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More than a decade since the start of the , scientists are still working to understand the extent of its impacts. This unprecedented heat wave, nicknamed "," stretched thousands of kilometers over North America's western coastal waters, affecting everything from the smallest plankton to the largest marine mammals.

Between 2014 and 2016, when this heat wave occurred, water temperatures soared between 2 and 6°C above average.

One would be forgiven for thinking this is no big deal. After all, temperatures fluctuate more than this on land most days. But not so in the ocean, where temperatures are normally much more stable because of the .

Although the duration of this multi-year warming event made it the first of its kind, it offers a glimpse into a future with climate change, .

In our , we synthesized the findings from 331 scientific studies documenting the ecological impacts of this marine heat wave across ecosystems, all the way from Alaska to Baja California.

Our results offer a stark warning for how profoundly ocean life can be upended by heat waves that are now a dominant signature of climate change.

Species on the move

One of the most common responses to this extreme event was that marine species moved into places where as they searched for cooler water. Most headed north.

In total, we identified 240 species that were found outside of their normal ranges. More than 100 of these were found further north than they had ever been recorded before, with some moving up to 1,000 kilometers.

These species on the move included everything from fish and invertebrates to seabirds and marine mammals. But species don't all have the same discomfort level with warm water, and some species are more mobile than others. So, marine communities didn't simply pick up and move together to avoid the heat.

Instead, marine heat waves like this one are causing a massive reorganization of ocean life, as new predators, prey and competitors intermingle for the first time. The newcomers with far-reaching consequences.

Heat wave effects on linked species and fisheries

One of the key lessons from this heat wave is that impacts on one species can have effects that ripple throughout entire ecosystems. For example, the shifting availability of key forage fish like anchovies and sardines contributed to and .

Warm, nutrient-poor waters also triggered unprecedented blooms of toxic algae. This led to the that cost local economies .

Widespread ecological disruption

The marine heat wave also transformed coastal habitats, including kelp forests and seagrass beds. Kelp forests, sometimes called , were impacted along several thousand kilometers of coastline.

In some cases, local extinctions of these habitats have persisted for years following the event, with on the critters that rely on them. We still don't know whether these changes represent permanent losses or whether any of these ecosystems will be able to recover.

Diseases flourished in the warmer waters. The previously abundant sunflower sea star was hit particularly hard. Warmer waters likely increased the susceptibility of this species to an . This led to losses severe enough to have it listed as a .

Similarly, increases in contributed to of the habitats these plants create.

Preparing for warmer water

Our review highlights how we are unprepared to respond to these challenges in real time.

With marine heat waves becoming more prevalent, we need to prepare for what is coming. Climate models indicate that these events will only get stronger as greenhouse gas emissions continue to warm our planet.

Global ocean temperatures have continued to rise over the decade since The Blob, with several years since being declared the in the ocean, only to be surpassed the following year.

Actions such as or reducing additional stressors, like overfishing, may help ecosystems cope with some of these shocks. However, and offer only a temporary solution to a problem that is worsening.

Our review demonstrates how unpredictably these heat waves can unfold across marine ecosystems and how widespread their impacts can be. In the face of such drastic change, climate adaptation measures will only get us so far.

To stave off the worst impacts of heat waves driven by climate change, governments and industry must urgently reduce greenhouse gas emissions. The 2014–16 marine heat wave was a warning. The question now is whether we will listen.

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