is truly assured any time soon at the scales and relative costs needed, especially not without firm, appropriate energy policy applied with great conviction - or the absence of inappropriate energy policy applied with great conviction. Nor do I think massive growth of robotics can occur without adding it's own environmental and economic burdens; they may have some potential to tackle some difficult problems but too many of the most critical problems are not presenting themselves as profit making opportunities. The political fallout of ever greater loss of employment opportunities to automation might be ever greater support flowing to the kinds of populist political opportunism that prevents foresight and planning being applied in a thoughtful and far-sighted manner.

Edited January 11, 2017 by Ken Fabian

[EdEarl]

According to Elon Musk, batteries are improving about 5% /yr, which means about 5 years to make NY-LA flights possible, unless I made a mistake.

 

Industrial robot arms are about $20K and up, with prices falling. My guess may be a bit too aggressive, but I think in five years we will see reasonably priced household robots. At the same time, jobs will be disappearing.

 

I believe whatever rate you envision would be easier on everyone; I hope you are right.

[Ken Fabian]

EdEarl, although I am very optimistic that crucial limitations of energy storage can be overcome I'm a bit wary of predictions of rates of battery improvements - it's going to be dependent on the actual tech developments feeding into the commercialisation pipeline rather than extrapolation based on observed rates of change. Some costs can be expected to come down in the near term due to improvements in manufacturing methods and economies of scale, including because of automation and I think we can see that with Tesla. These tend to be improvements that reduce the costs of existing and incrementally improving technologies but I suspect there will be diminishing returns over time; the most significant innovations, the real game changers - such as major improvements in energy density that commercial electric aircraft would require - are going to remain unpredictable.

[Eclipse]

Back on the seaweed. I initially wondered if nutrients far out to sea would be a limiting factor, but the seaweed ecology wiki only mentions sunlight and a shallow enough anchorage point to grow seaweed. Nutrients may not be the limiting factor I thought.

https://en.wikipedia.org/wiki/Seaweed#Ecology

 

The seaweed ecology wiki only mentions sunlight and a shallow enough anchorage point to grow seaweed. Nutrients may not be the limiting factor I thought.
https://en.wikipedia.org/wiki/Seaweed#Ecology

Tim Flannery again:-

“The most exciting, if least well understood, of all the biological options involve the marine environment. Seaweed grows very fast, meaning that seaweed farms could be used to absorb CO2 very efficiently, and on a very large scale. The seaweed could be harvested and processed to generate methane for electricity production or to replace natural gas, and the remaining nutrients recycled. One analysis shows that if seaweed farms covered 9% of the ocean they could produce enough biomethane to replace all of today’s needs in fossil fuel energy, while removing 53 gigatonnes of CO2 (about the same as all current human emissions) per year from the atmosphere. It could also increase sustainable fish production to provide 200kg per year, per person, for 10 billion people. Additional benefits include reduction in ocean acidification and increased ocean primary productivity and biodiversity. Many of the technologies required to achieve this are already in widespread use, if at a comparatively minuscule scale.”
https://www.theguardian.com/books/2015/nov/20/climate-crisis-future-brighter-tim-flannery

 

 

If big oil get wind of this, then let them at it!
1. 200kg of seafood per person! That’s over half a kilo per day!

 

2. Some of our NPK nutrients – normally flushed out to sea via the toilet – recaptured for land farming.

 

3. And biogas energy backup for a renewable world that the politicians and pundits seem so intent on! \

 

(If we DON'T have seaweed biogas as backup, and on this kind of scale, I'm not convinced renewables can do it. I'm an EcoModernist, and with James Hansen when he says believing in 100% RENEWABLES is like believing in the Easter Bunny or Tooth Fairy. He's aware of all the 'studies' that say we can do 100% renewables, but still thinks storage is ridiculously expensive and cannot do the job. http://goo.gl/8qidgV )

 

4. Maybe some biochar left over to help retain that NPK and moisture in our farmlands.

Edited January 22, 2017 by Eclipse

[RiceAWay]

Kelp only grow in particular areas and at depths no more than about 10 meter or so. This is a TINY portion of the Earth's oceanic areas.

 

What's more - kelp farming would completely destroy the ecosystems of the area in which it doesn't naturally grow.

[mistermack]

To grow Kelp in the deep ocean, you would need some kind of anchorage for it. That would have to be some floating device that supported some kind of submerged plastic netting. That would have to be extremely robust to survive a storm.

Then there's the problem of nutrients, I doubt if you could get much of a yield without some kind of upwelling.

I think it's a non-starter, on cost grounds. The infrastructure costs would be enormous, and the returns tiny.

My own suggestion for capturing carbon using the oceans would be to pump up nutrients from the sea floor to the surface.

You would then get huge plankton blooms, where once there was ocean desert. This would provide a huge increase in fish stocks, which could finance the operation, and the plankton would be producing hard shells, which would sink to the ocean floor, locking carbon in the carbonates for many thousands of years.

 

This happens naturally, wherever there is a natural upwelling. We would just be replicating that in areas of ocean desert.

[EdEarl]

Instead of kelp, you can grow salt water algae at ground level, either using sea water or saline ground water. Anything grown for CO2 remediation will be expensive, but not growing it will also be expensive.

[mistermack]

The cheapest form of carbon fixing is planting trees. Oaks, redwoods, yew, and hundreds of other species live for hundreds of years.

That would take us well past the time when fossil fuels would be running out.

 

Trees cost hardly anything to plant, or maintain, and there is spare land all around. But we are actually reducing trees.

If you want to lock up carbon, why would you go for expensive CO2 capture, when you can plant trees for a fraction of the cost?

 

An oak tree lives at least 200 years. In 200 years time, we will probably have nuclear fusion energy on tap, and CO2 will be falling.

The danger then will be an ice age, not warming.

[Ken Fabian]

The cheapest form of carbon fixing is planting trees. Oaks, redwoods, yew, and hundreds of other species live for hundreds of years.

That would take us well past the time when fossil fuels would be running out.

 

Trees cost hardly anything to plant, or maintain, and there is spare land all around. But we are actually reducing trees.

If you want to lock up carbon, why would you go for expensive CO2 capture, when you can plant trees for a fraction of the cost?

 

An oak tree lives at least 200 years. In 200 years time, we will probably have nuclear fusion energy on tap, and CO2 will be falling.

The danger then will be an ice age, not warming.

 

Forests will never be able to fix much more carbon than was released by prior deforestation; it is unrealistic to expect it to sequester the far greater additional carbon from continued fossil fuel burning as well. Since the land that hosted the greatest forests are where the best soils and greatest amount of agricultural production is occurring it will not be re-planted. Reforestation will also face problems and risks due to climate change including greater risks of limited growth and tree deaths from things like drought, heatwave, fire. There is no certainty such forests will survive over the long term let alone permanently store excess carbon. There are good and sound reasons to encourage reforestation where possible but it won't work as a means of avoiding the real solution - which is making the energy we use with minimal or zero emissions.

 

Given what is known about climate and climate change dangerous and irreversible climate change will result long before fossil fuels run out. There is no danger of an imminent ice age even if AMOC shutdown could result in decades of strong regional cooling - and no certainty that nuclear fusion energy will ever be reliable, abundant or cheap. Climate action requires more urgent and effective actions than reforestation. Innovation will be essential and we will probably never have the comfort of certainty of means, costs or effectiveness for a full climate solution but the starting point is where we are now. I think we do have what we need to make a serious start on emissions reduction.

Edited January 30, 2017 by Ken Fabian

[mistermack]

I think we do have what we need to make a serious start on emissions reduction.

That's not going to happen. That's becoming more and more obvious.

Unless the economics change drastically. But the Sun will never shine at night, and the wind will always be intermittent, so fossil fuel will always be the cheapest option till it starts to run out. Unless nuclear gets more economic, which might possibly happen with fusion.

Like I said, if governments are seriously looking at carbon capture and storage, planting trees is the most economic way they could do it. It's not going to remove all the carbon in the environment, it's just one way of contributing. A very cheap way.

One acorn grows an oak in 200 years that removes 4 tons of carbon. And has a cash value at the end of it.

You don't have to plant over farmland, you just plant on empty land and hedgerows.

 

Of course, that's not going to happen either. Politicians would rather spend millions on ludicrous CO2 capture schemes, than doing it the easy and cheap way.

[Ken Fabian]

Mistermack - currently more capacity of renewables are being built than nuclear and coal combined and the price point where they are cost effective has recently been passed. Cost estimates for new RE projects keep going lower. Intermittency is an issue but it isn't insurmountable and can actually become a defacto carbon price by forcing fossil fuel plant into greater intermittency, increasing incentives for solution.

 

The PV and battery combination is just reaching the point here where it's use makes our household power bill cheaper - not 100% supply of course but forcing coal and gas into the role of backup and used more intermittently, in smaller amounts, rather than continuously is a step we can make and should make using the technology we have. Doing so will create economic incentives that favour investment in storage technologies.There are soundly based projections of battery costs coming down a lot - from very close to economically viable now to become economically sensible over the next few years. I struggle to understand why fusion, with extraordinary technological hurdles to overcome, that has such a poor record is held up as a potential saviour whilst renewable energy that has such a good record of successful, rapid improvement is - still - treated like it can't ever overcome it's limitations. Fission using proven nuclear technology will surely have a role however it's problems remain principally economic ones (and political ones that derive mostly from the economic ones) - renewable energy may not be a complete solution at low enough cost yet but it can and is being deployed in ever growing amounts even within the mire of conflicted climate and energy politics; as long as that mire persists nuclear, which requires strong, clear, persistent policy far more than renewables do, will be the loser. Nuclear needs a greater minimum threshold of support than renewables and the largest base of existing support can't be used effectively because of it's overlap with anti-climate action politics.

 

Climate science denial prevents that clear, strong policy and did what the anti-nuclear activists could never do - got the captains of commerce and industry, that would be nuclear's most potent backers, to give up on serious climate action and give fixing it with nuclear a collective shrug. That was an economic decision - not fixing emissions appeared to be cheaper - not a position arrived at by assessing the validity of climate science but by assessing the impacts of addressing the problem on their near term costs and profitability. Climate science denial is justification and excuse for that position, propped up with a strong dose of alarmist economic fear.

 

Even if climate change appears intractable to such "leaders" the choice of obstructing strong policy and avoiding a burden of climate responsibility has been within their power, using a well developed toolkit - judicious donating, lobbying, PR, advertising, tankthink. Perhaps disconnecting the collective lobbying of commerce and industry from obstruction of climate policy, by disarming the economic fear, will be the most significant thing that low cost renewable energy (even with intermittency) can deliver in the near term. That political shift may give nuclear a belated kickstart from people with great influence but solar and wind are already cost effective part of the time and storage is on the cusp of cost effectiveness; nuclear will not be competing hour by hour with solar and wind, but with hydro storage and batteries during the combined wind lows and evenings. The whole of RE system's success in emissions terms will come more slowly, as existing emitting plant displaced and shut down, but I think there is reasonable expectation that it will.

 

Millions (billions) wasted on capture schemes - true. Including dubious reforestation schemes that simply cannot make a significant difference if the fossil fuel burning continues. As long as the area dedicated to reforestation is less than the preceding deforestation it will struggle to sequester the carbon released by that deforestation. It does not store endlessly - it becomes part of the carbon cycle - it will reach a point where it releases as much as it stores, well short of sequestering what fossil fuel burning releases.

[Endy0816]

There have been successful conventional kelp projects.

 

Submerged floating rafts could also work though. I know they have been looked at for mobile fish farms. Idea is sound. Steer them clear of hurricanes and straits and kelp should be fine.

[mistermack]

Ken, I agree that solar and wind are getting closer to fossil in price, and they are indeed saving a chunk of carbon going into the atmosphere. However, I really doubt that they will ever replace enough fossil to reverse the trend of rising CO2 in the atmosphere.

 

Like planting trees, they aren't enough, but make a contribution. But your point about trees eventually returning carbon to the atmosphere isn't really valid. Of course that will happen, but if you plant long lived trees, it will be in about 2 to 300 years.

By that time, by your own reasoning, fossil fuels will be long extinct and renewables will be super efficient, and generating all the power we need.

Also, quality wood gets used, and might keep the carbon out of circulation for yet another hundred years.

And the rest can be used in wood burners, instead of dug-up fossil fuel.

 

To say that trees can't replace this or that, and therefor are not worth bothering with, could be said today about wind and solar.

Because the global demand keeps rising, wind and solar aren't going to replace fossil. They will replace SOME.

But CO2 will keep rising in the atmosphere for probably the next fifty years at least. Because of rising demand in the poorer countries.

 

I personally have more confidence in fusion than you. It's a shame that they aren't putting more money into it to speed it up.

I won't live to see it, but it will eventually be providing all the power we need, I'm sure of that.

In the next fifty years, at the current rate of investment, it won't. But long-term, talking 100 years plus, it will.

Investment will take off, once the big doubts start to reduce.

[Eclipse]

Hi all,

I found the answer. Let me summarise!

Seaweed farms could revolutionise the world. 2% of the world's oceans are nutrient rich enough for these farms. Nutrients come from coastal erosion or oceanic upwelling. Sometimes there is nutrient pollution which causes algal blooms and dead zones. Seaweed farming can help mop up excess nutrients and restore ocean health. A new vertical column method of farming the oceans grows both kelp and shellfish and oysters and even encourages fisheries to grow in an ocean ecosystem based approach. Watch this 15 minute TED talk about seaweed feeding the world, and even bringing some of that seaweed back up onto our farmlands to help our farmers.

Many seaweeds are a rich source of vegetarian super-food in their own right, and help form a whole variety of seaweed ice-creams, salads, sauces, and other food ingredients. Kelp farms also stimulate ocean ecosystems, and there are a variety of oysters and shellfish and even wild fish that will grow in amongst the kelp farms. We could feed the world from a small fraction of the 2% of the world's oceans that have their own nutrients. Not that we would be limited to only seaweed and seafood! Think of all the seaweed fertiliser this industry could grow.We could grow so much seaweed that we bring some onto land, get the salt out, and use it as fertiliser. Seaweed could bring our soils back to life. There is even a special seaweed that cows love and eliminates their methane burps! Methane burps are bad news, and cattle lose 15% of their growth to these energy losing burps. But a special seaweed cuts their burps by 99%, solving cattle's infamous methane climate emissions, *and* helping the cows grow faster!

https://theconversation.com/seaweed-could-hold-the-key-to-cutting-methane-emissions-from-cow-burps-66498

Now here's where it gets really bizarre, and potentially planet-saving. Some peer-reviewed work has been done imagining extending kelp farming out into the nutrient-poor open ocean. They start farming the nutrient rich waters. Then a previous season's kelp is biodigested to collect methane gas out the top, leaving the digested kelp nutrients behind. They then recycle those nutrients out in nutrient poor waters. They use slow drip feed hoses and 'tea-bags' that slowly fertilise the kelp, extending the kelp farms out into what was nutrient poor water. This means that nutrients are not a limit to where we can grow kelp!

What if we really went crazy and farmed about 9% of the world's oceans this way?

It would give:-

* a world of 10 billion people half a kilogram of seafood per person per day!

* all the biofuels and biogas we could need to backup a renewable grid (and this is coming from someone who is usually pro-nuclear because of the intermittency and unreliability of renewables!)

* remove ocean acidity

* restore our atmosphere to 350ppm by 2085

In other words, seaweed is a silver bullet to feed the world, save the oceans, and save us from climate change, all in this free PDF. "Negative carbon via Ocean Afforestation". Just register, and download it for free.

http://www.psep.ichemejournals.com/article/S0957-5820(12)00120-6/abstract

[mistermack]

I'm quite sold on that. I've always felt that there's huge potential in the oceans, as most of the ocean areas are literally unproductive desert, due to lack of nutrients.

The main problem would be getting the nutrients there, and keeping them there, in the areas with low nutrient levels.

I favour stirring up the ocean floor, and pumping the cloudy water up to the surface.

 

Ocean storms would also be a problem. But if people are doing it now, and making money, then that is the proof of the pudding.

 

What if it took off in a big way? Would we end up with too little CO2 and too much oxygen? You never know !

[Moontanman]

I'm quite sold on that. I've always felt that there's huge potential in the oceans, as most of the ocean areas are literally unproductive desert, due to lack of nutrients.

The main problem would be getting the nutrients there, and keeping them there, in the areas with low nutrient levels.

I favour stirring up the ocean floor, and pumping the cloudy water up to the surface.

 

Ocean storms would also be a problem. But if people are doing it now, and making money, then that is the proof of the pudding.

 

What if it took off in a big way? Would we end up with too little CO2 and too much oxygen? You never know !

 

 

I would propose using air pumps to bring cold nutrient rich deep sea water to the surface, it could be tested at a small scale in a limited area to see what effects it might have...

[mistermack]

 

 

I would propose using air pumps to bring cold nutrient rich deep sea water to the surface, it could be tested at a small scale in a limited area to see what effects it might have...

I'm guessing you would run into problems using air. It's not very energy efficient compressing air to the sort of pressures that exist at the bottom of the ocean.

My idea would be to adapt a ship to pump a jet of water at the bed, stirring up the silt, and then suck it up to the surface with another pump. You only have the friction in the pipe to overcome, so it shouldn't be too costly in energy. You could maybe use wave motion to power the pump.