We’re In Hot Water…
While we’ve been focused on “climate warming,” we’ve primarily been talking about the rising temperatures in the earth’s atmosphere and its land masses. It’s understandable – after all, that’s where we live. But the continents are only 30% of the earth’s surface; the other 70% comprises the global ocean, and it’s heating up too.
Global Warming of the Oceans
Global warming results from our collective chemistry experiment with the earth’s atmosphere (aka greenhouse gas emissions), which has upset the energy balance between incoming sunlight, and heat radiated to space. Over time, the resulting increase in the Earth’s Energy Imbalance (EEI) leads to melting ice and more energy accumulating in the atmosphere, land, and oceans. Accumulating energy inevitably results in rising temperatures. In a 2025 article, scientists tracking the EEI reported that the imbalance has more than doubled in recent decades. All that extra energy has to go somewhere…
More than 90% of the extra energy is absorbed by the oceans, vastly limiting how hot the atmosphere would otherwise become. Ocean currents and turbulent mixing distribute the heat horizontally and vertically within the ocean, gradually moving it from surface waters into deeper layers over time. Scientists track this using “ocean heat content” or OHC, a measure of total heat stored throughout the full depth of the ocean, not just the sea surface temperature.
As a measure of energy, Ocean Heat Content is measured in joules, the standard measure of total thermal energy. (One joule is quite small—a 100 watt light bulb consumes 100 joules in a second.)
What Happened in 2025?
The annual increase in global ocean heat content in 2025 reached the highest level ever recorded, continuing a nine‑year streak of record‑breaking warmth in the upper 2,000 meters of the ocean. An international study found that the OHC of the global upper 2,000 m of the ocean increased by 23 zettajoules in 2025 relative to 2024. This was a dramatic increase over the previous annual record of 16 zettajoules added in 2024.
I know you’re wondering “what’s a zettajoule?” It’s a whole lot of joules – a sextillion of them, in fact. Still wondering? The 23 zettajoules the ocean gained in 2025 can be written as 23,000,000,000,000,000,000,000 joules. This is a lot of energy—equivalent to more than 200 times the total amount of electricity used by the global population in a year.
The 2025 increase of 23 zettajoules marked a stunning acceleration relative to the eight prior record years, which logged annual increases of 13 to 16 zettajoules. The acceleration in ocean heat content is clear in Figure 1 below.
Figure 1: Increase in OHC since the mid-2oth century Credit: Carbon Brief/Zeke Hausfather
As shown in Figure 1, overall global energy storage, or OHC, has increased by more than 500 zettajoules since the 1940s, and the rate of change in OHC is accelerating.
Regional Hot Spots
The 2025 change in OHC was not uniform, as shown in Figure 2. Record high OHC was recorded in the Tropical and South Atlantic, the Mediterranean Sea, the North Indian Ocean, and the Southern Ocean (around 40°S.)
Figure 2: Global distribution of OHC change in 2025, relative to the 1981-2010 average.
In all, about 16% of the global ocean surface reached its highest heat content on record. Although the global mean sea surface temperature (SST) in 2025 was slightly lower than in 2024, ranking about third warmest, subsurface heat content still set a new record, reflecting ongoing heat accumulation even as surface conditions evolved toward a cooler La Niña.
OHC Impacts in 2025
Sea Level Rise
Sea Level Rise (or SLR) is perhaps the most widely known impact of ocean warming. Thermal expansion of the warming oceans is the principal driver of SLR, boosted by accelerated land ice melt from Greenland and Antarctica. The overall rate of sea level rise has more than doubled over the past three decades, with a total rise of roughly 10 cm (4 inches) since 1993.
Interestingly, despite the record increase in global OHC, 2025 global sea level rise was below average. How is this possible?
Sea level researchers at NASA/JPL found that the late 2025 La Niña event, although relatively mild, triggered extreme precipitation, particularly over the Amazon River basin, and thus a shift of water from oceans to land. (We’ve previously discussed the relationship between the water cycle and sea level here.) However, as the JPL researcher put it, “… that cycle is short-lived. The extra water in the Amazon is going to reach the oceans in less than a year, and rapid [sea level] rise will soon return.”
Extreme Weather Events
Record‑high OHC in 2025 acted as extra fuel for many extreme weather events by supercharging evaporation, moisture, and storm energy in the atmosphere. Increasing heat in the oceans is linked to:
- Heavier rain and flooding. Warmer surface waters evaporate more, loading storms with extra moisture that then leads to extreme rainfall.
- Stronger tropical cyclones. High OHC means deep warm layers, so hurricanes and typhoons can keep intensifying even as they churn the surface, increasing peak winds and rainfall.
- Hotter, longer marine heatwaves. Extra stored heat produces persistent marine heatwaves that disrupt fisheries, coral reefs, and coastal weather patterns.
- More energy in the global water cycle. With oceans absorbing about 90% of excess heat, rising OHC boosts the overall hydrological cycle, amplifying both very wet and very dry extremes.
Examples linked to 2025’s ocean heat include:
- Severe flooding in Southeast Asia and Mexico and in the U.S. Pacific Northwest, tied to moisture‑rich storms over unusually warm oceans.
- Drought conditions in parts of the Middle East, linked to shifted circulation patterns influenced by warm oceans and the El Niño/La Niña transition.
- Intense flooding and landslides in central China and destructive floods in Nigeria and West Africa, where warmer regional seas increased atmospheric moisture feeding heavy rains.
- Hurricane Melissa: Rapid intensification to Category 5 over the very warm Caribbean Sea made it the strongest tropical cyclone in the world in 2025 and the third-most intense Atlantic hurricane on record.
- The 2024-2025 coral bleaching event is the biggest and most serious to date. Bleaching-level heat stress impacted 84% of the world’s coral reef area and mass coral bleaching occurred in at least 83 countries and territories.
Rising OHC does not cause a single specific weather event, but it does raise the baseline so that storms, floods, heatwaves, and even some drought patterns in 2025 were more intense and damaging than they would have been in a cooler‑ocean world.
What Can We Expect in 2026?
January 2026 was the fifth warmest January on record and saw contrasting temperature extremes in the Northern and Southern Hemispheres, according to the Copernicus Climate Change Service (C3S). C3S also recorded the month as being 1.47°C above pre-industrial January temperatures.
Extreme Weather Persists
According to the World Meteorological Organization (WMO), extreme weather was the story of the first month of 2026. The Northern Hemisphere experienced severe cold waves as a meandering polar jet stream spilled icy air into Europe and North America. Europe had its coldest January since 2010. Nonetheless, monthly temperatures in January were above average over much of the globe, including large parts of the Arctic, in Greenland and in western North America.
In the Southern Hemisphere, record breaking heat provided fuel for extreme conditions, including wildfires in Australia, Chile and Patagonia. Heavy rains in Southern Africa in the final week of the month led to severe flooding, particularly in Mozambique, with catastrophic impacts.
Ocean Warming Continues
With regard to the oceans, scientists expect 2026 ocean heat content to remain extremely high and likely set yet another record or stay near the 2025 peak, which in turn will keep the “background risk” for extreme weather elevated. Because OHC changes slowly and 2025 saw a dramatic increase in heat content, the extra heat already stored in the upper ocean would likely keep OHC elevated in 2026 regardless of the effects of periodic weather phenomena, such as the El Niño Southern Oscillation (ENSO) in the equatorial Pacific Ocean. In fact, the Columbia Climate School’s International Research Institute predicts an ENSO shift from January’s weak La Niña to a warmer El Niño by mid-2026.
Global Temperature Remains High
Following record-breaking heat in 2023, 2024 and 2025, global temperatures are expected to remain at historically high levels in 2026. Environment and Climate Change Canada (ECCC) forecasts a global mean temperature of 1.44°C above pre-industrial levels, within a range of 1.35°C – 1.53°C. This will make 2026 one of the four hottest years on record, along with 2023, 2024 and 2025.
The Bottom Line?
With the likelihood of record or near‑record OHC and continued high global temperatures, the “table is set” for another year of extreme weather around the world. Buckle up!