Impacts of Changes in Atmospheric O2 on Human Physiology. Is There a Basis for Concern?

Abstract

Concern is often voiced over the ongoing loss of atmospheric O₂. This loss, which is caused by fossil-fuel burning but also influenced by other processes, is likely to continue at least for the next few centuries. We argue that this loss is quite well understood, and the eventual decrease is bounded by the fossil-fuel resource base. Because the atmospheric O₂ reservoir is so large, the predicted relative drop in O₂ is very small even for extreme scenarios of future fossil-fuel usage which produce increases in atmospheric CO₂ sufficient to cause catastrophic climate changes. At sea level, the ultimate drop in oxygen partial pressure will be less than 2.5 mm Hg out of a baseline of 159 mmHg. The drop by year 2300 is likely to be between 0.5 and 1.3 mmHg. The implications for normal human health is negligible because respiratory O₂ consumption in healthy individuals is only weakly dependent on ambient partial pressure, especially at sea level. The impacts on top athlete performance, on disease, on reproduction, and on cognition, will also be very small. For people living at higher elevations, the implications of this loss will be even smaller, because of a counteracting increase in barometric pressure at higher elevations due to global warming.

Keywords: V.O 2max; atmospheric oxygen; evolution; fossil fuels; global change; high altitude; human health; hypoxia.

FIGURE 1

The global oxygen cycle, from Keeling (1988), showing short-term and long-term sources and sinks and coupling with the reservoirs of organic carbon in units of 10¹⁵ moles and 10¹⁵ moles year^{– 1}. Oxygen fluxes and reservoirs are denoted by solid lines and solid boxes. Organic fluxes reservoirs are denoted by gray boxes with dashed perimeter, and organic fluxes with dashed lines. Organic matter is expressed in terms of O₂ equivalent, i.e., the amount of O₂ consumed when the material is fully oxidized. Organic reservoirs other than surface biota and sedimentary rocks have been updated using recent estimates from Ciais et al. (2013) using O₂/C oxidative ratios of 1.1 (vegetation, soils, permafrost), 1.3 (dissolved organic carbon), and for fossil-fuel by fuel type from Keeling (1988). Fluxes and reservoirs other than fossil-fuel burning are notionally for a pre-industrial steady state. Fossil-fuel burning is for year 2019 (Friedlingstein et al., 2020).

FIGURE 2

Modeled changes in atmospheric O₂ from Shaffer et al. (2009) versus observed global averages from the Scripps O₂ program (Keeling and Manning, 2014). The O₂ levels are shown on left as fractional change in O₂ partial pressure relative to a preindustrial reference and on right as absolute partial pressure. The observations, originally reported as changes in O₂/N₂ ratio in per meg units (see Appendix A), were converted to dP′_{O_2}/${\text{P}}_{{\mathrm{O}}_2}^{\prime }$ and offset with an additive constant to align with the model results. Global averages are based on the data from Alert (82.5°N), La Jolla (32.9°N), and Cape Grim (40.7°S) stations, following Keeling et al. (1996).

FIGURE 3

Predicted changes in atmospheric O₂ partial pressure (${\text{P}}_{{\text{O}}_2}^{\prime }$), temperature, and CO₂ mole fraction from model simulations of Shaffer et al. (2009).

FIGURE 4

Predicted changes in O₂ partial pressure (${\text{P}}_{{\mathrm{O}}_2}^{\prime }$) at different elevations. The curves for sea level are from Shaffer et al. (2009) and repeated from Figure 2. The curves for other elevations were calculated by accounting both for global O₂ loss and for changes in barometric pressure with warming (Table 1), scaled by the modeled temperature changes from Shaffer et al. (2009).

FIGURE 5

Maximal O₂ consumption (V̇_{O_2max}, as % of the maximum value at P_O2 = 150 mmHg) measured at different ambient P_O2 conditions for isolated mitochondria in a saline suspension (open symbols) and humans on a bicycle ergometer (closed symbols). V̇_{O_2max} decreases at inspired P_O2 levels in exercising humans much greater than those necessary to decrease V̇_{O_2max} in isolated mitochondria, which can be explained by P_O2 around the mitochondria in exercising humans falling to less than 1 mmHg (see text). After Gnaiger (2001) and Pugh et al. (1964).

FIGURE 6

Oxygen cascade, showing how P_O2 decreases from the atmosphere to mitochondria along the physiological O₂ transport chain at sea level (red) and high altitude (blue). (A) person at rest, (B) person at maximum exercise.

FIGURE 7

O₂-hemoglobin equilibrium curve plotting the % hemoglobin saturation with O₂ (left) or O₂ concentration in blood (right) versus P_O2. The sigmoidal shape of the curve allows a similar change in O₂ concentration for a smaller change in P_O2 in hypoxia versus normoxia.