GB2617178A - A method and system for carbon sequestration - Google Patents

Abstract

A system and method are provided for sequestering carbon. The system comprises apparatus for sinking plant material, such as seaweed in batches, each batch containing a known amount of plant material. The sinking of a batch of plant material represents a carbon sequestration event. The system further includes an apparatus for imaging the sinking of a batch of plant material to confirm a carbon sequestration event and an apparatus for determining the time and location of the carbon sequestration event. The system further includes a computer system for creating and storing a verification data record for the carbon sequestration event. The verification data record includes the imaging of the carbon sequestration event, the timing and location of the carbon sequestration event, and the known amount of plant material that was sunk for the carbon sequestration event. Advantageously carbon is stored at the bottom of the sea rather than on land and is therefore a relatively irreversible sequestration.

Description

A METHOD AND SYSTEM FOR CARBON SEQUESTRATION

Field

The present application relates to a method and system for sequestering carbon to help prevent global warming.

Background

The wavelength A in metres of peak electromagnetic radiation from a black body of temperature T (in Kelvin) is given by A = 0.0029/T. The sun has a surface temperature of -5800K so A = 5x10-7 m = 500 nm in the wavelength range of visible light. In contrast, the earth has a surface temperature of -300K, so A = 1x10, m = 10 pm in the infrared region of the electromagnetic spectrum. Carbon dioxide, which is a minor constituent of the earth's atmosphere, is generally transparent at optical wavelengths (and so is invisible to the human eye). Accordingly, carbon dioxide does not inhibit solar radiation from passing through the atmosphere to the surface of the earth.

However, carbon dioxide absorbs certain bands in the infrared range. Therefore, as the earth emits infrared (heat) radiation back into space, a proportion of this infrared radiation is absorbed by the carbon dioxide in the atmosphere. This absorption causes the carbon dioxide, and hence also the other components in the earth's atmosphere, to warm up. This increase in temperature caused by transparency to incoming solar radiation, but (partial) absorption for outgoing thermal radiation from the earth, is known as the greenhouse effect.

Plants absorb carbon dioxide from the atmosphere as their main source of carbon, as well as water from the ground, to form carbohydrates in order to grow plant structure and material. The chain of chemical reactions to create carbohydrates from atmospheric carbon dioxide is endothermic.

Accordingly, plants generally use chlorophyll to absorb sunlight which then powers a process known as photosynthesis for plant metabolism and growth. The reverse reaction, an oxidation back from carbohydrate back to water and carbon dioxide, is exothermic. Animals that eat plants primarily utilise aerobic respiration to perform this oxidation, and hence derive energy such as for movement and warmth.

Some plants may die in anaerobic conditions, such as a peat bog, which can limit decay (oxidation) of the plant material. This material can then become incorporated into geological processes such as sedimentation, subduction, and so on, leading to the production of fossil fuels, for example, oil and coal. On burning (oxidation), these fossil fuels release significant energy as they are converted back into water and carbon dioxide.

The use of fossil fuels has increased very significantly over the past couple of centuries following the industrial revolution, which has led to an anthropogenic rise in the level of carbon dioxide in the atmosphere. Due to the greenhouse effect, the increased amount of carbon dioxide acts to warm the atmosphere in a process known as global warming. This presents a significant challenge for humanity, since global warming may have serious adverse effects, including a rise in sea levels as the polar ice caps melt, and potentially a runaway increase in planetary temperature (which may have happened on the planet Venus).

In June 2019, the UK parliament passed legislation defining a 'net zero' target to reduce the net emissions of greenhouse gases by 100% relative to 1990 levels by 2050. One way to address this target is to replace the use of fossil fuels with renewable alternatives such as wind or solar power (or possibly by increased nuclear power). Such actions reduce the amount of carbon dioxide that is generated and then released into the atmosphere.

Another approach to help meet the net zero target is carbon sequestration, in which carbon is stored (sequestered) so that it does not enter the atmosphere as carbon dioxide to contribute towards global warning. One type of carbon sequestration is associated with buildings, such as coal-fired power stations, that produce a lot of carbon dioxide. It is contemplated that instead of the carbon dioxide being released from such buildings into the atmosphere, rather it is pumped or otherwise saved beneath the earth's surface, for example in some suitable geological formation. However, the implementation of this type of carbon sequestration is challenging from an engineering perspective, and progress has been relatively slow.

Another type of carbon sequestration is based on growing plants, e.g. forests, to absorb and retain carbon dioxide from the atmosphere. This approach is sometimes used in the context of carbon offsets, whereby an activity that adds carbon dioxide to the atmosphere, such as an aeroplane flight using conventional hydrocarbon fuel, e.g. kerosene, is matched (offset) against an activity that removes a corresponding amount of carbon dioxide from the atmosphere, such as growing plants.

It has been recognised in the literature that seaweed (macro algae) may be used for carbon sequestration by sinking the seaweed to the bottom of the sea. The sea may be divided into multiple layers or zones. The top layer is known as the euphotic (sunlight) zone and is home to many familiar species such as tuna fish. This layer extends from the surface down to a depth of around 200 metres.

The euphotic zone absorbs (or reflects) nearly all (around 99%) of the sunlight that is incident on the surface of the sea and represents the region in which net photosynthesis may occur. Below the euphotic layer is the mesopelagic (twilight) zone, which extends from around 200 metres down to around 1000 metres and is home to species such as shrimps and swordfish. The light level that penetrates through to the mesopelagic zone is too low to support photosynthesis. Beneath the mesopelagic zone is the aphotic zone, i.e. at a depth of 1000 metres or more, which is home to species such as the giant squid and the angler fish. The aphotic zone is too deep to receive sunlight from the surface and so is shrouded in permanent darkness. Most schemes for performing carbon sequestration at sea involve sinking seaweed to a depth corresponding to the aphotic zone.

The use of seaweed for carbon sequestration is further discussed, inter alia, in: "Tracers in the Sea" by Broecker and Peng, January 1982 (see https://www.amazon.co.uk/Tracers-sea-Wallace-S-Broecker/dp/B0000EHBZ3): "Sequestration of macroalgal carbon: the elephant in the Blue Carbon room" by Krause-Jensen et at, in Biological Letters, 14, 20180236; "Substantial role of macroalgae in marine carbon sequestration" by Krause-Jensen and Duarte, pages 737-742 in Nature Geoscience, volume 9, October 2016; and "Testing the climate intervention potential of ocean afforestation using the Great Atlantic Sargassum Belt" by Bach et al, published on-line in Nature Communications, 7 May 2021, as well as: "Removing 10 Gigatons of Carbon Dioxide" by Tim Flannery, see https://www.youtu be.co m/watch?v=SRVn tJ1r2c.

The use of plants, especially forests, for carbon sequestration has led to commercial arrangements in which a plant grower may sell the carbon offset corresponding to the sequestered carbon to a business which wants to reduce its net carbon emissions. Such a transaction provides a commercial motivation for growers to increase their plant holdings, and a commercial opportunity for a business to reduce its carbon footprint to (or at least towards) net zero.

However, although the provision and use of carbon offsets is now well-established, certain aspects of the implementation remain problematic. For example, there is continued pressure on land-use for many different activities (including agriculture and development), which can increase the price of land available for carbon sequestration. Furthermore, the sequestered carbon of a forest may in fact be released back into atmosphere if the ownership of the forest is subsequently transferred to a new owner who wants to use the land for a different purpose.

Summary

A system and method are provided for sequestering carbon as disclosed herein. The system comprises apparatus for sinking plant material in batches, each batch containing a known amount of plant material. The sinking of a batch of plant material represents a carbon sequestration event. The system further includes an apparatus for imaging the sinking of a batch of plant material to confirm a carbon sequestration event and an apparatus for determining the time and location of the carbon sequestration event. The system further includes a computer system for creating and storing a verification data record for the carbon sequestration event. The verification data record includes the imaging of the carbon sequestration event, the timing and location of the carbon sequestration event, and the known amount of plant material that was sunk for the carbon sequestration event.

Brief Description of the Figures

Various implementations of the claimed invention will now be described by way of example only with reference to the following drawings.

Figure 1 is a schematic diagram of one example of collecting seaweed in accordance with the

present disclosure.

Figure 2 is a schematic diagram of one example of lowering seaweed to the bottom of the sea in accordance with the present disclosure.

Figure 3 is a schematic diagram of one example of a mechanism for sinking seaweed to the bottom of the sea in accordance with the present disclosure.

Figure 4 is a schematic diagram of one example of a computer system and other equipment for creating a record of sinking seaweed to the bottom of the sea in accordance with the present disclosure.

Figure 5 is a flowchart illustrating one example of a method for carbon sequestration in accordance with the present disclosure.

Detailed Description

The present disclosure relates to a system and method for carbon sequestration which involves sinking plant material, typically seaweed (macro algae), to the bottom of the sea. This form of carbon sequestration has certain advantages over other forms of plant-based carbon sequestration such as growing a forest. For example, approximately 70 per cent of the earth's surface is covered by water compared to just 30 per cent covered by land, so intrinsically there is more space for storing carbon at sea than on land. Furthermore, whereas there are many existing forms of land use, such as agriculture, recreation, urbanisation, and so on, the use of the sea bottom is much more limited in nature and extent.

A further advantage of using the bottom of the sea for carbon sequestration relates to the irreversibility and verification (authentication) of the sequestration procedure as disclosed herein. Thus a single event, namely sinking of the plant material, corresponds directly to the sequestration and is, for practical purposes, irreversible. In contrast, for forest growing there is no such single event corresponding to the sequestration, rather such sequestration depends on the ongoing life of the trees in the forest and is reversible, for example, if the forest is subsequently cut down and burnt for fuel.

As described in more detail below, an authentication or verification procedure can be formulated around the single event for sea-bed sequestration to provide confirmation of an irreversible sequestration that is not available (or only partly available) for other sequestration techniques.

Another advantage of using seaweed for carbon sequestration is that in certain parts of the world there has been a significant, and generally undesired, growth in the amount of seaweed. For example, the Caribbean Sea has seen a significant increase in the amount of sargassum seaweed, thought to be due at least in part to excessive use of fertilisers on land, which then run off into rivers for discharge into the sea. This leads to an increasing level of sargassum being washed up onto the beaches of Caribbean islands, which can be unsightly and may also detract from recreational activities on the beach. The growth in sargassum seaweed may also have other adverse consequences on the general marine environment. Accordingly, the removal of such seaweed can be regarded as an ecological benefit in its own right (separate from, but additional to, the resulting sequestration of carbon).

The carbon sequestration procedure disclosed herein supports the removal of carbon dioxide from the atmosphere by allowing seaweed to accumulate carbon dioxide from the atmosphere (as part of the natural growth of the seaweed) and then sinking the seaweed for long-term storage on the seabed. Note that the sequestration procedure itself typically generates a certain amount of carbon dioxide, for example by powering boats. The sequestration procedure disclosed herein has been developed so that the carbon dioxide generated as part of this procedure is significantly less than the carbon deposited on the seabed as part of the sequestration procedure.

Figure 1 is a schematic diagram (not to scale) of one example of collecting seaweed in accordance with the present disclosure. The collection is performed using a boat 100 to trawl across the sea surface. Behind (downstream of) the boat 100 is a bag 120 which is used to accumulate the seaweed. The bag may be made of coconut fibre or any other suitable material. The open end of the bag 120 faces towards the rear of the boat, and together the boat 100 and bag 120 can be considered as defining a longitudinal axis, with the boat 100 and bag 120 then both travelling along this axis (towards the right hand side of Figure 1). In a typical implementation, a bag may have a capacity for around 1 tonne of seaweed, corresponding approximately to a volume of between 1 and 4 cubic metres. However, larger or smaller bags may be used instead according to the circumstances of any given implementation, e.g. with a capacity somewhere in the range 0.5-10 metric tonnes.

The bag may be connected to two booms 130, one boom on each side of the bag 120. The bag 120 and the two booms 130 typically float on the surface of the sea. Each boom 130 is slanted so as to extend partly forwards (towards the boat 100) and partly away from longitudinal axis defined above. In other words, the boom 130 on the left (port) side of the longitudinal axis slants forwards and further left compared to bag 120, while the boom on the right (starboard) side of the longitudinal axis slants forwards and further right compared to the bag 120. Each boom 130 is connected at one end to the bag 120 and at an opposite end to a corresponding (respective) tow line 140. Each tow line 140 is connected at one end to the respective boom 130, and at the opposite end each tow line 140 is connected to the boat.

It will be appreciated that the configuration and/or connections shown in Figure 1 are by way of example, and many variations may be adopted according to the circumstances of any given implementation. For example, in some implementations, the two booms 130 may both be connected at their trailing end to a coupling device (not shown in Figure 1), and the bag 120 may likewise be fitted to this coupling device. Other potential variations and modifications will be apparent to the skilled person.

In operation, as the boat progresses in a forward direction, the tow lines 140 pull the booms 130 along and also the bag 120 in a form of trawling operation. In the above configuration, the surface water containing the seaweed is generally unable to flow past, i.e. over or below the booms 130. Instead, the booms 130 act as a form of funnel, so that the water flows along (parallel to) the booms 130 towards and into the bag 120. The bottom (tail) end of the bag 120, i.e. the end furthest from the boat 100, may be provided with some form of filter to allow water to flow out of the bottom end of the bag 120. The filter may be implemented, for example, as some form of netting or mesh which allows the exit of water from the bottom end of the bag 120, but which retains seaweed in the bag 120 because the seaweed is unable to pass through the filter and hence unable to flow out of the bag 120.

In some implementations, the (direct or indirect) attachment of the bag 120 to the booms 130 may be configured to break at a predetermined force level. As the trawling operation is performed, and more seaweed is collected into the bag 120, the force on this attachment increases because the bag is becoming heavier (and also potentially because the collected seaweed partly obstructs the flow of water through the bag). The predetermined force level is set to correspond to the force experienced by the attachment when the bag 120 is full (or nearly so) with seaweed. Accordingly, at this point the attachment breaks, and the (approximately) full bag detaches automatically from the booms 130. The detached bag of seaweed then floats on the sea surface before being collected for sequestration.

Various modifications and/or refinements to the above approach may be adopted by the skilled person. For example, in some cases the mechanism for breaking the attachment between a bag 120 and the booms 130 may be configured to close the bag automatically as the attachment breaks. In some cases, there may also be a facility to deploy a new bag automatically into position with respect to the booms 130 to allow a sequence of bags 120 to be filled automatically in turn with seaweed. In some cases, a continuous line may be provided (e.g. from the boat) which passes through (or is otherwise attached to) each bag that is filled and released during a given phase of operation. Such a line may be used to facilitate subsequent operations with the released bags, such as movement and/or collection for sequestration. It will be appreciated that any given implementation may combine one or more of the various modifications described above.

Figure 2 is a schematic diagram (not to scale) of one example of lowering seaweed to the bottom of the sea in accordance with the present disclosure. In particular, Figure 2 shows a boat 200 which uses a winch line 250 to assist in lowering a bag 120 (which is full of seaweed) towards the bottom of the sea. In some implementations the boat 200 shown in Figure 2 may be the same vessel as the boat 100 shown in Figure 1, while in other implementations they may be different vessels.

There are various reasons why different vessels 100, 200 may be used for the seaweed capture and then sequestration respectively. For example, in some cases it may be easier (and/or more cost-effective) to provide the trawling equipment on one boat and the lowering equipment on another boat, rather than trying to accommodate both sets of equipment on a single boat. Also, the sequestration of the seaweed (i.e. the lowering shown in Figure 2) is generally performed in relatively deep water, such as water having a depth of 1000m or more. In order to access such deep water, the boat 200 may be designed for ocean travel to allow the boat to reach locations where the ocean floor has the desired depth (such as 1000m or more, i.e. in the aphotic zone or below); in contrast, the boat 100 may be designed for operation closer to land in shallower water, for example, to help remove seaweed that is most likely to otherwise be deposited on beaches. Accordingly, it will be understood that between the processing shown in Figures 1 and 2, the bag 120 may be transferred from boat 100 to boat 200, and boat 200 may then travel to the selected location for performing the sequestration. There are various ways in which the bag 120 may be deposited on the sea (e.g. ocean) floor. In one example, the bag is jettisoned overboard and allowed to sink to the sea floor; in another example, a winch line 250 may be used to lower the bag 120 all the way to the sea floor. In both of these cases, ballast may be added to the bag 120, since the bag may otherwise float on the sea surface (rather than sinking as desired).

Typically, it has been found that as the bag is lowered into the sea, the increasing pressure acts to compress the air bladders in the seaweed and to expel any remaining air from the bag. The result of such compression is that at a given depth (referred to herein as a transition depth), for example, 50m, 100m, 150m, 200m, 250m, or 500m, or in the range 10-1000m, or 20-500m, 50-250m or 100-200m, the density of the bag and seaweed becomes greater than the density of the surrounding water, and therefore the bag 120 is able to sink to the bottom unaided from this given transition depth.

The schematic diagram of Figure 2 shows an implementation in which the bag 120 for disposal is placed in a cage 260 and the winch line 250 is attached to the cage (rather than necessarily to the bag 120). The combination of the cage 260 and bag 120 may be heavy enough to cause this combination to fall (sink) under its own weight as the winch line 250 is let out. Once the transition depth has been attained, the cage 260 is opened, e.g. by automatically releasing a latch or bolt to open the bottom of the cage 260. The bag 120 is thereby released or jettisoned from the cage to fall (sink) under its own weight down to the sea floor. The winch line 250 can then be retracted back up to the boat 200, taking cage 260 with it, to allow the next bag 120 for sequestration to be placed into the cage for lowering and release.

Many other implementations for releasing and/or lowering the bag will be apparent to the skilled person, with or without the use of cage 260. For example, the bag 120 might be compressed (on boat 100, boat 200, or elsewhere) to remove air and to pack the seaweed more tightly in the bag 120. In this approach, the bag 120 may already be dense enough to sink directly from the sea surface when released into the sea, with or without the use of a winch line and without having to rely on compression by water pressure during the initial phase of lowering the bag 120 into the sea until the transition depth is reached. Another approach is to control the buoyancy of the cage 260 by supplying gas to or removing gas from the cage via a gas supply line (not shown in Figure 2, but typically extending in parallel and adjacent to winch line 250). In this implementation, the cage 260 may be brought to the surface by providing increased gas (such as air) in the cage so that the cage in effect becomes lighter (less dense) than the surrounding water. Conversely, the cage 160 may be submerged and sunk by removing and replacing the gas with water.

The configuration shown in Figure 2 may be operated in or close to a condition of neutral buoyancy. Neutral buoyancy occurs when the density of an object in a liquid matches the density of the liquid itself, so that the depth of the object can be altered without any impact on potential (gravitational) energy. In such a configuration, the energy consumption of the winch is minimised, in that the winch does not expend power to overcome gravity, but only to overcome friction.

Figure 3 is a schematic diagram of another example of a device for sinking seaweed to the bottom of the sea in accordance with the present disclosure. In this approach, the winch line 350 extends over a winch wheel 380. On each side of the winch wheel, the winch line 350 is attached to a respective cage 360A, 360B. In operation, the cages 360A and 360B, which are generally the same as one another, alternate as being a rising component or a falling component -i.e. if cage 360A on one side is the rising component, then cage 360B on the other side of the winch wheel 380 is the falling component, and vice versa.

In the situation shown in Figure 3, the cage 360B is the falling component and is lowering bag 120 for jettison to the sea floor, while the cage 360A is the rising component, returning to the sea surface having already released its bag to sink to the sea floor. The length of the winch line may be arranged so that when the cage 360B is deep enough to release bag 120, cage 360A has been raised back up to the sea surface for access by boat 200 (not shown in Figure 3). Accordingly, once the cage 360B has been emptied of bag 120, and a new bag has been inserted into cage 360A, the roles are reversed, and the cage 360A becomes the falling component and cage 360B becomes the rising component.

The configuration shown in Figure 3 may be operated in or close to a balanced fashion with both sides, e.g. cages 360A and 360B having the same (or similar) weight. In such a configuration, the energy consumption of the winch is minimised, since again the winch does not expend power to overcome gravity, but only to overcome friction. In some implementations, the configuration of Figure 3 may be designed to provide both balance and also neutral buoyancy (or close thereto).

In a typical implementation of the approach shown in Figure 3, the cages 360A, 360B may be designed to be just heavy enough to sink in water irrespective of whether or not the cage is loaded with a bag 120. In this situation, the cages 360A, 360B may be moved in combination by powered rotation of the winch wheel 380. Thus with clockwise rotation of the winch wheel 380, the cage 360A is the rising component and cage 360B is the falling component. Conversely, with anti-clockwise rotation of the winch wheel 380, the cage 360A becomes the falling component and cage 360B is the rising component.

In some cases, the operation of the configuration of Figure 3 may be powered primarily by the gravitational potential energy released by the falling bags and cages, rather than powered rotation of the winch wheel 380. For example, each time a cage is filled with a new bag, sufficient ballast may be included such that the newly filled (first) cage is heavier than the other (second) cage which has just been emptied of a bag (but now contains water instead). Accordingly, the newly filled first cage will fall as desired, and the second cage will rise as desired. When the first cage arrives at the transition depth, the bag in the first cage is released, but the ballast is retained. At the same time, the second cage arrives at the surface to be refilled with a bag, and slightly more ballast than is in the first cage, to ensure that the second cage is heavier than the first cage and hence now becomes the falling component as desired. The above iterative operation may continue through multiple cycles, for example until all the bags 120 have been released or until a cage has been loaded with the maximum ballast. In such a situation, the ballast may be removed from the cages, either by sinking to the sea floor, or by return to the boat 200, e.g. by using a powered rotation of the winch wheel 380. As a variation on this approach, the ballast may be removed from the cages at each iteration (either to the bottom of the sea or to the boat) so the cages do not have to contain increasing amounts of ballast.

The above procedure of Figures 1-3 may be subject to various modifications and/or refinements which may be adopted by the skilled person according to the circumstances of any given implementation. For example, in some cases, the winch wheel 380 and winch line 350 may operate directly on the bags 120 to lower and release such bags, without the involvement of any cages 560A, 560B. Another possibility is that rather than sinking the seaweed in a bag 120, the seaweed may be released from the bag, e.g. at or below the transition depth, for example at a release depth of 100m, m, 250 m, or 400 m, or at a depth in the range 10-1000 m, 100-600 m, or 150-500 m. This opening of the bag 120 might be mechanically triggered once the winch line 350 for the falling component extends down to a desired depth (equal to or greater than the transition depth). This approach allows the emptied bags 120 to be brought up again to the boat 200 for reuse.

In one operating scheme, a bag 120 may firstly be used with boat 100 to collect seaweed. When the bag 120 has been filled with seaweed, it is transferred to boat 200 and lowered into the sea for releasing the collected seaweed to fall to the bottom of the sea. The empty bag 120 is then raised by the winch line back to boat 200 and returned in due course to boat 100 for re-use in collecting seaweed. Accordingly, this represents a cyclic operating process in which the bags 120 can be repeatedly reused.

Figure 4 is a schematic diagram of one example of a computer system and other equipment for creating a record of sinking seaweed to the bottom of the sea in accordance with the present disclosure. Thus compared to other forms of carbon sequestration, the sinking of a bag 120 of seaweed can be considered as a single, specific event representative of carbon sequestration having occurred. The equipment shown in Figure 4 helps to capture the details of this specific event, and can then be used as a verification or confirmation that the carbon sequestration has occurred.

Such verification supports an audit process in respect of the carbon sequestration for tracking and authenticating the amount of carbon that has been sequestered by the event. For example, a company may want to fund (buy) a particular sequestration event to obtain a reliable and trackable amount of carbon offset. As another example, a state body may want to fund carbon sequestration in general to support net zero, and the details captured for specific seaweed sinking events can be used to demonstrate (measure) that a particular amount of carbon sequestration has indeed been performed.

To support such tracking, the equipment shown in Figure 4 may include at least one computing system 420, at least one storage facility 430, at least one communications (comms) link 440, at least one imaging system 460, at least one positioning system 450, at least one weighing system 470, and at least one depth measurement system 480 (although not all implementations will necessarily include all the above types of tracking or monitoring devices). The equipment shown in Figure 4 is generally located on boat 200 which is used to sink the bags of seaweed for performing the carbon sequestration, but at least some of these facilities may be located elsewhere, such as on a support vessel which accompanies boat 200, and/or by using a remote server facility which may for example be accessed via communications link 440.

It will be understood that the computing system 420 may be implemented as appropriate by one or more machines, such as computer laptop(s), tablet(s), smartphone(s), and so on. The storage system 430 may be implemented in any suitable manner, for example by hard disks and/or flash memory provided in the one or more machines used to provide the computing system 420. The various components depicted in Figure 4 are shown as connecting to the computer system 420. Such connections may be implemented by any appropriate wired or wireless link. For example, in the latter case, the wireless link might be provided by Bluetooth and/or by a wireless local area network (WLAN).

The communications link 440 shown in Figure 4 may comprise one or more devices for linking with one or more different communications system, such as mobile telephone (smartphone) networks, satellite communications systems, terrestrial radio networks, and so on. It will be appreciated that the networks and communications links available to boat 200 may change or become more limited as it travels out to sea. The communications link may be provided, at least in part, by communications facilities already available as part of boat 200; in other cases the communications link 440 of Figure 4 may provide alternative or additional communications facilities compared to those already available within boat 200. Note that communications link 440 may allow at least part of the computer system 420 and/or storage 430 to be located remotely, e.g. in the cloud.

The imaging system 460 may comprise one or more cameras for obtaining still pictures and/or videos of the sequestration process described herein. The imaging system 460 may be used for imaging different stages of the process, such as filling a bag 120 with seaweed on boat 100, the transfer of the bag to boat 200, and the release of the bag (or batch of bags) into the sea for sinking the seaweed (either within a bag 120 or emptied out from the bag). If the bag of seaweed is released or jettisoned directly from the sea surface, then the imaging can generally be performed above water. However, if the (bag of) seaweed is released or jettisoned below the surface, such as described above in relation to a transition depth, then underwater imaging (filming) may also be utilised. For example, the cages 250, 350A, 350B, may be provided with one or more underwater cameras (as part of the imaging system 460) to show the bags being released from the cages and starting to sink to the seabed.

One possibility is that one or more cameras for use underwater are attached to the winch line 350 adjacent to the cages 360A, 360B (or to the point of attachment of the bags 120 of seaweed to the winch line if cages are not being used). Accordingly, the camera and the bags of seaweed are lowered together by the winch line 350 to the point of release of the seaweed (at or below the transition depth). In particular, the camera is positioned to view and acquire a video of the seaweed being released (either within the bag 120, or by emptying from the bag) and sinking down towards the seabed. The camera(s) used for capturing such underwater video may be provided with lights for illumination (given that typically very little sunshine reaches the transition depth). For example, such light(s) may be attached directly to the camera(s), and/or to the winch line 350, and/or to a cage 360A, 360B, to provide suitable illumination of the carbon sequestration event for the acquisition of a video record.

In some cases, the imaging system 460 may include handheld cameras, such as smartphones or more specialist cameras. Greater consistency and reliability is generally achieved with one or more fixed cameras positioned at suitable locations on the boat 200, for example, near the winch system, to image the sequestration procedure (as well as any underwater cameras provided on winch line 350 as discussed above). In some cases, the camera(s) may produce a continuous stream of video (or pictures) which is recorded onto storage 430. This continuous image stream could then be segmented (manually or automatically) into portions, each portion corresponding to a separate sequestration event. In other cases, the camera may start a new image stream for each separate sequestration event through manual or automatic control. For example, a new image stream may begin automatically on boat 200 when a bag is loaded onto the winch system and terminate when the bag 120 has submerged and is no longer visible (or has been released if the camera is also used underwater). Similarly, the cameras in the one or more cages may be automatically triggered to start imaging as the cage door is opened to release a bag 120 and to cease imaging after a suitable predetermined time interval or when the bag or bags of this batch have sunk out of view.

The imaging system 460 is generally designed to be able to picture any visible ID 410, e.g. bar code, provided on a bag 120 to allow identification of the specific bag 120 that is being released.

This then allows the pictures and/or video capture of the bag to be saved to the data record for that specific bag 120, along with other appropriate information, such as current location and depth of release.

In some cases, it may be feasible to use a picture or image of a bag 120 acquired by imaging system 460 to determine the approximate mass of the bag. For example, bags which are full of seaweed may generally have a reasonably consistent weight, and hence hold a consistent, known amount of carbon. The pictures and/or video acquired by the imaging system may be able to confirm that each of the bags is indeed filled with seaweed to capacity. In some implementations, this may provide a close enough determination of the weight of the bags that a separate weighing system 470 (such as described below) may not be required.

Figure 4 further shows a positioning system 450 to determine the location of the boat for saving into the data record for the sequestration event. In general, the positioning system may be implemented based on a global satellite navigation system, such as the global positioning system (GPS) and/or Galileo, etc. Furthermore, some level of positioning information may be derived from other satellite constellations, such as Inmarsat. In some locations it is also possible to determine location at least in part from one or more terrestrial beacons and/or from mobile telephone (cellphone) base stations. Positioning system 450 may therefore utilise any suitable space and/or terrestrial location determination facilities, and may include or combine results from multiple such facilities if appropriate.

We note that the radio signals from such positioning systems generally do not penetrate underwater. Therefore, the location determined by and obtained from positioning system 450 generally corresponds to the location of the boat 200 on the sea surface (rather than the location of a bag 120 underwater), but of course the positions of the boat on the surface and the bag underwater are very closely related. Accordingly, recording the position of the boat into the verification data record is sufficiently accurate to support confirmation of the validity of the carbon sequestration event.

The positioning system 450 may be provided as a stand-alone system (but with a data connection to the computer system 420, etc.), or may be incorporated into other systems. For example, the boat 200 may already include a satellite navigation system and/or some other positioning system for use in navigating the boat 200, and the boat navigation system may be utilised to provide some or all of the positioning system 450. An additional (or alternative) approach is that some imaging systems 460, e.g. cameras or smartphones, may include a positioning facility, for example GPS, to save location information for acquired pictures or videos. Thus in some cases, the imaging system 460 may be used to implement or supplement the positioning system 450.

If desired, an additional level of location authentication can be based on recording the raw signals (the spreading codes) received from GPS and/or other satellite navigation systems. These signals are very difficult to predict (or spoof) in advance, but can be verified subsequent to their transmission. Accordingly, in some implementations, the positioning system may also receive and encrypt the spreading codes from different satellites in the network, and send this encrypted information on an ongoing basis to a trusted third party via the communications link 440. The information received by the trusted third party can be used to provide an additional layer of verification concerning the location of the ship at the time of the sequestration event.

The boat 200 may also include a depth measurement system 480 which may use a sonar or other suitable depth measurement facility to confirm that the bag is being released (or emptied) at a location having sufficient depth (>1000m) for the seaweed to sink into the aphotic zone (or to any other desired depth). Note that alternatively or additionally the depth may be determined based on the measured GPS position and existing charts of sea depth (hence in some implementations, the depth measurement system may be omitted if so desired).

The depth measurement system 480 may also be used to determine the depth at which a bag 120 of seaweed is released or emptied to perform sequestration. Suitable underwater depth sensors are readily available, for example as utilised by divers. Alternatively (or additionally), the release depth may be determined from the position of the winch line used to lower the bag below the sea surface. For example, the winch line may be provided with suitable markings to provide a depth determination for the end of the winch line (and any bag attached thereto); a depth determination may also be made by tracking rotation of the winch wheel backwards and forwards during operation to lower bags for release (and then to raise e.g. an empty cage or bag).

Figure 4 also shows a bag 120 which includes an ID 410. The ID 410 is a unique identifier for that bag. The ID might be applied to the bag at various stages, for example, when the bag is first created (prior to any use), when the bag is filled with seaweed (for example, on boat 100), or when the bag is ready for sequestration on boat 200. The ID 140 might come in various forms, including as a barcode (or OR code or similar) printed on or otherwise attached to the bag 120; some other alphanumerical identifier, which may correspond to a bar code, and which may be printed or otherwise attached to the bag 120; and/or a radio frequency identifier (RFID) chip which may be attached to the bag 120.

In some implementations the bag may be provided with multiple identifiers, either at the same time, or at different stages in the handling of the bag. For example one identifier might be provided at the time of bag manufacture, and another identifier as the bag is lowered into the sea for carbon sequestration. It is also possible for some identifiers, such as an RFID chip, to be updated one or more times during the overall procedure. For example, an RFID chip may be attached to the bag 120 after manufacture, and further information is subsequently stored to the chip to indicate when the bag was filled with seaweed on boat 100, when the RFID chip (and associated bag) were loaded onto boat 200, and when the RFID and bag 120 were prepared for sinking, e.g. by loading into cage 260.

The computer system 420 may be used (inter alia) to track and store information sensed by other devices such as a GPS unit 450, a weighing system 470, a depth monitoring system 480 and an RFID reader (not shown in Figure 4). For example, the computer system 420 may store sensed information relating to a sequestration event corresponding to a batch of one or more bags of seaweed (or other plant material for sequestration). The information about a sequestration event may be saved into storage 430, where the saved information may comprise a respective data record for each batch of bags of seaweed that have been sunk. Note that typically the bags of seaweed may be sunk one by one on an individual basis (with or without the bags themselves being sunk), hence a batch may correspond to a single bag 120. In other implementations however, a batch may correspond to multiple bags 120 of seaweed being sunk together, for example if a cage 260 such as shown in Figure 2 is large enough to accommodate two or more bags.

In tracking the sequestration events, the computer system 420 typically generates a separate data record for each sequestration event, i.e. for each sinking of a batch of plant material. As part of this data record, the computer system may generate a new batch reference for the sequestration event, and also store in the data record the identifiers 410 for the one or more bags contained in the batch. It is generally more efficient (and less error-prone) if the identifiers 410 are read electronically from the bags, e.g. by scanning a bar code, or by using an electronic reader to access the RFID tag, since this then allows direct saving of the bag identifiers 410 into the data records maintained in the computer system 420. Note that if the batches only contain single bags, i.e. one bag per batch, then in effect a bag represents a batch and vice versa (so the tracking can be described as operating at the bag level without additional reference to batches).

The data record for a sequestration event may further include the weight of the bag or bags sunk in this event. The weight of a bag may be recorded on one or more occasions during the processing of a bag, for example, when a bag is first filled with seaweed on boat 100, when the bag is transferred to boat 200, and/or when the bag is lowered into the sea for sequestration. In some implementations the weighing system of Figure 4 may be provided as a standalone system (but with a data connection to the computer system 420, etc.). In other implementations the weighing system may be incorporated into the winch system, such as may be provided by winch line 150, 350 and/or winch wheel 380. For example, the winch system may suspend a bag in air prior to sequestration to determine the weight of the bag 120.

The measured weight information may then be added to the data record for the sequestration. This weight information can generally be mapped to the amount of carbon contained in the bag. In particular, the relevant type of plant material for sequestration, such as a type of seaweed, can be analysed (in advance, or retrospectively, or on-board ship 200 at the time of sequestration) to determine the typical amount of carbon per unit mass of seaweed.

Although Figure 4 shows various systems for acquiring information and measurements to incorporate into the data record for each sequestration event, it will be appreciated that further information may be captured as desired into the data record. Examples of such data may include sea conditions, such as wave size and direction, plus any speed and direction information available on sea currents. Such information might be useful, for example, to estimate where the released bags (or the bundles of seaweed emptied therefrom) are most likely to sink to and settle on the sea floor. The additional information in a data record may also indicate other parameters of interest, such as air and water temperature, and/or other data associated with the bag ID 410-such as capacity of the bag, material from which the bag is made, manufacturer of the bag, etc.. (This bag information may be provided by the bag supplier and already linked to the different bag IDs 410).

There may also be interest in the original location of the seaweed which has now been sequestered. In other words, this corresponds to the location of the trawling operation shown in Figure 1 to fill the bags 120. This information may be obtained, for example, by also providing boat 100 with a GPS facility or similar, and then associating the trawling location (or sequence of locations) with the bag ID 410 of the bag that was filled by this trawling. Such information can then be added into the data records for the bags based on their respective bag IDs.

Knowing the locations where seaweed has been trawled, and how much seaweed has been removed from those locations, may be helpful for addressing the ecological issues discussed above in relation to excessive amounts of seaweed. For example, a study may be made for a given set of trawling locations of which beaches did, or did not, experience a reduction in seaweed following the trawling operations. This information can then be used to determine whether to continue trawling at the same general locations or whether to move, at least in part, to other locations for trawling.

Once the seaweed has been released as described above, it falls to the bottom of the sea. The sequestration locations are typically chosen so that the depth of the sea floor corresponds to the aphotic zone, i.e at least 1000m below sea level, and potentially much lower. At these depths, there is essentially no sunlight and very little oxygen available in the water, so any decay of the seaweed is very slow and there is very little formation and release of carbon dioxide. It is estimated that the carbon sequestration is typically effective for a few hundred years if the bags or seaweed bundles just sit on the sea bed. However, if the seaweed sinks into (or is covered by) sediment, then the sequestration may become effective on geological timescales, such as millions or tens of millions of years (akin to the duration of existing deposits of coal).

Figure 5 is a flowchart illustrating one example of a method for carbon sequestration in accordance with the present disclosure. The method commences with collecting seaweed (or other plant material to be sequestered) at operation 510. This collection of seaweed may be performed, for example, as described above with reference to Figure 1. Note that the collection of seaweed at operation 510 may be performed as a separate task, e.g. at a different location, and/or by a different boat, and/or by a different operator and/or at a different time compared to the sequestration itself. For example, a boat 100 may collect the seaweed at operation 510, and this seaweed may potentially be stored on boat 100, or on land, e.g. in a warehouse, or in any other suitable manner prior to the sequestration itself.

The procedure of Figure 5 then progresses to operation 520, in which a batch (e.g. one or more bags) of plant material (typically seaweed) is released from a boat to sink to the bottom of the sea, such as shown for example in Figures 2 and 3 above. Note that in this context, references herein to the sea generally include the oceans or any other bodies of water which may be suitable for carbon sequestration. Sinking the batch of plant material to the bottom of the sea represents a carbon sequestration event. Accordingly, at operation 530 data is collected relating to the sequestration operation of 520 (hence at least part of operation 530 will generally run concurrently with operation 520, for example to perform a video recording of the carbon sequestration event). Further details about the type of data to be acquired in operation 530 are provided in relation to the discussion above of Figure 4.

In operation 540, a verification data record is created for the carbon sequestration event. The data record serves as clear evidence that the carbon sequestration event has been performed, and also indicates the amount of carbon that has been sequestered. Such a verification data record may be used, for example, if a carbon offset process is audited to ensure that money spent on the offset reflects the sequestration of a particular amount of carbon.

The verification data record may hold collected data for one carbon sequestration event, so that each data record corresponds to a single respective carbon sequestration event. Another possibility is for the verification data record to correspond to multiple carbon sequestration events, for example as performed by a boat over a more prolonged period such as a day. In some cases, each verification data record may hold collected data for multiple events which in combination have sequestered a predetermined amount (mass) of carbon. This predetermined amount of carbon may correspond to (or define) a recognised unit of carbon offset; accordingly, a given number of units of carbon offset will be supported by the corresponding number of verification data records.

In some implementations, the verification data records created from a carbon sequestration process may be incorporated into a blockchain -e.g. each verification data record corresponds to a block to be appended onto the chain. Such a blockchain is a data structure which comprises a sequential set of data records (blocks). Operations on the blockchain are generally limited to appending new blocks to the end of the chain; editing or removal of blocks already accepted onto the chain is not permitted. More particularly, the data structure of the blockchain ensures that editing or removal of blocks already accepted onto the blockchain can be detected. This allows users of the blockchain to verify (authenticate) the blocks stored sequentially on the blockchain to confirm that they have not been subject to tampering or other modification.

The protection of the blockchain against modification is based on a hash function which is calculated with respect to the data held in a block. A hash function produces an output which depends on (and is usually much smaller than) the original data held in the block. The hash function is designed to be one-way, in that calculating the hash from the original data is relatively quick, whereas the reverse mapping from a hash value back to the original data is generally intractable (for practical purposes). Therefore any modification (e.g. corruption) of the data in a block can be detected because there is no longer a match between a stored hash determined from the original data and a hash newly created from the corrupted data.

An important aspect of blockchains relates to the control of how new blocks are appended onto the blockchain. In blockchains used to support crypto-currencies such as Bitcoin, the ordering of blocks to be newly appended to an existing blockchain typically relies upon different users performing a mining operation (a complex mathematical calculation). This is known as a Proof of Work strategy.

The first user to complete the mining operation is rewarded with a unit of crypto-currency and this incentivises many users to perform the mining operation. However, the large-scale performance of mining operations in Proof of Work implementations is inefficient in terms of computational processing resource since multiple users all perform the mining operation in parallel with one another until one user is successful. Having many users all involved in mining requires significant amounts of electrical power.

Accordingly, the blockchain used for holding the verification data records for the carbon sequestration may be based on a different approach for blockchain management, known as Proof of Authority, which does not use large-scale mining. In this approach, only known validators are permitted to add new blocks to the blockchain. In some implementations, there may be only a single validator, such as the party operating the carbon sequestration procedure, i.e. the party responsible for performing operations 520 and 530 in Figure 5. In this situation, there is a just a single party which generates the data records for verification and then appends them to the blockchain. The public keys used by this single party for generating and appending blocks to the blockchain can be made available to third parties, thereby allowing third parties to confirm that the blocks were indeed created by the single party.

In other implementations, an alternative (or additional) validator might for example be a trusted third party, independent from the party responsible for performing operations 520 and 530 in Figure 5. In a situation in which multiple nodes (parties) may participate in the blockchain to add new blocks, the Proof of Authority approach may be supplemented by using a Round Robin consensus to select which party (validator) is to add a new block to the blockchain. With a Round Robin consensus, this selection is deterministic based on when each participant last mined a block.

(For more information about using a Round Robin consensus, see "Don't Mine, Wait in Line: Fair and Efficient Blockchain Consensus with Robust Round Robin" by Ahmed-Rengers and Kostiainen, available from https://arxiv.org/pdf/1804.07391.pdf. Note that this paper relates to the use of a Round Robin consensus in the context of a Proof of Stake approach, which is an alternative to using a Proof of Authority, however, the skilled person can readily map the use of a Round Robin consensus from a Proof of Stake context to a Proof of Authority context).

The energy consumption used by the blockchain for a Proof of Authority approach is comparable to any standard computer application. If only a single party can add to the blockchain, there is no selection process based on multiple users performing a mining operation in parallel with one another, so the energy consumption is much lower than for a Proof of Work approach. Even if multiple parties (nodes) may potentially add blocks to the chain, the deterministic nature of the Round Robin consensus selection avoids any significant increase in energy consumption.

The energy expended on use of the blockchain will grow if an increasing number of parties use the blockchain for verification purposes. Each verification involves downloading the data for a block and calculating the hash value for comparison with a stored value. In the present context, the largest component of the verification data record will generally be the video of the carbon sequestration operation being performed. These videos are still relatively short and the hashing is reasonably fast, e.g. typically in the range of milliseconds to seconds. Moreover, the total number of verifiers is likely to remain small because there is no incentive for the blockchain to be accessed other than by those directly involved in the carbon sequestration and subsequent carbon offset arrangements. Accordingly, the energy consumption involved with the creation and subsequent use of the blockchain as described herein remains relatively small.

In some implementations, the blockchain cryptography is based on Ed25519 digital signatures, which is a public/private (asymmetric) key system -see RFC 8032 for more details. Each block is signed using the private key of the party (node) responsible for adding the block to the blockchain. The public key corresponding to this private key is included in the block after serialisation. If a third party buys a carbon offset (corresponding to a given number of CO2 units), the initial owner (the first party) signs a transfer of the relevant block (or blocks) to the public key of the third party. The effect of this transfer is that the third party becomes the new owner of the CO2 units corresponding to the transferred block(s). Note that in some cases the blockchain may be configured to allow (only) the new owner to further transfer the block(s) and associated CO2 units to another party; in other cases the blockchain may be configured to prevent such onward (further) transfer of the block(s).

Users of the blockchain system (in effect, clients of the carbon offset facility) may choose to keep their private keys offline. In such a case, a seed mnemonic may be provided for storage (analogous to the situation with Bitcoin cryptocurrency). The seed mnemonic may be used as a deterministic source of new private keys.

The blockchain provides proof of carbon removal (sequestration) because the blockchain provides a chain of ownership for each block (e.g. each CO2 unit of offset) leading back to the provenance of the block, i.e. to the sequestration event corresponding to the block. In particular, the blockchain typically contains two types of blocks: a first type of block that represents the creation (performance) of a carbon sequestration event (this is sometimes referred to as a minting block by analogy with crypto-currency blockchains such as Bitcoin); and (H) a second type of block that each represent a transfer of ownership of a first type of a block (where ownership of a first type of block reflects who owns the carbon offset associated with the first type of block). A user who receives a CO2 offset unit, e.g. as per a transfer recorded in the second type of block, is able to click their way back to the published transaction (i.e. first type of block) that incorporated this CO2 offset unit into the blockchain. The published transaction will include a link to the verification data record, such as a URL of a proof video for the sequestration, a hash to show that the block is both unique and hasn't been altered, and the reviewer that signed off on approval of the block.

The blockchain uses the keys described above to ensure that at any given time there is only a single owner of each first type of block (and the carbon offset associated with that block). In other words, the blockchain prevents an owner of that block from transferring the block to more than one party, thereby ensuring there is no double-counting of carbon offset. In other words, the blockchain supports and in effect maintains accurate accounting of the carbon offsets, and this is underpinned by incorporating into the block the physical evidence (e.g. video) of the sequestration event that led to the generation of this carbon offset.

Note that this use of a blockchain to show effective ownership of a CO2 unit of offset has some similarities with the use of non-non-fungible tokens (NFTs), which are also implemented on a blockchain, see https://en.wikipedia.org/wiki/Non-fungible_token). The verification data records disclosed herein may potentially be implemented as NFTs if so desired.

Although the use of a blockchain as described above provides an effective way of handling and storing verification data records from carbon sequestration events, other approaches are possible. As an example, a trusted party may maintain a database of verification data records from carbon sequestration events, including relevant video, etc. Again a hash (or some other form of digital certificate) may be used to demonstrate the integrity of the stored data record, and a digital signature may be provided to support provenance. The database may also track updated ownership of the block in terms of the carbon offset associated with the block.

More generally, the approach described herein supports an irreversible and verifiable carbon sequestration (carbon dioxide removal) based on creating and providing data records, e.g. in the form of a blockchain, to document and authenticate each sequestration event. This approach supports a more reliable development and adoption of carbon offset trading, which in turn facilitates increased carbon sequestration to help reduce atmospheric carbon dioxide.

In conclusion, while various implementations and examples have been described herein, they are provided by way of illustration, and many potential modifications will be apparent to the skilled person having regard to the specifics of any given implementation. Accordingly, the scope of the present case should be determined from the appended claims and their equivalents.

Claims (14)

1. Claims 1. A system for sequestering carbon comprising: apparatus for sinking plant material in batches, each batch containing a known amount of plant material, wherein the sinking of a batch of plant material represents a carbon sequestration event; apparatus for imaging the sinking of a batch of plant material to confirm the carbon sequestration event; apparatus for determining the time and location of the carbon sequestration event; and a computer system for creating and storing a verification data record for the carbon sequestration event, the verification data record including the imaging of the carbon sequestration event, the timing and location of the carbon sequestration event, and the known amount of plant material that was sunk for the carbon sequestration event.
2. 2. The system of claim 1, wherein each batch comprises one or more bags of plant material.
3. 3. The system of claim 2, wherein the bags of plant material are lowered to a transition depth in the sea, and then released to fall to the seabed under gravity.
4. 4. The system of claim 2, wherein the bags of plant material are lowered to a transition depth in the sea and then emptied of the plant material for the plant material to fall to the seabed under gravity.
5. 5. The system of claim 3 or 4, wherein the imaging of the sinking of a batch of plant material includes underwater imaging of the release or emptying of the one or more bags of plant material to sink under gravity.
6. 6. The system of any of claims 2 to 5, wherein each bag has a respective identifier, and wherein the verification data record for a carbon sequestration event includes the respective identifiers for the one or more bags of plant material included in the sequestration event.
7. 7. The system of any of claims 2 to 6, wherein the one or more bags are configured to be trawled across the sea surface to collect plant material for sequestration.
8. 8. The system of any preceding claim, further comprising a winch wheel supporting a winch line, wherein the winch wheel rotates to provide a lifting action on one side of the wheel and a lowering action on the other side of the wheel.
9. 9. The system of claim 8, wherein the apparatus for imaging the sinking of a batch of plant material comprises a camera which is mounted to the winch line for lowering with the batch of plant material
10. 10. The system of any preceding claim, further comprising apparatus for determining the depth underwater at which a batch of plant material is released for sinking, and wherein the determined depth is included in the verification data record.
11. 11. The system of any preceding claim, wherein the plant material is seaweed. 10
12. 12. A method for sequestering carbon comprising: sinking plant material in batches, each batch containing a known amount of plant material, wherein the sinking of a batch of plant material represents a carbon sequestration event; imaging the sinking of each batch of plant material to confirm the carbon sequestration event: determining the time and location of the carbon sequestration event; and using a computer system to create and store a verification data record for the carbon sequestration event, the verification data record including the imaging of the carbon sequestration event, the timing and location of the carbon sequestration event, and the known amount of plant material that was sunk for the carbon sequestration event.
13. 13. The method of claim 12, further comprising forming a blockchain from the verification data records, wherein each block of the blockchain corresponds to one or more carbon sequestration events.
14. 14. The method of claim 12 or 13, further comprising an initial operation of trawling one or more bags across the sea surface to collect the plant material for sinking as part of the carbon sequestration event.