It appears that people are finally seeing that climate change is real, although the depth of their realization leaves much to be desired. Thus German politicians are going to close down their nuclear reactors and presumably burn more carbon. Not exactly constructive. A number of US politicians simply deny it, as if to say that if you deny it often enough, it will go away. Here in New Zealand we have politicians who say, yes it is real, but what they are doing about it tends to be to encourage electric vehicles and bicycles, with a bit of tree planting. Good intentions, but perhaps the commitment is a little less than necessary, but still better than the heads in sand approach. So, consider the size of the problem: the Intergovernmental Panel on Climate Change has stated that to limit global warming to 1.5 degrees compared with pre-industrial levels could require the removal of 20 billion tonnes of CO2 from the atmosphere each year until 2100. That is a much bigger than average ask. However, planting trees is a start, and the good news is they keep working at it, year after year. So, what to do? In my opinion, there is no one big fix. The concept of beating climate change with a thousand cuts is more appropriate. Part of the problem is to persuade people to do something. They turn around and say, why me? Who pays?
As an example, it has been argued that in the US the application of biochar to soils could improve grain harvests by 4.87 – 6.4 %. The carbon tends to last for maybe hundreds of years, at least to some extent, so the argument goes that it will eventually pay for itself, but initially it is a cost. This works particularly well in acidic heavily weathered soils, where the yields are generally somewhat low because they do not hold nutrients well. This is also not exactly a single bullet solution, since with good uptake, it would sequester and offset about 0.5% of US emissions.
There was an article in a recent edition of Nature that summarised marine geoengineering. Rather pickily, they stated that none of the proposals have been rigorously tested scientifically nor published in peer-reviewed journals. Part of this gripe is fair: they complain that results have been published, but in places like websites that no longer work. That is a separate issue really, and provided the work is properly done, peer-reviewed journals, following editorial contractions to save space, may not be the best. But let us leave that for the moment. The oceans are an attractive place for one reason: they are not doing much else other than being a place for fish to live in. Land tends to be owned, and much is either required for environmental reserves or food production. Certainly, there is a lot of land that is little better than waste, often left over from previous forest harvesting, and there is no reason why this could not be planted. Another useful contribution, but what are the options for the sea?
The first approach noted by Nature is to try to reduce the albedo, by reflecting incoming sunlight. Two ways proposed for doing this would be to put films on the water, or to spray water upwards and let it form clouds. The latter should be reasonably harmless, leaving aside the problem of whether some places might be adversely affected, a problem that applies to any such proposal. The former could have a serious adverse effect on marine life. Squirting water into the air to form clouds would seem to reasonably easily tested, but it also leaves the question, who is going to do it because ultimately this concept involves a cost for which there is no return.
Two more processes noted in the article are the spreading of alkaline rock into the sea to absorb CO2, and the spreading of iron-rich fertiliser to promote the growth of microalgae. The problem for the first is what sort of rock? A billion tonne of burnt lime per year would do, but first it would have to have its CO2 pyrolysed off, so that would emit as much as it saved. We could try basalt, such as peridotite, but if we powdered that it would make more sense to apply it to land where previously we had applied lime because it does much the same job, but also absorbs carbon dioxide. The iron fertiliser case is more interesting. There have been experiments to do this. An example: a ship sailed around, spread the crushed rock, and found that yes, there was a microalgal bloom. However, they also concluded that the amount of carbon that was fixed by sinking to the bottom of the sea was insufficient to justify the exercise. That, however, omits two other thoughts. First, what happened to the algae? If it was eaten by fish (or mammals) that would increase the food supply, and an increase in animal biomass also fixes carbon. The second thought is that if it were harvested, it could well be used to make biofuels, which would reduce the requirement for oil consumption, so that is equally useful. Can it be harvested? That is a question that needs more research. As a general rule, if there is just one thing that needs doing, there is usually a way, if you can find it. The making of fuel is easy. I have done it. There is, of course, the problem of making money from it, and with the current cost of oil that is impossible. Also, scale-up is still a problem to be solved.
The final two proposals were to cultivate macroalgae and to upwell deep water and cool the top. The latter does nothing for the carbon problem, so I shall not think too hard about that, but it is almost essential for the former. In the 1970s the US Navy carried out experiments on growing macroalgae on rafts in deep water, and they only grew when deep water was brought to the surface to act as a fertiliser. These algae can also be used as fuel, or the carbon absorbed somewhere else, and some algae are the fastest growing plants on Earth. It is quite fascinating to watch through a microscope and see continual cell division. This may be easier than some think. Apparently floating Sargassum is filling up some sections of the Atlantic and off the coast of Mexico.
So the question then is, should any of this be done? The macroalgae probably have the lowest probability of undesired side effects, since it is merely farming on water that is otherwise unused. However, to absorb enough carbon dioxide to make a serious difference an awful lot of algae would have to be grown. However, the major oceans have plenty of area.