Global Carbon Budget
When the growth of the continents began (about 3 billion years ago)
The picture above shows how atmospheric carbon dioxide had decreased to 0.003 (atm) by about 600 million years ago. Still, it is about ten times as much as it is today.
In the Phanerozoic (540 million years ago), plants began to flourish on land and some carbon was sequestered underground as fossil (coal and oil). This led to atmospheric carbon dioxide levels approaching those of today. (However, considering fossil (coal and oil) as the main reason for the decrease in atmospheric CO2 assumes that the amount of carbon held by the oceans is the same today as it was in the early Phanerozoic.)
The tiny fraction of carbon left on the Earth's surface is now distributed as mobile carbon to the atmosphere, oceans, and terrestrial vegetation. Climate change over past time scales of hundreds of millions to tens of millions of years is determined by the balance between the amount of carbon supplied to the surface from carbonate minerals in the crust (e.g., volcanism and weathering) and the amount removed from the ocean as carbonate particles and fixed in the crust. In the present ocean, carbonate particles are mostly thought to be formed by biological action.
(The data in the above figure is quoted from IPCC report chapter 3, figure 3.1. The amount of carbonate minerals in the crust is given as 107-108 Pg C, based on the values in Table 2 of KITANO, Geochemistry, (1997). Note that there is about 108 PgC in the upper mantle. This course contains ambiguous statements because the information is not fully organized. Since the carbon in the mantle is probably not carbonate rock, it is likely that there are remnants of carbonaceous chondrites in the mantle that were supplied as meteorites at the beginning of the Earth's life. Carbonaceous chondrites accumulated and brought carbon to Earth.)
Before humans were born, the release of carbon dioxide from carbonate rock weathering and volcanoes was balanced by the deposition and burial of calcium carbonate particles by marine organisms. However, as shown in the figure above, humans have now greatly upset that balance. In recent years, anthropogenic carbon dioxide emissions have been highlighted as a cause of global warming. If all the carbon dioxide emitted by humans over the past hundred years had accumulated in the atmosphere, atmospheric carbon dioxide would have doubled, to the order of 1000 ppm. Fortunately, it is still stuck at 380 - 400 ppm. While anthropogenic carbon dioxide emissions are relatively easy to estimate from the amount of fossil fuels in circulation, it is not easy to know where the released carbon dioxide is distributed on the Earth's surface. Since the oceans are the largest reservoir for temporary storage of carbon, we can imagine that the oceans are absorbing anthropogenic carbon dioxide. From an oceanographic standpoint, the question must be answered: how much carbon dioxide is sequestered in the oceans?
To face that question, you must understand how seawater absorbs carbon dioxide and be able to calculate it. That is what you will learn.
References: "The 4.6 Billion Year History of Earth's Environmental Changes, TAJIKA Eiichi, Kagaku doujin, 2009", "Evolution of the Earth's Surface Environment: From the Precambrian Era to the Near Future, KAWABATA Hodaka, University of Tokyo Press, 2011", "Encyclopedia of the Global Environment, YOSHIZAKI Masanori, edited by NODA Akira et al, Asakura Shoten, 2013", "Study on Carbonate Materials in the Earth's Environment, KITANO, Geochemistry, 31, 211-226, 1997" etc.
Book Introduction
"The 4.6 Billion Year History of Earth's Environmental Changes, TAJIKA Eiichi, Kagaku doujin, 2009"
The book is an easy-to-read history of the Earth's evolution. The oceans have played a decisive role in the evolution of the Earth. It is important to learn from the past in order to understand the present-day oceans, the future oceans, the global environment, and what global warming is all about.