The Suess Effect Revisited: It's not what you may think

Abstract:

The Suess effect is commonly characterized as a dilution, or even a decrease, of atmospheric ¹⁴CO₂ due to emission of ¹⁴C-free CO₂ from fossil fuel combustion over the period of industrialization prior to nuclear weapons testing in the 1950's and 1960's (pre-bomb Anthropocene). The Suess effect served as an important early demonstration of the increase of atmospheric CO₂ by anthropogenic emissions prior to measurements of CO₂ in the atmosphere (Keeling) and glacial ice cores (Etheridge). For reasons of measurement convenience and historical precedent the atmospheric abundance of ¹⁴CO₂ is generally quantified as the departure from a standard of the fractionation-corrected ratio of ¹⁴CO₂ to total CO₂, Δ¹⁴CO₂. From dendrochronologically dated tree rings, Suess, and subsequent investigators (notably Stuiver), established systematic decrease of atmospheric Δ¹⁴CO₂ over the pre-bomb Anthropocene, Figure 1a, although less than expected from dilution. However, over this period the amount of ¹⁴CO₂ in the atmosphere actually increased, Figure 1b, as did that of total CO₂, the relative increase ¹⁴CO₂ being slightly less than that of total CO₂, with the difference manifested as the Suess effect in the isotopic ratio. Here the increase in atmospheric ¹⁴CO₂ over this period is shown to be due mainly to decreased solubility of CO₂ in ocean water, caused by the increase in atmospheric CO₂ with resultant increase in oceanic acidity, leading to outgassing of ¹⁴CO₂ from the world ocean, and to lesser extent, to introduction of ¹⁴CO₂ into the atmosphere from emission of modern CO₂ associated with land-use change. While recognition that atmospheric ¹⁴CO₂ actually increased over the pre-bomb Anthropocene does not change the interpretation of the Suess effect as a demonstration of anthropogenic CO₂ emissions, the notion of ¹⁴CO₂ as having been passively diluted or as having decreased over the pre-bomb Anthropocene should be put to rest. The decrease in ocean solubility of ¹⁴CO₂ due to increasing total CO₂ has important implications for understanding the course of atmospheric ¹⁴CO₂ as the burden of bomb ¹⁴C distributes among the several compartments of the biogeosphere.

Figure 1. a, Δ¹⁴CO₂, and b, ¹⁴CO₂ amount and CO₂ mixing ratio over the pre-bomb Anthropocene. Ordinates (a, right; b, both) are logarithmic with identical scales.