5 Facts on Marine Heatwaves

As Earth's temperatures rise, so do its waters—especially its oceans. In fact, oceans are bearing the brunt of climate change and its impacts. Though some of those impacts might be more visible, like marine pollution, unfortunately not all of them are. Marine heatwaves are one such event that the human eye cannot always see. With climate change impacts in full swing, these heatwaves are wreaking havoc on our oceans and are putting marine species and coastal communities at risk.   

Here are five facts on marine heatwaves:

1. Like on land, heatwaves occur in our marine environments, too

   A marine heatwave is an extreme event occurring in our oceans, where unusual increases in temperature above seasonal thresholds occur over a prolonged period. For example, just a few degrees centigrade higher than normal, for longer than five days classifies as a heatwave in the ocean. Heatwaves are driven by hot and windless conditions, that are needed to mix upper and lower layers of the ocean with cooler water. The most intense heatwaves occur at the surface but can occur in areas over 1 km deep. These deep heatwaves are likely underreported, leaving more to be learned about their impact. Additionally, El Niño, which is classified by extreme heat and dry conditions, influences the occurrence of marine heatwaves. 

2. Human-induced climate change is driving intensity and frequency 

   Oceans absorb atmospheric heat, which makes them a valuable buffer against climate change. However, as the Earth warms, so does the ocean, which in turn pushes temperature to go up further. But the changing climate also means changing weather patterns and ocean currents, which also affect how heatwaves come and how intense they are. 

3. They have a devastating impact on ecosystems and humans 

   While heatwaves might not be fully visible and their impacts might not be so drastic to humans living on land, it can be catastrophic for underwater organisms and ecosystems. Because climate change increases the intensity and frequency of marine heatwaves, we are seeing increased mortality of species like fish, birds, corals and seagrasses, which we depend on for a range of critical ecosystem services. These impacts directly and indirectly affect risks for people and the world. Marine heatwaves are also linked to increasing intensities of coastal storms and cyclones, which are causing higher amounts of damage to vulnerable coastal communities. 

4. They can occur anywhere, even in the coldest water on Earth

   Marine heatwaves can occur in cold waters like the Arctic and Antarctic Oceans. Between 2007 and 2021, 11 marine heatwaves were recorded near the Arctic. One such event in 2020 lasted for 103 days and temperatures peaked at 4 degrees Celsius above the normal average. Another event known as the “Blob” occurred from 2013-2016 in the cold northern Pacific Ocean. Initially reported at 800 km (500 miles wide and 91 metres (300 feet) deep, it eventually formed into patches reaching from Canada to Mexico, hugging the US states of California, Washington and Oregon in between. 

5. Reducing CO2 emissions and increasing nature-based solutions are the best solutions

   While developing early-warning systems and continued monitoring are important, solutions to reduce marine heatwaves must tackle the root causes. Mitigating climate change impacts by immediately reducing greenhouse gas emissions is the most crucial step. In the meantime, nature-based solutions that conserve and restore marine ecosystems can make them more resilient to heatwave impacts, and in turn help them to support us in the future.