Negative Carbon Options
What can we do to pull CO2 out of the atmosphere?
It is useful to categorize negative carbon options into broad categories, because techniques in each category share enough important characteristics that you can reason about the group as a whole.
The breakdown (shown in the diagram) is
We will look at each of these categories in more detail below, but the key take-away is that large seaweed farms have the most promising negative carbon potential that can SCALE.
Chemical / Industrial Negative Carbon
Examples of Chemical/Industrial approaches include
Direct Air Capture - Using chemical 'sponges' to absorb the CO2 and then extract it from the sponge using heat, chemicals, or other processes. These often work best when near a source of CO2.
Enhanced Weathering - Basically crushing rocks and allowing the rocks to absorb the CO2 (natural weathering does this, we are enhancing it).
BioEnergy with Carbon Capture and Storage BECCS - This is a hybrid biological/industrial approach. The idea is to grow things (trees or grass or seaweed or something), and then burn it in an industrial process and collect the carbon (probably using a carbon sponge), and then sequester the CO2 deep underground (or in the deep ocean). This solution is categorized here because it shares the high cost of the other chemical/industrial solutions.
The problem with industrial methods is that they require humans to do the 'heavy lifting' of supplying the energy and raw materials for the process, as well as building infrastructure that is of sufficient scale. This makes chemical/Industrial approaches relatively expensive compared to biological solutions in which natural processes do much of the work for us. Typical costs are greater (often much greater) than $100 per ton of CO2 sequestered.
Biological (photosynthesis) Negative Carbon
Techniques that use biological approaches should be cheaper than industrial approaches because the energy can come from the sun, and the plant 'builds and maintains itself' with relatively little outside help from humans. We can further categorize the biological solutions on whether they work on land or the ocean.
Land Based Biological Negative Carbon
Techniques that work on land include
Trees - Plant trees or other plants, or encourage natural reclamation of degraded lands. In general this is a VERY GOOD source of negative carbon and we should invest in it NOW since it is uncontroversial and can be implemented immediately. However there are some drawbacks
It is not enough. Roughly speaking if trees or other plants could grow on land, they were doing so before humans came along. Thus replanting the trees is simply fixing the agricultural fraction of our CO2 problem, and we will not fix it completely (since we are using some of that land for non-carbon sequestration purposes). There is no enough to compensate for the (LARGE) fossil fuel part of our CO2 problem.
It is limited. Plants sequester carbon while they are actively increasing their biomass (growing). When that biomass dies, it will decompose and release the carbon. Thus a GROWING forest will sequester carbon, but a MATURE forest is roughly carbon neutral.
There is a risk that the carbon will be released. If the forest dies for whatever reason (e.g. disease, fire, or humans turning it into lumber) then the carbon goes back into the atmosphere.
It is less efficient then it could be. Trees produce leaves and other biomass that dies and decomposes. Things that decompose represent inefficiency since energy was used to extract the carbon from the atmosphere and turn it into leaves, but ultimately it decomposes and ends up back in the atmosphere.
Other Land Based Sources of Negative Carbon
Soil Management - This idea is about changing agricultural practices so that biomass in soils tend to sequester more carbon. Like the planting trees solution, this solution is GOOD and we should just DO IT (see 1, 2, 3). But it has most of the same disadvantages as the planting trees solution (not enough, limited, carbon may be released). The biggest problem, however, is that its maximum impact will be relatively small (there is just not that many opportunities to do it).
Bio-Char - is the idea of growing plants and then PARTIALLY burning them to form charcoal, and then using the charcoal as a soil additive. The charcoal only slowly gets converted to CO2 (only unusual microbes will decompose the char, and only slowly), so the sequestering happens, but it is not ideal. There is also the human expense associated with transporting, burning, and then distributing the char, so it has the same cost problem as industrial solutions.
Blue Carbon - The idea is that coastal ocean ecosystems (bogs, peat lands, tidal marshes) store a lot of carbon, and if you stop destroying it (and maybe enhance it) you can store carbon. Again, GOOD idea, DO IT, but it is mostly about repairing damage we have done (The negative carbon is not new). It is a variation of the 'trees' approach (just by the ocean with different plants) The land involved tends to be very valuable (humans like coasts)
Fundamentally the issue with all land-based negative carbon is that humans value fertile land, so land based biological system will always have a significant expense for the land itself. In order to use 'cheap' land (like deserts) we need to 'fix' the land (e.g. bring in water), which is expensive (if we could do it cheaply we would have already).
The other issue is that to a rough approximation, before humans started deforesting, nature HAD already grown things on all land that can grow things. Thus we are only putting back what was at one time there, and thus we can only fix part of the problem (we need NEW negative carbon to remove the fossil fuel emissions).
This is not to say land based solutions are not helpful. Perhaps the MOST IMPORTANT thing we can do is to AVOID FURTHER DAMAGE. The cheapest way of getting trees is to NOT CUT THEM DOWN / BURN THEM in the first place. The next cheapest way is to find land that HAD supported a forest/grassland and allow that forest/grassland to reestablish itself. In places where the land is of little value otherwise, this works, and we should do that, but we simply can't get enough cheap land to do this. This is the fundamental problem with land based solutions: good land is expensive.
Marine (Ocean Based) Negative Carbon
There is a lot of potential
The good news about the ocean (at least with respect to negative carbon), is that the vast majority of the ocean is UNDERUTILIZED. Here is a satellite image from nasa.gov that shows the concentration of chlorophyll on the surface. It is a reasonable approximation of how photosynthetically active any particular place in the ocean is. Green means active, and blue means inactive. You can see that there is a large region in the Arctic and Antarctic oceans that are active but the vast interior parts of the Pacific, Atlantic and Indian oceans are 'deserts' with little going on. If this area could be filled with "Plants" to fix carbon, THAT would represent VERY LARGE (continent sized) areas of NEW negative carbon. This is very promising. See Why Most of the ocean is a desert for more.
Plankton vs Seaweed Farms
There is a basic choice of what kind of marine 'plant' we use
The basic problem with microscopic plants is that they are difficult for humans to control. The plankton goes where the currents pull it and only expensive 'tanks' of one form or another can prevent that. Moreover even very high densities of plankton are measured in a few kilograms per cubic meter of water, so if you want to harvest the plankton it means filtering out a lot of water (which is expensive). If you don't 'harvest' it (some ocean fertilization proposals propose this), the fixed carbon from the plankton does not end up sequestered. Instead, it enters the food web, and ultimately some animal consumes it, releasing the carbon as CO2 again.
Macroscopic "plants" (seaweed) do not have these kinds of control problems. Harvesting it is a as simple as pulling it out of the water. Most seaweed naturally anchors itself to something, which allows us to (mostly) keep it where we want it while it is growing. Finally unlike plankton, it is easy to make seaweed sink which makes for an easy way to sequester carbon.
Thus one of the most promising ideas for LARGE scale negative carbon involve 'farming' large (continent sized) amounts of seaweed in the open ocean.
Review: Negative Carbon Options
Chemical/Industrial Techniques are Expensive
If our goal is large scale negative carbon, industrial/chemical techniques are likely to always be more expensive because humans have to do 'all the work' including providing the needed energy.
Land based Biological Techniques are not Enough
Land-based biological techniques are a good option, but don't represent NEW negative carbon (just undoing deforestation), and also face the issue that humans just value fertile land too much (making it expensive). Some 'spent' land WILL be cheap enough and we should take advantage of those opportunities, but they will not be enough alone.
But Ocean-based Biological Techniques are Promising
Ocean-based biological techniques offer REAL POTENTIAL for LARGE amounts of NEW negative carbon. Oceans are big enough, and are not fixing as much carbon as they could. Raising microscopic algae is difficult to control, but farming large amounts macroscopic algae (seaweed) is very promising.