The Very Modest Carbon Capture Potential of the Massive Sargassum Blooms.

[NNadir]

Last edited Fri Mar 17, 2023, 10:53 PM - Edit history (1)

The paper I'll discuss in this post is this one: The great Atlantic Sargassum belt (Wang et al, Science , Vol. 365, Issue 6448, pp. 83-87 (2019))

Recently, in this space, I speculated that it might be possible to remove carbon dioxide from the atmosphere by supercritical water oxidation (SCWO) reforming the massive Sargassum blooms which began in the Atlantic Ocean in this decade and are now proceeding at a massive rate.

My previous post is here: Can We Recover Carbon Dioxide From the Atmosphere Using Sargassum Seaweed?

I often remonstrate, again in this space, against a failure to embrace and understand scale when presented with a scheme to address climate change as well as remonstrating against wishful thinking.

It is time to hoist me with my own petard, as Shakespeare had it.

Having engaged in this speculation in my never ending search for schemes to remove carbon dioxide from the air, I decided to look up how much Sargassum there actually is as well as how it compares to the annual dumping of the dangerous fossil fuel waste carbon dioxide, which is roughly on the order of 35 billion tons per year and rising while we all wait for the grand renewable nirvana that has not come, is not here, and will not come.

The mass of the massive "Great Atlantic Sargassum Belt" is discussed in the paper referenced at the outset.

From the introductory text of the paper:

The Sargasso Sea is named after the floating mats of Sargassum seaweed, first reported by Christopher Columbus in the 15th century. These seaweed attract fish, shrimp, crabs, birds, and turtles (1–3), providing essential habitats and serving as hotspots for biodiversity and productivity. Two species of Sargassum, S. fluitans and S. natans, are the most abundant in the Sargasso Sea and the Gulf of Mexico (1, 4), which are notably connected by ocean currents.

Large quantities of Sargassum have recently been reported in the central Atlantic Ocean and the Caribbean Sea (5–14), accompanied by frequent beaching events that have caused serious environmental, ecological, and economic problems (15, 16). Numerous workshops have been held to develop strategies to respond to Sargassum inundations (17, 18). A critical question is whether a regime shift in the atmospheric and/or oceanic climatic conditions has led to the recent changes. Several hypotheses have been proposed concerning the relative roles of warming temperatures, climate change, and nutrient enrichment (19–23), but the lack of large-scale Sargassum data has prevented investigators from reaching a solid conclusion.

We attempt to address this question using long term satellite data, numerical models, and field measurements...

The first graphic of the paper gives a history of the extent of the ocean surface covered by Sargassum over this decade:



The caption:

Fig. 1. Sargassum distributions in the Gulf of Mexico and the Atlantic Ocean. (A) Monthly mean Sargassum areal coverage in the Caribbean Sea and the central Atlantic Ocean, with a maximum of ~ 6000 km2 or >20 million tons in June 2018. The year mark starts from January. (B) Monthly mean Sargassum density (% cover) in January, April, July, and October of 2011–2017 after excluding the nonbloom year of 2013. (C) Monthly mean Sargassum density for the month of July from 2011 to 2018. The GASB is observed in all years except 2013.

As is well known, our complete indifference to climate change, denial on the right and wishful thinking and selective attention on the left, has resulted in dramatic changes to the environment, particularly with respect to temperature and the effect of temperature on biomes throughout the world.

The effects on these biomes is also complicated by nutrient flows, and these in turn are effected by agricultural practices (particularly with respect to the use of fertilizers), river flows, and the (generally declining) health of forests.

Here, from the second graphic in the paper effects on the ocean surface temperature.




The caption:

Fig. 2. Environmental conditions and climate indices used to explain interannual changes of GASB. (A) Mean NAO index averaged from December to February (winter NAO) for 1990–2018. (B) Seasonal mean discharge anomaly of the Amazon River from 1990 to 2018 measured at the Obidos station. (C) Latitude-averaged (from 5°S to 23°N) Sargassum monthly areal coverage density from 2009 to 2018. (D) Latitude-averaged monthly mean SST anomaly from 2009 to 2018. In (C) and (D), the vertical lines mark the locations of 88°W, 61°W, 50°W, 38°W, and 15°W, representing the Yucatan peninsula coast, Barbados coast, the Amazon River mouth, the middle of the central Atlantic, and the West Africa coast, respectively

And now the "money" graphic, the mass of the sargassum belt:



The caption:

Fig. 3. Sargassum biomass and change rate from April 2011 to December 2018. (A) Monthly mean Sargassum biomass in the Caribbean Sea and central Atlantic. These estimates represent lower bounds because satellite measurements are insensitive to Sargassum accumulations in the vertical direction. The inset shows the correlation between the mean change rate in November and December (derived from the mean biomass change from October to December) with the annual mean Sargassum biomass in the next year. The red dot marks the data from 2019 (biomass averaged between January and April 2019) (B) Sargassum monthly change rate since 2011. The gray dashed line marks the climatological change rate between 2011 and 2018 except for 2013

Thus we see that the mass of Sargassum, even with explosion, is relatively modest compared to the amount of dangerous fossil fuel (and biomass combustion) waste we dump each year, particularly when one recognizes that sargassum is wet and consists of considerable amounts of fixed nitrogen, oxygen, and hydrogen as well as sulfur, as evidenced by the smell when it rots on beaches.

We can see that if all of the Sargassum in the massive 2018 bloom were collected and reformed to produce figures comparable to the "down" year of 2013, the amount of carbon captured would be on the order of perhaps 100 million tons, a drop in the bucket compared to our orgy of dumping.

It is not clear either, that an industry based on Sargassum harvesting as a carbon source would be reliable, or entirely unaffected by things like the Brazilian fires now underway or further radical changes in temperature, the latter now being inevitable.

Some further comments from the authors on the cause and predictability of Sargassum blooms:

The following conditions appear to be associated with massive Sargassum blooms at magnitudes comparable to those in 2015 and 2018: (i) large seed populations during winter as a result of the previous year’s bloom; (ii) higher nutrient supply from theWest Africa upwelling in winter months, which can be inferred from higher chlorophyll levels and lower SSTs in satellite imagery; and (iii) higher nutrient supply from the Amazon River input but normal or lower SSTs during the current year. If these conditions are met, then a massive bloom is likely to occur in the central Atlantic, followed by severe beaching events in the Caribbean Sea in later months…
…Finally, we recognize that there are active discussions within the research community on the mechanisms driving the recent trends of Sargassum blooms. The explanation presented here is based on the physical connectivity across several regions, on the analysis of several environmental factors, on limited field studies, and on the satellite-based Sargassum observations. These modeling and observationally based analyses, although reasonable to the best of our knowledge, still require validation in the future and admittedly may not rule out other explanations. Conversely, the recurrent GASB clearly shows a regime shift after 2011 in bloom patterns and possibly in oceanographic conditions as well. A critical question is whether we have reached the point where recurrent GASB and beaching events may become the new norm. Under continued nutrient enrichment due to deforestation and fertilizer use in agriculture (fig. S4), along with the substantial mass of Sargassum seed populations lingering in the tropics (movie S1), the answer is likely positive, and more recent satellite observations between January and April 2019 also support this interpretation. However, the considerable Sargassum accumulations along the pathway of the GASB underline the need for multidisciplinary research to better understand their ecological and biogeochemical impacts (24, 38), as well as their impacts on coastal environments, tourism, economies, and human health (39)…

The above realism does not mean that it is will always be worthless to reform Sargassum using high temperature nuclear plants utilizing supercritical seawater as a source of hydrogen. Such reformation would prevent the use of dangerous fossil fuel carbon sources, and the seawater would of course contain more CO2 from the solubility of this dangerous fossil fuel waste in it, as well as suspended algae and microplastics, all of which would be oxidized under these conditions. In local regions there may be local advantages.

The point is however that any effect will be modest and relatively trivial in terms of scale.

It is well to check one's assumptions.

Have a nice weekend.

Edit: Restored graphics links 03/17/23