If you're already getting nightmares from the picture scientists are painting of the future blighted by heatwaves and rising seas, now might be the time to look away. The latest scenario is unlikely, but involves a worst case that is even worse than we imagined. If carbon emissions are allowed to run unchecked there is a possibility that the planet's main source of oxygen could grind to a halt, suffocating even those animals – humans included – high enough not to be drowned by rising seas.
"Global warming has been a focus of attention of science and politics for about two decades now," said Professor Sergei Petrovskii of the University of Leicester in a statement. "A lot has been said about its expected disastrous consequences; perhaps the most notorious is the global flooding that may result from melting of Antarctic ice if the warming exceeds a few degrees compared to the pre-industrial level. However, it now appears that this is probably not the biggest danger that the warming can cause to the humanity [sic]."
He continued: "About two-thirds of the planet's total atmospheric oxygen is produced by ocean phytoplankton – and therefore cessation would result in the depletion of atmospheric oxygen on a global scale. This would likely result in the mass mortality of animals and humans."
Petrovskii modeled what would happen if ocean temperatures increase too rapidly for plankton communities to adapt. The model indicated that under extreme conditions oceanic ecosystems can undergo dramatic shifts, leading to massive die-offs and depletion of oxygen from the upper ocean, and eventually the atmosphere. This could occur if phytoplankton levels got too low, or unsustainably high, either of which could occur with rising temperatures.
"The rate of oxygen production depends on water temperature and hence can be affected by the global warming," Petrovskii wrote in the Bulletin of Mathematical Biology.
The modeling is preliminary, with the paper noting that it "leaves many features of real marine ecosystems out of the scope." The effects did not arise until temperatures exceeded pre-industrial levels by 5 to 6 °C (9 to 10 °F). This is just beyond the upper end of predictions for 2100, if we make no serious efforts to cut back on the greenhouse gasses we emit, and well beyond what is likely if the most polluting technologies are phased out. Moreover, any scenario that brought Earth close to such a large increase in such a short time would have such devastating consequences for society that emissions would likely slow.
"A decrease in the oxygen concentration below a certain level can result in hypoxia and mass mortality of marine fauna because of asphyxiation," the paper states. "Moreover, this is an issue of crucial importance... for terrestrial ecosystems on the global scale."
The fact that such a scenario is even faintly possible indicates the extent to which we are messing with the planet's life support systems through unchecked emissions.