As they neared publication, they had a lingering concern. Their research showed two very different outcomes for ocean life in the next three centuries depending on greenhouse gas emissions. The study's title seemed to only focus on the negative aspects, so they added a word to emphasize that the bleak future they described could still be prevented. On Thursday, an article titled "Avoiding Ocean Mass Extinction From Climate Warming" was published in Science. This new research highlights the critical moment that humanity is currently facing, where decisions about greenhouse gas emissions will have a significant impact on the future of life on Earth. Despite the fact that the worst consequences may seem distant, if pollution from fossil fuels continues to increase as projected in the high emissions scenario, warming could lead to a level of ocean species loss by 2300 comparable to the five mass extinctions that have occurred in Earth's history, including the one that caused the extinction of the dinosaurs. Penn, a postdoctoral researcher at Princeton, described the moment he first saw a graph comparing past extinctions to future forecasts as more of an "oh, my God" moment than a sudden realization. Scientists have found that reducing emissions to comply with the upper limit of the Paris climate agreement could decrease ocean extinction risks by over 70%. If this goal is achieved, climate change would result in a 4% loss of species by the end of the century. According to Deutsch, our choices have significant consequences, and while transitioning from coal to wind and solar energy could prevent the worst-case scenario, the continued use of oil and gas is causing the world to fall behind in meeting lower-emission targets. The recent research expands upon the previous work of Deutsch and Penn, who developed a computer simulation outlining the most severe extinction event in Earth's history around 252 million years ago. Referred to as "the Great Dying," this event resulted in the loss of over 90% of species in the oceans. The primary cause was global warming, brought about by volcanic eruptions, leading to a decrease in oxygen levels in the oceans and causing fish to perish from heat stress, lack of oxygen, or both. The computer model showed a higher rate of extinctions at the poles compared to the tropics, a finding that was supported by the fossil record. The scientists used a complex model to predict the impacts of global warming caused by human activity, considering factors like sunlight, clouds, ocean and air currents, and chemical reactions between heat, oxygen, water, and air. They also studied how fish habitats might shift and estimated survivability thresholds. The study focused on warming and oxygen loss, but warned that ocean acidification and other effects could exacerbate species loss. The ocean has historically helped mitigate climate change by absorbing large amounts of carbon dioxide and trapped heat from human activities like burning fossil fuels and deforestation. The provision of that service has had consequences. The ocean experienced its highest temperature and lowest oxygen levels last year since humans began recording data. The changes in the ocean's chemistry are currently endangering fish, and coral reefs are deteriorating rapidly. Deutsch frequently hears questions about how much trouble we are in. "If we fail to take action, we are in serious trouble." Countries have not yet taken the necessary measures to prevent devastating climate change. UN Secretary-General António Guterres recently warned that the goal of limiting global warming to 1.5 degrees Celsius above preindustrial levels is in jeopardy. The International Energy Agency has urged countries to halt new fossil fuel projects, but this has not happened. The situation has been further complicated by calls for increased drilling following Russia's invasion of Ukraine. Scientists feel like the ignored protagonists in the movie "Don't Look Up," where a comet represents the urgent threat of climate change. Just like in the film, the world is at a critical juncture, with current generations having a significant impact on the future. Deutsch stated that with great power comes great responsibility, emphasizing the need to consider future generations and all other life on the planet. Pippa Moore, a marine science professor at Newcastle University, praised the study for its thoroughness and warned that without action to reduce greenhouse gas emissions, marine ecosystems could face significant changes and extinction events.Brad Plumer contributed to the reporting.