Article image

Marine protected areas are not immune to ocean heatwaves

Ocean temperatures are reaching a boiling point with marine heatwaves wreaking havoc across the Pacific. This phenomenon spells disaster for marine ecosystems, according to a recent study from the University of California, Santa Barbara (UCSB). 

The research, published in the journal Global Change Biology, demonstrates that despite the many social and ecological benefits provided by marine protected areas (MPAs), these parts of the ocean are not immune to the pressures of global warming.

Marine heatwaves

Across the world, marine heatwaves, characterized by extended periods of anomalously warm sea surface temperatures, are generating mass species mortality and displacement events, triggering economic downfalls, and causing extensive habitat loss. The researchers’ findings challenge the notion that areas of the ocean protected from human activities such as fishing are safe from the impacts of climate change.

Marine heatwaves are a burgeoning global issue, and unlike their terrestrial counterparts, these sweltering oceanic periods can persist for months or even years. This phenomenon has taken center stage in marine biology, as even the protective shields of MPAs prove ineffective against these heatwaves.

“MPAs in California and around the world have many benefits, such as increased fish abundance, biomass and diversity,” stated Joshua Smith, the study’s lead researcher. Smith is a former postdoctoral researcher at UCSB’s National Center for Ecological Analysis & Synthesis (NCEAS). He continued, “But they were never designed to buffer the impacts of climate change or marine heatwaves.”

Focus of the study

As part of a broader review of California’s MPA network, the UCSB team compiled data spanning several decades to assess the long-term effects of marine heatwaves on a variety of habitats in the Pacific Ocean. 

The goal of the research was to equip California’s policymakers and natural resource managers with actionable data on climate change’s impact on marine ecosystems.

Jenn Caselle co-led the team. She is a researcher with UCSB’s Marine Science Institute, and Kerry Nickols, a professor from California State University Northridge now affiliated with non-profit Ocean Visions.

The Blob

The analysis coincided with the largest marine heatwave ever recorded, which surged across the Pacific Ocean from 2014-2016. Unprecedented ocean warming – colloquially dubbed “The Blob” – coupled with a subsequent major El Niño event, resulted in a prolonged period of abnormally high sea temperatures. 

The heatwave spanned the entirety of the West Coast, causing significant alterations to food webs, the collapse of fisheries, and a shift in marine life populations, among other consequential disruptions.

As MPA managers worldwide grapple with an escalating number of climate shocks, they must question how protected areas can withstand these events. In light of the severe marine heatwave that lasted several years, the research group set out to determine how the ecological communities within California’s MPAs would fare.

Little to no resistance

The team analyzed over a decade’s worth of data from 13 MPAs, each located within diverse ecosystems along the Central Coast, to understand how the marine heatwave affected various species populations within these areas. Their study revealed that the MPAs provided little to no resistance to the marine heatwave’s effects.

“The MPAs did not facilitate resistance or recovery across habitats or across communities,” noted Caselle. “In the face of this unprecedented marine heatwave, communities did change dramatically in most habitats. But, with one exception, the changes occurred similarly both inside and outside the MPAs.”

“MPAs are effective in many of the ways they were designed, but our findings suggest that MPAs alone are not sufficient to buffer the effects of climate change,” said Smith. 

Ocean under siege

The results underscore the grim reality that the entire ocean is under siege from climate change. Post-heatwave, many ecological communities experienced a “pronounced decline in the relative proportion of cold-water species and an increase in warm water species.” 

An increase in subtropical species like the señorita fish, previously rare in central California, notably influenced the shift in marine communities. Whether these species will continue to thrive in their newfound habitats remains uncertain.

Continued monitoring of ocean heatwaves

Yet, MPAs retain value, even as they struggle under the weight of these climate-driven events. They are invaluable sites for ongoing research into the complex impacts of climate change, unencumbered by human fishing activities. Continued monitoring will show if future shifts in marine communities occur at different rates or to different base states in MPAs compared to fished areas.

“Some of these time series are longer than 25 years at this point and the data are critical to understanding and readying human communities for the changes occurring in our marine communities,” stressed Caselle. Despite the heatwave, Smith also points out, “the ecological communities in MPAs are still being protected, even if they are different as a result of the heatwave.”

“With the devastating impacts of climate change already apparent, it is very important that we are upfront about climate solutions – as long as we are burning fossil fuels and warming the globe, marine ecosystems will be at risk, even if they are protected from fishing.”

This study is the first in a series led by the NCEAS working group, exploring human engagement with the California MPA network, the effect of MPAs on fish populations and fisheries, and a synthesis of marine protected areas that benefit both people and nature.

We must take swift climate action and implement nature-based solutions to safeguard the health of our oceans, as we expect marine heatwaves to increase in frequency and intensity.

More about ocean heatwaves

Ocean heatwaves, akin to their terrestrial counterparts, are characterized by prolonged periods of unusually high sea surface temperatures. These marine thermal anomalies last for at least five days, with temperatures exceeding the local 30-year maximum monthly mean by at least 0.5 degrees Celsius.


These intense oceanic temperature spikes are largely driven by global warming and natural climate variability. Greenhouse gas emissions, primarily due to human activities such as the combustion of fossil fuels, amplify the warming of the Earth’s atmosphere. This atmospheric heating then transfers to the oceans, as they act as a significant heat sink, absorbing approximately 93% of the Earth’s excess heat.

Effects on marine ecosystems

The repercussions of ocean heatwaves are far-reaching and often devastating for marine ecosystems. Corals, for instance, experience bleaching—a condition where they expel symbiotic algae due to heat stress, leading to potential coral death. Ocean heatwaves also pose significant threats to marine species, disrupting their reproductive cycles and shifting their geographic distribution towards the poles. Additionally, heatwaves can damage kelp forests and seagrass meadows, essential carbon sinks, hence affecting their ability to mitigate climate change.

Impact of ocean heatwaves on human communities

Ocean heatwaves can heavily impact human communities that rely on marine resources. They can lead to economic hardships in industries such as fishing and tourism due to the reduction of marine biodiversity and the destruction of underwater landscapes. Also, the decrease in fish populations can disrupt food chains, leading to global food security issues.

Scientific research and prediction tools

Scientific communities are making continuous efforts to better understand, predict, and assess the impacts of ocean heatwaves. Predictive tools are being developed to provide early warnings, allowing for timely adaptation and mitigation efforts by marine resource managers and coastal communities.

Mitigation and future directions

Addressing ocean heatwaves requires global action to reduce greenhouse gas emissions, adopt renewable energy, and promote sustainable practices. As ocean heatwaves continue to increase in frequency and intensity due to climate change, strategies for understanding, preventing, and preparing for these marine heat events will remain a critical focus in the years ahead.


In sum, ocean heatwaves pose a growing threat to marine ecosystems and human communities. The global community’s task is to reduce the causes of these phenomena while enhancing our ability to predict and mitigate their impacts. As we move forward, protecting our oceans from the devastating effects of heatwaves will be a vital element of our climate action plans.


Check us out on EarthSnap, a free app brought to you by Eric Ralls and

News coming your way
The biggest news about our planet delivered to you each day