The ocean and climate change : relations with marine chemistry and biology (1)

As measurements at Hawaii's Mauna Loa observatory show, the partial pressure of atmospheric_CO2 has risen sharply since 315 ppm in the late 1950s, and today stands at 400 ppm (parts per million by volume). Apart from a small seasonal variation ( 10 ppm), the continuous evolution of_CO2 corresponds to an annual increase of around 1 to 2 ppm. Taking into account equivalent measurements from other stations, this increase can be directly converted into billions of tonnes (GtC) of carbon per year (or petagram:¹⁰¹⁵g PgC). At present, the evolution of the atmospheric stock is around 4 to 5 GtC/year. This increase can be directly compared with various anthropogenic emissions, notably fossil fuel combustion and deforestation. Current emissions are estimated at around 10 GtC/year. If we consider emissions balances since the beginning of the industrial era, we can also estimate a cumulative total of around 350 GtC injected into the atmosphere in the form of_CO2 (or ≈ 530 GtC if we include deforestation). A direct comparison between_CO2 emissions and the evolution of the atmospheric stock shows that around half of the_CO2 emitted is no longer in the atmosphere. This first-order observation is true both for the current increase (10 vs. 5 GtC/year) and for the cumulative evolution since the pre-industrial level of 280 ppm measured in the bubbles of Antarctic ice cores. In fact, excess carbon dioxide molecules have diffused to other reservoirs in the carbon cycle, notably the terrestrial biosphere and soils, as well as the ocean, which is the subject of this lecture.

The ocean is an enormous carbon reservoir, containing around 60 times more carbon than the atmosphere. Annual gross fluxes between the atmosphere and the ocean are an order of magnitude greater than anthropogenic disturbance. Nevertheless, these gross fluxes led to an equilibrium that maintained carbon stocks at a steady state over the course of centuries and millennia. Despite the relative smallness of the anthropogenic carbon flux, it is possible to track its penetration into the ocean and quantify the annual storage to verify the balance established by calculating the difference between emissions and the atmospheric stock.

The first proof of the injection of anthropogenic_CO2 into the atmosphere comes from measuring the carbon-14 content of_CO2. Since fossil fuels no longer contain ^14C, these measurements enable us to study the mixing of anthropogenic carbon with natural carbon. In a way, ^14Cprovides ^uswith a negative photo of_CO2 contamination. In fact, this is how the invasion of fossil carbon was first detected in the 1950s by Hans Suess, who measured the ^14Ccontent in tree rings going back beyond the ^19th century. This ^14Cdetection preceded that of the increase in atmospheric_CO2 by Dave Keeling at the Mauna Loa observatory. The data show a gradual decrease of around - 25 ‰ in the ^14C^/12Cratio of the atmosphere until the mid-1950s. From that date onwards, the atmosphere was suddenly enriched by ^14Ccreatedby aerial testing of thermonuclear bombs. Just over a ton of ^{artificial 14C}was injected into the stratosphere. After homogenization in the troposphere, we were able to follow the decrease in the ^14C^/12Cratio due to the diffusion of_CO2 molecules marked by ^{14Cthermonuclear}and by the dilution due to the combustion of fossil carbons, devoid of ^14C.

A first method for calculating the amount of_CO2 entering the ocean each year is to measure the net flux of_CO2 at the air-sea interface. This flux depends on the imbalance between the atmospheric partial pressure of_CO2 and its fugacity at the ocean surface. The flux depends not only on this partial pressure gradient, but also on the solubility of_CO2 and a gas transfer velocity coefficient, itself a function of sea surface conditions linked to wind speed. _PCO2 analyses are carried out in the laboratory (IR absorption, gas chromatography) or in situ using automated colorimetric sensors. Over the past 40 years, some three million measurements of surface ocean _pCO2 have made it possible to map local seasonal fluxes throughout the world ocean. The map of_CO2 fluxes shows very marked gradients, with sink zones at mid-latitudes (i.e. positive net fluxes : of_CO2 from the atmosphere to the ocean) and source zones at low and high latitudes (i.e. negative net fluxes: from the ocean to the atmosphere). These differences in behavior are due to ocean dynamics and the natural distribution of carbon in the ocean. To estimate a global flux, it is crucial to take into account the seasonality of temperatures and winds, which control solubility and the exchange coefficient. Using this method of local exchange fluxes, it is possible to estimate an average annual flux of around 2 ± 1 GtC/year for the year 2000, when fossil carbon emissions were 7 GtC/year (and 8 GtC/year including deforestation). The ocean therefore pumps out an average of 30 % of_CO2 emissions from fossil fuels.