56 million years ago, Earth underwent the greatest rise in temperature in the last 100 million years. In just 20 to 50 millennia, the global temperature had risen by five to eight degrees Celsius (nine to 14.4 degrees Fahrenheit), and this spike would plateau for 200,000 years. This still-mysterious phenomenon, known as the “Paleocene-Eocene Thermal Maximum”, or “PETM” for short, was a double-edged sword in evolution. Whereas the global jungles spiked the diversity of certain modern clades of mammals, including the primates, this same heat wave warmed up the oceans, weakening their abilities to hold oxygen and rendering them vulnerable to ocean acidification. Deep-sea variations of single-celled organisms called foraminiferans suffered a species loss no greater than half. Coral reefs were cut down, as the greater acidities of the water stunted the production of calcium carbonate. Dinoflagellate cells ballooned, resulting in more common occurrences of harmful algal blooms. Ocean currents took opposite directions, transporting warm water to the depths, reducing the overall temperature gradient and thus worsening the issue. It was so hot that sea levels rose back to mid-Mesozoic levels. The lysocline, the point in which carbonates can dissolve, had risen to a shallow depth of two kilometers, or almost a mile and a quarter. In short, the Paleocene-Eocene Thermal Maximum was both a godsend for life on land and absolute hell for life in the oceans.
On this alternate Earth, the PETM lasted a lot longer than back home. Three to four times longer, as a matter of fact. Another difference is that this was the point in which the entire global supply of methane hydrates, or frozen natural gas—all two trillion metric tons of it—were released into the atmosphere, turning the hothouse into an even hotter house. This didn’t happen all at once, as there wasn’t any evidence of a terrestrial mass extinction. What we may be looking at instead was a gentle upward slope spanning the entirety of the PETM—600 to 950 millennia. Sea levels rose to the extent that the amount of land relative to the rest of the planet’s surface to between 12 and 17 percent. As higher temperatures hold more water vapor than lower temperatures, such a spike would mean that so much moisture would be retained that constant rainfalls would destroy mountains. Which ranges had survived such outcomes and which had suffered big time would be explored in due time. It was so hot that the lysocline had climbed up to an even shallower depth of one kilometer, almost two-thirds of a mile. A band of water spanning 15 degrees from both sides of the equator would have been an impenetrable barrier of sickly purple water, inhabited only by anti-oxygen purple bacteria. Beyond that, the temperate seas were mottled with large masses of marine heatwaves, whereas polar seas had smaller but more numerous heatwaves. Between each heatwave, the waters would suffer repeated short-term episodes of harmful algal blooms, which would rob the waters of their precious oxygen. Life on land was about to get harder, but life in the oceans would suffer another major mass extinction—the absolute worst since the Great Dying, and right in the middle of the process of recovering from the fall of the dinosaur empire one million decades prior! 99% of all marine species were gone. Which marine organisms would make up the surviving one percent?