Around April of 2023, we saw a remarkable jump in global sea surface temperatures that have persisted through to the present day. These ocean temperatures help explain why global average temperatures were warmer than any previous record in 2024.

These unprecedented temperatures manifest as extreme marine heatwaves across many parts of the ocean. And scientists have been frantically trying to understand what has caused these heatwaves.

We know that marine heatwaves have become increasingly common and more intense over time because of global warming. We also know that the El Niño that kicked off in 2023 allowed more heat to enter the ocean. But these factors alone can't explain the incredible scale of the jump that began in 2023. This new global study, along with some more regionally focused studies, helps explain what happened.

Interestingly, the causes seem to be different in different regions. In some places, we have been getting less cloud cover, so more solar energy, entering the ocean. In other regions reduced winds mean that cooling from evaporation has diminished - leading to warming. In others still, the upper layer of the ocean has become thinner, making it more easy to warm, or ocean currents have shifted, transporting more heat.

While this and other studies are now able to explain most of the individual heatwaves, it still leaves the question of why this all happened over the last couple of years. Was it just a perfect storm of factors - a coincidence - or are we seeing some abrupt and unexpected shift in the climate system as the planet warms?

We know the Earth is warming, but it's not happening evenly in time or space. Marine heatwaves are just one example of how our future will play out in ways that we are still figuring out. While we urgently need to reduce our GHG emissions, it is critical that we also continue to measure, monitor and model what our future Earth is going to be like – if we don’t, we can’t prepare, and we are walking into the unknown with dire consequences for our future food, health and security.

An increase in marine heatwaves is one example of the impacts of climate change, and another reason why we need to reduce greenhouse gas emissions.

Marine heatwaves are becoming more frequent, with notable occurrences off Australia’s East Coast and Tasmania, as well as the northeast Pacific coast and North Atlantic.

This latest research on global marine heatwaves aligns with what we have found in Australia and our State of the Climate Report, which shows that the warming of the ocean has contributed to longer and more frequent marine heatwaves.

It is critical that we continue to measure, monitor and model future changes to protect our environment, food security and health.