As they neared publication, they felt a sense of unease. Their research showed two very different outcomes for ocean life in the next three centuries depending on how greenhouse gas emissions were managed. The title of the study seemed to only focus on the negative aspects, so they decided to add a word to emphasize that the bleak future could still be prevented. The latest research article titled "Avoiding Ocean Mass Extinction From Climate Warming" was published in Science on Thursday. This study highlights the critical moment that humanity is currently facing, where decisions made about greenhouse gas emissions will have a significant impact on the future of life on Earth. Even though the worst consequences may seem distant, under the high emissions scenario modelled by scientists, continued pollution from fossil fuels could lead to a level of ocean species loss by 2300 comparable to the five mass extinctions in Earth's history, including the event that caused the extinction of dinosaurs. "It wasn't exactly a moment of sudden realization," explained Penn, a postdoctoral researcher at Princeton, when reflecting on the first time he saw a graph comparing past extinctions with their grim predictions. "It was more of a moment of shock." On the contrary, scientists discovered that reducing emissions to align with the upper limit of the Paris climate agreement would decrease ocean extinction risks by over 70%. In this scenario, climate change would result in the loss of around 4% of species by the end of the century, when warming would cease. "Our decisions have significant consequences," stated Deutsch. Despite the general agreement that transitioning from coal to increased use of wind and solar energy would make the worst-case scenario less likely, the use of oil and gas continues to rise, and the world is not on course to achieve the lower-emissions scenario outlined by the scientists. The recent study expands on the previous research by Deutsch and Penn, which involved developing a computer simulation to analyze the most severe extinction event on Earth approximately 252 million years ago. Known as "the Great Dying," this event resulted in the extinction of over 90% of ocean species. The primary cause was global warming, brought on by volcanic eruptions. As a result, the oceans experienced a drop in oxygen levels, leading to the demise of fish due to heat stress, lack of oxygen, or a combination of both. The computer model revealed a higher number 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 such as sunlight, clouds, ocean and air currents, and chemical interactions among heat, oxygen, water, and air. They also assessed potential shifts in fish habitats and estimated survivability thresholds. The study primarily focused on the effects of warming and oxygen depletion, but warned that ocean acidification and other cascading effects could exacerbate species loss as predicted. The ocean has historically served as a buffer against 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 resulted in negative consequences. The ocean experienced its highest temperature and lowest oxygen levels in recorded history last year. Alterations in the ocean's chemistry are putting fish at risk and causing a sharp decline in coral reefs. Deutsch frequently hears concerns about the severity of the situation. "If we fail to act, we are in serious trouble." Countries are still not taking the necessary actions to prevent disastrous climate change. The UN Secretary-General recently warned that the goal of limiting global warming to 1.5 degrees Celsius is in jeopardy. The International Energy Agency has called for an immediate halt to new fossil fuel projects, but countries have not complied. The recent conflict in Ukraine has led to calls for increased drilling for energy security. Some scientists feel like the ignored experts in the movie "Don't Look Up," where a comet symbolizes climate change. Just like in the film, the world is facing a critical moment where current actions will shape the future. Deutsch emphasized the importance of understanding the responsibility that comes with great power, particularly in terms of preserving the planet for future generations and all other forms of life. Pippa Moore, a marine science professor, praised a study for its thoroughness in highlighting the impact of climate change on marine ecosystems. She warned that without significant action to reduce greenhouse gas emissions, marine species could face major shifts in their habitats and potential extinction events similar to past mass extinctions. The reporting was contributed by Brad Plumer.