Marine Heatwaves (MHWs)

Scientists have reported that marine heatwaves (MHWs) affected 96% of the ocean surface in 2023, raising alarms about the potential for a permanent temperature shift that could severely disrupt marine ecosystems and affect life both in oceans and on land.

What are Marine Heatwaves (MHWs)?

• Definition:
  A marine heatwave (MHW) is an extreme weather event characterized by sea surface temperatures rising 3 to 4°C above average in a specific region for at least five days. MHWs can persist for weeks, months, or even years, causing long-lasting changes to marine environments.

Key Causes of Marine Heatwaves

1. Global Warming:

   • Rising CO₂ levels trap heat in the atmosphere, causing ocean warming. Oceans absorb about 90% of excess heat from global warming, increasing the likelihood of MHWs.

2. El Niño:

   • This climate phenomenon warms the upper ocean layers, weakens upwelling, and traps surface heat. The Pacific Decadal Oscillation (PDO), a long-term climate pattern, also amplifies MHWs by shifting Pacific temperature patterns.

3. Reduced Cloud Cover:

   • With fewer clouds, more sunlight directly reaches the ocean surface, causing rapid warming. For example, the Atlantic heatwaves of 2023 were exacerbated by this factor.

4. Changing Ocean Currents:

   • Shifts in ocean currents, like the weakened Gulf Stream, raise regional sea temperatures, contributing to MHWs. This also affects hurricanes and sea-level rise along affected coastlines.

5. Human-Induced Feedback Loops:

   • Arctic warming melts reflective sea ice, exposing darker waters that absorb more heat, contributing to further warming. Similarly, coral die-offs reduce CO₂ absorption, accelerating global warming.

Projected Trends

• The ocean's average temperature has increased by 1.5°C over the last century, and by 2100, marine heatwaves could be 50 times more frequent than pre-industrial times.

Effects of Marine Heatwaves on Oceans and Climate

1. Climatic Impacts:

   • MHWs contribute to extreme weather events like tropical storms and hurricanes. They also disrupt the water cycle, leading to increased flooding, droughts, and wildfires. A notable example is Hurricane Ian in Florida.

2. Economic Impact:

   • MHWs significantly affect aquaculture and fisheries, as farmed species rely on stable temperatures. For example, key species like lobster, snow crab, and scallops have seen declines in affected regions like the Northwest Atlantic and Western Australia.

3. Ecological Consequences:

   • Marine ecosystems face mass mortality of invertebrates, disruption of food webs, and changes in wildlife behavior. Species like kelp forests, seagrass meadows, and coral reefs are particularly vulnerable, with ecosystem collapses and local extinctions witnessed during MHW events (e.g., 2011 Western Australia).

4. Compounding Environmental Stressors:

   • MHWs often coincide with other oceanic stressors like ocean acidification, deoxygenation, and overfishing, intensifying the environmental destruction.

How Can We Prevent and Mitigate Marine Heatwaves?

1. Strengthen Ocean Monitoring:

   • Expand marine monitoring systems and improve climate models to predict MHWs in real time, providing early warnings to coastal communities.

2. Protect and Restore Marine Ecosystems:

   • Conserve coral reefs, mangroves, and other key ecosystems by establishing Marine Protected Areas (MPAs) and restoring degraded coastal zones like seagrasses and salt marshes. These ecosystems help absorb CO₂ and enhance resilience.

3. Advance Sustainable Fishing and Aquaculture:

   • Promote climate-resilient aquaculture by using heat-tolerant species and providing sustainable feeds. Early warning systems can help alert fishing communities about upcoming MHWs, allowing them to adapt.

4. Enhance Global Cooperation:

   • Strengthen commitments under the Paris Agreement, support vulnerable nations with climate finance, and promote global ocean governance through frameworks like the UN Decade of Ocean Science (2021-2030).

5. Reduce Greenhouse Gas Emissions (GHGs):

   • Transition to renewable energy, implement carbon pricing, and promote sustainable transport and industries to curb emissions and fight climate change.

6. Reduce Local Stressors on Oceans:

   • Curb ocean pollution through agreements like the UN Plastic Treaty and sustainable farming practices. Innovations like reflective infrastructure and artificial upwelling can cool ocean surfaces and reduce heat absorption.

Conclusion

Marine heatwaves pose an increasing threat to ocean health, weather stability, and coastal economies. Combating them requires global emission reductions, protection of marine ecosystems, and the development of advanced monitoring systems. Without immediate and concerted action, MHWs will continue to intensify, leading to irreversible damage to biodiversity and human livelihoods. A coordinated climate-ocean policy is essential to safeguard our oceans and the future of life on Earth.