Marine extremes are sudden, intense deviations from the average physical or chemical conditions of the ocean that persist for days to months, creating hostile environments for marine life. These events are defined not by their absolute value, but by how far they exceed the typical range of historical variability for a given location and time of year. Since the ocean has absorbed over 90% of the excess heat from human-caused greenhouse gas emissions, baseline conditions are steadily changing. This makes it easier for temporary fluctuations to become extreme events. Understanding these disturbances is now a central focus in marine science, as they represent a significant threat to the health and stability of the world’s ocean ecosystems.
Categorizing Marine Extremes
Marine extremes manifest in three primary forms, each representing a sharp departure from normal ocean chemistry or temperature. The most recognized are Marine Heatwaves (MHWs), periods where sea surface temperatures exceed the 90th percentile of historical readings for at least five consecutive days. These temperature spikes can last for weeks or months and often extend deep into the water column, affecting both shallow and deep-sea ecosystems.
Another category is extreme Ocean Acidification Events, which involve a rapid and localized drop in the seawater’s pH level. This occurs because the ocean absorbs atmospheric carbon dioxide, which increases acidity. The ocean’s average pH has dropped by approximately 0.1 since the 1950s, representing a 26% increase in acidity.
The third major type is Ocean Deoxygenation, the severe reduction in dissolved oxygen concentration in the water column. This loss is driven by two factors: warmer water holds less dissolved gas, and increased stratification prevents oxygen-rich surface water from mixing with deeper layers. In coastal areas, nutrient runoff exacerbates this issue by fueling algal blooms whose decay consumes vast amounts of oxygen, creating hypoxic zones, sometimes called “dead zones.”
Direct Physiological Impacts on Ocean Life
Acute changes in ocean conditions immediately stress organisms, forcing them to expend energy simply to survive. For species exposed to Marine Heatwaves, the primary impact is thermal stress, which pushes organisms past their physiological limit. Corals, for instance, respond to sustained high temperatures by ejecting symbiotic algae, a process known as bleaching that leaves them vulnerable to starvation and disease.
Invertebrates that cannot escape the heat, such as oysters and mussels, often suffer mass mortality events. Mobile organisms also experience severe metabolic disruption, as higher temperatures increase their oxygen demand while deoxygenation simultaneously reduces the available supply. This limits their growth, impedes reproductive success, and alters fundamental behaviors like feeding and migration.
Extreme Ocean Acidification directly threatens calcifying organisms by interfering with their ability to build and maintain calcium carbonate shells and skeletons. Species like pteropods and the larvae of oysters exhibit reduced calcification and can develop thinner or malformed shells. To cope with increased acidity, some organisms must divert energy to maintain their internal acid-base balance, energy they can no longer use for growth or reproduction. In fish, exposure to acidified waters can impair neurological functions, reducing the ability of larvae to detect and avoid predators.
Large-Scale Ecosystem Disruptions
When individual physiological stress becomes widespread, the consequences cascade, leading to systemic disruptions that fundamentally alter entire marine ecosystems. The loss of foundation species due to Marine Heatwaves results in widespread habitat collapse. This has been observed in the rapid die-off of kelp forests and seagrass meadows, which serve as essential nurseries and shelter for countless species.
The destruction of these habitats forces large-scale shifts in species distribution, as organisms attempt to relocate to find more suitable conditions. During the major heatwave known as “The Blob” (2013–2016) in the Northeast Pacific, researchers documented hundreds of species moving beyond their historical geographic ranges, with some traveling over 1,000 kilometers north. This forced migration introduces species into new areas, disrupting established local food webs and creating novel competitive pressures.
These events also trigger destructive trophic cascades, where the loss of one species affects multiple levels of the food chain. The decline of cold-water forage fish or krill during MHWs can starve higher predators like seabirds and marine mammals. Reduced survival of calcifying zooplankton due to acidification removes a primary food source at the base of the food web, impacting larger predators such as whales. The systemic outcome is a less diverse and less resilient ecosystem structure.
The Increasing Frequency of Extreme Events
The urgency of addressing marine extremes is underscored by the evidence of their rising frequency and intensity. Since 1982, the global frequency of marine heatwaves has approximately doubled, meaning events once considered rare are now commonplace. This trend is a direct consequence of the ocean absorbing vast amounts of heat, which steadily raises the baseline ocean temperature.
This elevated baseline means that normal, short-term weather fluctuations are now far more likely to cross the high-temperature threshold that defines an extreme event. Globally, the number of days per year the ocean experiences extreme heat has nearly tripled, soaring from about 15 days in the 1940s to approximately 50 days today. Human-caused warming is responsible for nearly half of the marine heatwaves observed between 2000 and 2020. This shift is moving the ocean into a “new normal” where extreme events become recurrent pressures, pushing ecosystems past their ability to recover.