# Phytoplankton as CO\({}_{2}\) Sinks: Redirecting the Carbon Cycle
## 2 Photosynthesis and Global Balances of Matter and Energy
### Carbon Cycle and Civilization  
Before the industrial revolution, the biological carbon cycle maintained its flows almost in equilibrium, and the CO\({}_{2}\) concentration in the atmosphere stayed stable between 260 and 285 ppm for almost 12,000 years [8]. In this cycle, approximately \(1.1\times 10^{6}\) TWh of sunlight is annually trapped, together with 115 Gt of carbon (GtC) in the form of atmospheric CO\({}_{2}\), as Net Primary Production (NPP) to synthesise approximately 210 Gt of organic matter [9; 10; 11], of which land biomes capture approximately 67 GtC (58%), and oceans sink 48 GtC (42%). Despite this enormous flow of matter and energy, 95% of this biomass is finally decomposed across the different trophic chains, releasing the stored energy, powering the life (macro and microscopic), and finally returning to the atmosphere as CO\({}_{2}\)[12]. These numbers imply that the planet's surface, which is covered by primary producers, captures approximately 0.07% of incident radiation as organic carbon, agreeing with the numbers proposed by [PERSON] [13]. Since the industrial revolution, mainly since 1960, an additional and artificial source of CO\({}_{2}\) has rapidly overloaded the atmosphere with this gas, unbalancing the above cycle (Figure 1).  
In absolute terms, the total CO\({}_{2}\) emitted to the atmosphere since 1960 is estimated at 2570 Gt (695 GtC), of which 65% comes from the growing fossil fuel use and the remaining 35% comes from land use (agricultural, residential, industrial, etc.). In this way, the atmosphere has gone from an almost stable load of 2154 Gt of CO\({}_{2}\) before industrialization to 3280 Gt (+52%), which means 304 GtC of additional carbon. Note the sharp difference between the total carbon emitted (695 GtC) and the carbon accumulated (304 GtC). Different natural and artificial sink mechanisms could explain this mismatch. The increase in atmospheric CO\({}_{2}\) results in a proportional increase in its partial pressure over ocean surfaces. This causes a shift in the chemical equilibrium toward a higher solubility rate of CO\({}_{2}\) in water. This general trend can be intermittently altered by many other climatic phenomena, such as El Nino (2018). However, a growing rate of CO\({}_{2}\) uptake by the oceans has been quantified from almost zero in 1750 to 1 GtC in 1960 until the current 2 GtC/year (2018; 2018). As absolute figures, since 1750, oceans have captured 168 GtC as dissolved inorganic carbon (DIC), becoming part of their CO\({}_{2}\)/HCO\({}_{3}\)\({}^{-}\) equilibrium.  
Consequently, marine waters are acidifying progressively and are expected to double in acidity before 2100 (2018). In this way, the effects of CO\({}_{2}\) gas accumulation become evident not only through excessive greenhouse effect and, consequently, a rise in global temperature but also through a decrease in the pH of water bodies. The uncontrolled and explosive variation of both parameters could result in catastrophic consequences for any ecosystem, such as coral reefs. Otherwise, the net land uptake of CO\({}_{2}\) has gradually increased from almost zero in the 1980s to the current 4 GtC/year. This fact can be explained by an increase in Gross Primary Photosynthesis (GPP) caused by the eutrophication of soils and atmosphere and the expansion of agricultural land. It is estimated that since 1750, almost 870 Gt of CO\({}_{2}\) (235 GtC) derived from human emissions has been locked as land carbon (vegetation and soils). In summary, the total emitted carbon by human activities since the beginning of the Industrial Revolution (695 GtC) is distributed among land (34%), oceans  
Figure 1: Adapted scheme of the fast carbon cycle and related energy flows (2018; 2018). Primary production (green lines), respiration and combustion (blue lines), carbon and fluxes derived from human activities since 1750 (red lines), energy flows and amounts (yellow lines and text).  
(24%), and the atmosphere (42%), meaning that global mechanisms of the planet have managed to lock 58% of these emissions [10; 18].