In this chapter, we focus on the land sink of anthropogenic CO2, because humans have a history of using the terrestrial biosphere for our purpose and because efforts to control atmospheric CO2 levels involve deliberate manipulation of the biosphere. We present atmospheric evidence for the land sink and use information about its interannual variations to infer its stability.
The Mauna Loa CO2 record is a clear documentation of the increasing concentration of CO2 in the atmosphere as a result of anthropogenic activities. By 1999, the atmospheric CO2 abundance had increased by 25% since the beginning of the preindustrial era. The cumulative increase, together with the concomitant increase in CH4, N2O, CFCs, and other greenhouse gases, presents a total radiative forcing of ∼2–3 W/m2 to the climate system in the 1990s. This forcing is countered to some degree by the increase in sulphate and other aerosols in the atmosphere.
The decreasing 14C/12C ratio in tree rings (Suess, 1955) proves that the atmospheric CO2 increase is due to the addition of fossil (14C-free) carbon. However, the CO2 increase rate, as determined from the atmospheric record, is only 50%–60% that emitted by fossil fuel combustion (Figure 3.1). Thus, the land and oceans have absorbed the remainder of the fossil fuel CO2 as well as the CO2 released due to land use modification.
The gradients in partial pressure of CO2 across the air–sea interface provide a starting point for estimating regional and global CO2 fluxes between the atmosphere and ocean. They also are critical constraints on global atmospheric and oceanic models used to infer the land–sea partitioning of CO2 uptake. Here, we assess the factors that contribute to uncertainties in the estimated CO2 fluxes.
We estimate measurement precision in pCO2 to be ±2 μatm, and extrapolation of the data to regions with no measurements yields uncertainties of ±0.8 μatm. The short duration of spring blooms in the North Atlantic diminishes the uncertainties arising from sparse seasonal coverage in the measurements. We estimate an oceanic uptake of 0.3 Gt C/yr due to spring blooms in the North Atlantic. It is difficult to quantify the extent to which pCO2 gradients may change by correcting the pCO2 measurements to skin instead of bulk temperatures, as skin–bulk temperature differences may be positive (negative) with strong surface heating (cooling), or may vanish under high wind conditions. Uncertainties in fluxes associated with gas exchange rates cannot be separated from the yet unknown flux contributions from the covariance between high-frequency wind and pCO2 fluctuations.
In addition to expanded spatial coverage, high-resolution and high-frequency sampling of the meteorology, hydrography, and carbon system in the atmospheric and oceanic boundary layers at a few locations is needed for improving estimates of air-sea CO2 fluxes.
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