Guest post by Rud Istvan,
As most WUWT readers know, the issue of carbon sequestration is an important but largely IPCC undiscussed ‘anthropogenic global warming’ question. I got to thinking about it again as a result of the Australian brush fires that are dramatically releasing sequestered brush carbon. And it has been years since the topic was discussed in any depth here at WUWT, insofar as I know.
A cautionary note to WUWT readers. This guest post is a high level review, rather than a typically detailed and highly referenced analytic post on some paper. It is intended mainly to guide your own further research into a fairly complex subject by providing basic concepts and keywords.
There is little doubt that combusting fossil fuel raises atmospheric CO2 in the ‘short term’ at some ‘rate’. This is provable several ways including C12/C13 isotope ratios governed by the differential photosynthetic uptake of the atomically lighter, therefore more ‘reactive’, C12. The experimental proof is simple: as fossil fuel combustion releases more photosynthetically sequestered C12, the residual atmospheric fraction of heavier (so less sequestered) C13 should decline. It does.
The relevant questions for global warming are the meanings of ‘rate’ and ‘short term’. We know the present rate from the Keeling Curve. That curve shows biological sink seasonality (mainly northern hemisphere terrestrial, because plants don’t grow in winter), and surprisingly slight acceleration—much less than the estimated rate of increase in gross CO2 emissions from fossil fuel consumption. (Wiki has good illustrations and discussion.) This belies the ‘saturated sinks’ assumption in the Bern sequestration model because the simple gross/net comparison shows carbon sinks must be growing significantly.
We also know from that same Keeling curve that ‘short term’ is at least decades. But is it several centuries as all the IPCC AR5 climate models predict?
Different Carbon Sink Rates
The rate at which the increase in fossil fuel combustion (and cement production, ~15%) derived C12 increases depends on the rate at which the biosphere can (again) sequester it into more stuff like the coal, oil, natural gas, or limestone (CaCO3) whence it came.
The highest sink rate of coal formation was during the Carboniferous, circa 350-300 mya after large (so cellulosic tissue had evolved ‘strong’ lignin) land plants became dominant, but before lignin digesting ‘white rot’ fungi evolved. That was during the high atmospheric CO2 Paleozoic era, with atmospheric concentrations significantly lowered at the end by photosynthetic coal sequestration.
The longest duration but arguably slowest sequestration rate was into petrochemical source rock–kerogen rich shales–mainly from dead marine phytoplankton sinking from the ocean’s eutrophic zone into anaerobic ‘shallow’ marine mud/clay (now shale) seafloor. Depending on the pressure and temperature at which these marine shale kerogens were later geologically ‘cooked’ (technical term, catagenesis), they formed oil, gas, or both. Natural gas is just overcooked oil, itself just cooked kerogen. So far as the oil and gas industry knows, that process has been going on for at least 500 million years since the Middle Cambrian. It continues today in the anaerobic depths of the Black Sea.
Our use of these fossil fuels frees their long ago biologically sequestered CO2 back into the atmosphere. Hence AGW, which alarmists project into CAGW.
By far the largest carbon sequestration sink (and probably the fastest) is not fossil fuel formation, It is the formation of marine carbonate exoskeletons by single celled photosynthetic organisms (now mostly phytoplankton) like coccolithophores. (The oldest [but rare] are bacterial stromatolites formed more than 2 billion years ago.) Marine carbonate exoskeletons formed the White Cliffs of Dover and numerous miles thick limestone (or dolomite) sedimentary rock formations. An image of the carbonate exoskeleton of the globally most abundant coccolyth species, E. Huxleyi, follows.
A tactile sense of limestone sequestration was gained from my Uplands dairy farm in southwest Wisconsin along Hwy 23 from my farm to Lands End HQ in Dodgeville. The exposed roadcut limestone is over 300 mya and still over a mile thick AFTER eons of erosion and glaciation. Was a mildly shelliferous sea floor; the infrequent shell vugs often have beautiful little calcite crystal linings my kids would find as we walked our freshly plowed fields searching the surface for overturned limestone fragments.
Without tectonic recycling (discussed below), this would be a BIG problem.
There are three basic considerations: (1) rate of CO2 production, (2) present rate of sequestration, and (3) changes in that rate.
(1) The rate of CO2 production has been accelerating with economic development despite the Paris Accord, thanks mainly to India and China. So atmospheric CO2 concentrations will rise. How much depends on the biological sink rate.
(2) The present rate of biological sequestration is about 48% (estimates vary depending on study, whether just marine or marine plus terrestrial, by latitude and region, and lots of other details. Lets stipulate, by about half.
(3) The rate of sequestration is increasing. There are several indicia. For oceans, the eutrophic zone sampled existence of coccoliths has increased by ~tenfold in the North Atlantic in the last decades. For land, NASA satellite remote sensing shows an 18% greening over the past three decades thanks to more CO2 plant food. Unfortunately, this was also unwisely true for SE Australia with their ‘green’ anti brush burning initiatives until a few months ago. No different that last years California wildfires proximally caused by lack of proper forestry.
Carbon “unsink” rates.
Given carbon sink estimates that most of the about half of the present global carbon sink is into marine carbonates, this would be a very long term Earth life problem. Even if humans eventually consumed all fossil fuels, the resulting CO2 will eventually be converted by ocean phytoplankton back into carbonates. And then the plants starved of it would die, and so would almost all life on Earth (except for black smoker deep marine life, feeding on Bacteria and Archaea metabolizing hydrothermal vent H2S via chemosynthesis).
If atmospheric C02 dips below about 150- 50 ppm (depends on plant and environment), plants fail utterly at the bottom of the global food chain and most life eventually expires. Greenhouse plants grown under otherwise ideal conditions except at at different CO2 concentrations graphically demonstrate this.
Interesting Digression: earlier evolved C3 photosynthesis is less evolved to low CO2 than the rarer (~15% of plant species) and much newer C4 photosynthetic pathway that first evolved about 30 mya, first in in dry region grasses as preindustrial CO2 levels became ‘dangerously’ low for those environmental conditions. C4 photosynthesis is both more CO2 AND water efficient, because leaf stomata do not have to open as much or for as long since the plant needs less CO2. The latter drove evolution of the former. C4 is also an interesting example of convergent evolution, because so far as now known it evolved over 45 separate times in 19 different families of angiosperms.
So, even without considering sequestration rate changes (ignoring the CAGW existential immediate doom stuff), we can be sure that biological sequestration will eventually solve the “AGW” problem—and kill most life on Earth. From that perspective, AGW is good—frees CO2 plant food while maybe also staving off the next ice age.
Plate tectonics, not humans, is the ultimate great recycler of sequestered CO2 since most is sequestered as limestone, not as fossil fuel. Subduction zones (like the Pacific Ring of Fire) take seafloor carbonates, decompose them, and spew the resultant freed CO2 back into the atmosphere along with a lot of other stuff. The Pinatubo eruption on the Pacific Ring of Fire is an explosive example. St. Helens in the US is another spectacular recent tectonic recycling event.
In fact, without this plate tectonic carbonate recycling mechanism it is estimated that most life on Earth would cease in about 2.3 million years, no matter CAGW.
As a plus to this basic fact, there is a fascinating theoretical debate presently going on among astronomers searching for exoplanets, and then those orbiting in ‘habitable’ zones around their stars (assuming liquid water is needed for life forms like on Earth, perhaps itself a dubious assumption). Do such liquid water zone exoplanets also need plate tectonics to be ‘habitable’?
Dunno. But am sure the present already incredible exoplanet astronomy techniques cannot say whether they might have plate tectonics. Any more than IPCC climate models can say what might happen to carbon sinks here on Earth the rest of this century.