Uncharted waters: what do the recent record marine heatwaves mean for the oceans?

August 17, 2023

Humanity has just lived through the hottest month its history, but the effects of the heatwaves stretch far beyond the land.

While many of us recently had a break during the summer here in the Northern Hemisphere, climate change was still very much showing up to work. In fact, humanity has just lived through the hottest month its history with record breaking global temperatures lasting from 3rd July to 7th August 2023. That means that for 36 days straight, each day was hotter than the previous average temperature record that was only set a year ago, on 24th July 2022. With stark consequences. The extreme heat and accompanying fires in places like Greece and Hawaii have caused devastation to homes and whole communities, including loss of life. But the impact of this heat stretch far beyond the land.

As is often the case, the ocean bears the brunt of humanity’s excesses.  The oceans have acted as a kind of global buffer to the climate crisis over recent decades, by absorbing vast amounts of the carbon dioxide that we have poured into the atmosphere. In storing around 90% of the excess energy and heat this has created, the oceans actually dampen some of the impacts of global heating on land. Marine heatwaves (defined as periods of five days or more of sea-surface temperatures (SST) warmer than the 90th percentile) doubled in frequency between 1982 and 2016, and have become longer and more intense.

But what we are seeing this year is without precedent: At the end of July, the Mediterranean Sea recorded its highest-ever median surface temperature at 28.71C. In July the sea surface temperature anomaly in parts of the Mediterranean was a staggering 5 degrees higher than usual, and in August the same anomaly occurred in the eastern Pacific Ocean. But simply attributing this to the unfolding El Niño weather event would be a mistake. This rapid heating actually began back in March 2023 before the El Niño system had even developed. Although El Niño is partly responsible for these recent heatwaves, the record-breaking temperatures are primarily being driven by human-induced climate change.

This graph shows that world’s oceans are warmer on average in 2023 than at any time since measurements began in the 1980s (SST = Sea Surface Temperature). Source: Climate Change Institute, University of Maine (12/08/2023)

It is already well known that hot waters can devastate coral reefs, and reports of mass coral bleaching will sadly continue to grow through this year and into the next. But warming waters affect almost every part of marine life, which has evolved to exist within highly stable, often very specific, temperature ranges. Any deviation from that can cause real problems for marine life, especially when those temperature changes are occurring so rapidly that organisms cannot realistically adapt.

Corals and other sedentary marine creatures have few options when they find themselves in hot water. Most corals that experience even one degree Celsius above what they usually tolerate for several weeks will bleach, which essentially means the coral is no longer able to feed itself. If the abnormal temperatures continue, they will remain bleached, and starve to death.

More mobile marine life, like fishes, may become “climate refugees” as they seek out more favourable environments, and there is already evidence of poleward migrations to cooler water of numerous marine species each year. Many species, however, have limited options, such as those living in geographically circumscribed regions and climate change hotspots, like the Mediterranean. The Mediterranean Sea is one of the world’s most diverse habitats for marine life, home to more than 17,000 species, but it is also warming 20% faster than the global average, adding pressure to already strained ecosystems.

Warmer waters increase the metabolism of most fish species, meaning they need to eat more to maintain their health. This is may be compounded by the fact that their prey have themselves moved away due to the warming water, or declined as a result of other impacts, such as overfishing. One example of how an entire food chain was affect by a marine heatwave comes from the west coast of North America, where in 2014-16, a notorious marine heat wave known as “The Blob” led to nutrient poor waters, dramatically reducing the plankton stock that the fish usually relied upon. The fish responded by migrating away from the coast, causing widespread starvation among sea lion pups and the death of almost one million seabirds.

It is often the case what is happening below the surface of the water goes unnoticed until it shows up on land. In late July this year, approximately 12,000 fish washed up dead in the Gulf of Texas due to low levels of dissolved oxygen in the water, a consequence of rapidly warming water. Warmer water stores less oxygen, and also promotes the growth of harmful algal blooms, which additionally deplete the water of oxygen as well as being toxic to other marine life.

While toxic algae blooms may prosper from marine heatwaves, much ocean plant life will undergo negative effects. Seagrasses, for example, photosynthesise less efficiently under heat stress, and can even undergo “burning” under extreme conditions. Neptune grass (Posidonia oceanica), is a type of seagrass that grows in vast underwater meadows. Endemic to the Mediterranean, it’s both an enormous carbon sink and a vital nursery for fish. But if the water gets too warm, its growth can be stunted or, even worse, it can die off, as has been observed in the Eastern Mediterranean over the past two decades.

Even large marine mammals like whales and dolphins, are not immune from the effects of ocean warming and accompanying heat waves. Anomalous temperatures can cause them significant physiological stress and increase their susceptibility to disease. Like fishes, their prey may decline or move away from their usual hunting grounds, forcing them to look elsewhere. Increased competition or failure to find alternative food sources can lead to reduced reproductive success and survival rates, as occurred in an unprecedented marine heatwave of the West Coast of Australia in 2011, which had severe impacts on a population of Indo-Pacific bottle-nose dolphins (Tursiops aduncus).

What OceanCare is doing to protect the oceans from climate change

Protection of our oceans equates to protection of our climate, as the oceans regulate our climate and weather systems. OceanCare firmly believes that this global problem, can and must be solved at the political level. OceanCare therefore applied for, and was granted, observer status at the United Nations Framework Convention on Climate Change (UNFCCC). This provides us with a strong voice in this critical international forum, enabling us to better raise awareness of the threats that climate change poses to the oceans and its inhabitants. We did just that in the run up to COP 27 UNFCCC conference, providing a comprehensive report on the climate crisis in the Mediterranean and the extent of further oil and gas development in the region.

OceanCare continues to address the climate change issue and call for concrete action in numerous other fora at the UN level and beyond.

  • Reducing shipping speed
    Many of the other issues OceanCare is working on are related to the effort to reduce greenhouse gas emissions. One example is our programme to call for speed reduction in shipping. Ships should slow down: Simply reducing the average cruising speed of the global shipping fleet by 10 per cent would make a significant contribution to lowering greenhouse gas and underwater noise emissions. It would also reduce the risk of collisions with whales by half. OceanCare is a leading voice in international bodies such as the International Maritime Organization (IMO) to reduce the ecological footprint of shipping.
  • Curtailing underwater noise
    Resistance to the exploitation of further oil and gas reserves is becoming increasingly important. It requires the transformation of the energy sector and a complete shift away from fossil fuels. A priority focus for OceanCare is the prevention of further oil and gas exploration in the Mediterranean. The oil and gas industry uses seismic airguns to survey the seabed in the search for oil and gas deposits, which generate sound waves that penetrate hundreds or thousands of metres of sea before reaching more than a hundred kilometres deep into the ground. Such seismic activities are extremely dangerous and can drive away or kill marine animals. The continuation of seismic activities in the sea for the development of new oil and gas deposits must therefore be prohibited, not only to protect the climate, but also for the conservation of marine species. After years of efforts on the part of OceanCare, the Spanish government banned the search for oil and gas deposits in Spanish waters in May 2021. Comparable bans exist in Denmark, France, New Zealand and Portugal.
  • Reducing plastic pollution in our oceans
    The climate crisis and plastic pollution are also linked. Plastic waste in the oceans inhibits the growth and photosynthetic performance of Prochlorococcus bacteria, which produce around ten per cent of global oxygen. In addition, scientific studies show that phytoplankton are ingesting increasing amounts of microplastics, which reduces their ability to absorb carbon dioxide. We use the example of “ghost nets” to illustrate the extent of plastic pollution in the oceans, which cause the death of millions of marine animals. At the recent international negotiations in Paris on a new global Plastic Agreement (INC-2), OceanCare called for the reduction of orphan fishing gear to be set as a central commitment. Such a commitment should apply nationally and internationally, including gear restrictions and the reporting and recovery of lost fishing gear. Furthermore, within the negotiations for a Plastics Agreement, OceanCare also advocates for reducing plastic along its entire life cycle so that it does not end up in the oceans in the first place. Reducing marine litter will help to reduce pressure on marine species from the effects of cumulative impacts, such as climate change and associated marine heat waves. Additionally, the production of plastic also requires a lot of energy and releases large amounts of CO2, further perpetuating the climate crisis.
  • Timely and reliable research data from the field
    One of the many valuable reasons OceanCare supports and works with research organisations is to ensure that what happens at sea does not go unnoticed until it is too late. Through its partner organisations, OceanCare regularly receives information about a wide range of marine species and the state of their habitats. We are able to use this information strategically to influence policy and decision-making at international and national levels. We also act as an early warning system, alerting international bodies, governments, and industry to emerging issues, including the impact of heat waves on marine life.