The world's oceans have reached their highest temperatures on record for June, surpassing previous highs set during the 2023–24 El Niño years. Currently, the average sea surface temperature across tropical and temperate oceans stands just below 21°C, compared to about 19.6°C before widespread industrialization in 1870. While this difference may seem modest, the energy required to heat the oceans to this extent is immense. More than 90% of the extra heat trapped by greenhouse gases from burning coal, gas, and oil has been absorbed by the oceans, leading to rapid warming. In 2025, the heat added to the oceans was equivalent to about 12 Hiroshima-scale nuclear bombs exploding every second of every day. To find a climate analogue comparable to current ocean conditions, we would need to look back around 120,000 years, before the last ice age, when slow orbital shifts caused gradual warming over millennia. Humans have achieved a similar result in just over a century.
Impacts of Warmer Oceans
Ocean heat does not remain isolated. Hotter oceans fuel stronger cyclones, a more humid atmosphere, more intense rainfall, and greater heat in air masses over the seas, which can increase the likelihood and intensity of heatwaves on land. The El Niño currently forming in the tropical Pacific is expected to be a significant event. As it develops, warmer temperatures and extreme events such as marine heatwaves are anticipated in the western Indian Ocean, tropical Atlantic, and eastern Pacific Ocean.
Current Hotspots
Europe is experiencing a record-breaking heatwave, with surrounding oceans and enclosed seas also exceptionally hot. Parts of the Mediterranean are up to 6°C warmer than the long-term average, while areas of the North Sea are up to 3°C above average. The forming El Niño has led to sea surface temperatures about 1.24°C above average across a large area of the central eastern Pacific, with subsurface conditions in the eastern Pacific more than 6°C above average. A typical El Niño lasts about a year, with the full effect on atmospheric heat becoming clearest towards the end of the cycle. This means that while 2026 is expected to be very hot—possibly a new record—next year is likely to be even hotter as ocean heat is released back to the surface, similar to patterns observed during the 2023–24 and 2015–16 El Niño events.
Threats to Marine Ecosystems
Steady ocean warming, coupled with longer-lasting and more intense marine heatwaves, poses significant threats to marine ecosystems such as coral reefs, sea grass meadows, and coastal reefs. Research on the 2023–24 El Niño and the warm 2024 year showed widespread impacts. In June 2023, a record-breaking marine heatwave in the North Atlantic Ocean preceded intense heatwaves across Europe, extreme rains triggering deadly floods in Spain, and severe bushfires around the Mediterranean.
Consequences for Land Weather
Rising ocean temperatures have numerous consequences. A warmer ocean is less effective at cooling the land during summer, and increased evaporation boosts humidity, leading to more intense and sudden extreme rain and floods. During El Niño events, a clear geographical pattern emerges: typical cyclone areas like the western Indian Ocean could see stronger cyclones with heavier rainfall upon landfall. El Niño tends to bring extreme rain and floods to western South America and dry conditions to parts of Australia and Southeast Asia. Global surface temperatures spike during strong El Niño years and fall back during La Niña years, even as climate change drives the baseline higher.
Preparedness and Forecasting
Understanding how major climate drivers like El Niño shape weather is improving, and ocean data from around the world is being used to develop better seasonal forecasts that authorities can use to prepare. Over the past two years, forecasting capabilities for marine heatwaves have improved, allowing predictions three to four months ahead in Australia, the United States, and other regions. These forecasts enable marine authorities to act early by reducing allowable fishery catches and initiating conservation efforts for vulnerable species.
However, this early success in ocean forecasting may be short-lived. The current US administration last year slashed funding for climate data gathering networks and has worked to dismantle the National Center for Atmospheric Research. This year, the administration announced it would end funding for a key ocean monitoring network before backing down. Ongoing collection of ocean data is crucial for ocean and land forecasts. If these networks are weakened or discontinued, we could face the challenge of dealing with worsening climate impacts blind. Ending the measurement of climate change will not stop it from happening. The only way to keep climate change from steadily worsening is to reach net zero as soon as humanly possible. Until then, forecasts must be used to prepare for what cannot be avoided.
