What's more, the ocean surface is warming, driven by the emission of carbon dioxide and other greenhouse gases. This keeps the deeper waters, which are rich in nutrients but low in oxygen, from mixing with the oxygenated surface. According to a 2007 report from the International Panel on Climate Change (IPCC), global surface temperatures increased by 1.1 degrees Fahrenheit (0.6 degrees Celsius) throughout the 21st century, and, according to the National Oceanic and Atmospheric Administration (NOAA), this decade is the warmest since record-keeping began in 1880.
At the time of the third of the Big Five extinctions, the Permian-Triassic, there was only one massive continent and one massive ocean, conditions that disrupted ocean circulation and inhibited oxygen circulation in an already warm world, according to Lee Kump, a geoscientist at Pennsylvania State University. That set the stage for the ultimate trigger, a series of massive volcanic eruptions in Siberia.
The eruptions pumped massive amounts of carbon dioxide into the atmosphere. This warmed the ocean further, exacerbating its oxygen problem. Meanwhile, more storms on land washed more oxygen-eating nutrients into the ocean. Bacteria began producing hydrogen sulfide, which was ultimately expelled into an atmosphere already toxic with carbon dioxide, according to Kump.
A comparison of carbon dioxide release then versus now is telling, Kump said. Siberian volcanoes emitted tens of thousands of gigatons of carbon dioxide into the air over what was probably thousands of years. Humans currently are producing 9 gigatons per year from fossil fuel reservoirs that contain up to 4,000 gigatons.