FAQ: Ocean Warming

Introduction

As human activities have released greenhouse gases into the atmosphere, trapping excess heat and warming the planet, the ocean has absorbed more than 90% of that excess heat since the 1970s. The ocean can do this because of its size and because it’s made of water, which can absorb four times as much heat as the same volume of air.

One estimate suggests that the heat energy absorbed by the ocean’s upper 2,000 meters (6,500 feet) between 1955 and 2010 would have warmed Earth’s atmosphere by 36°C (64.8°F). Instead, greenhouse gases piling up in the atmosphere have so far led to a 1.2°C (2.2°F) increase in global average air temperatures compared to preindustrial times.

The absorption of all this heat by the ocean has protected land dwellers like humans from temperatures that would make life impossible in certain regions, but that energy, almost impossible to quantify in comprehensible terms, will remain present for decades, if not centuries.

This accumulation has major implications for the entire planet, ranging from extreme weather and sea-level rise to coral die-offs and oxygen depletion in parts of the sea. 

Unless humanity rapidly reduces its greenhouse gas emissions these impacts are expected to get worse as climate change progresses. But there is still a lot left to learn about how the ocean is responding to these changes — especially in deeper waters where scientists have much less data to draw on. 

The article below leverages the expertise of Scripps scientists to answer some common questions about ocean warming.

How much have the oceans warmed?

Different parts of the ocean have warmed different amounts, but between 1901 and 2020, average global sea-surface temperatures increased an average of 0.08°C (0.14°F) each decade, leading to a total increase of around 0.8°C (1.5°F). Within this timespan, scientists note a marked acceleration beginning in the 1970s. Both 2024 and 2023 were the hottest years ever recorded in the ocean. A 2013 study found evidence that the current period of human-caused warming is the fastest the ocean has heated up in at least 8,000 years.

Over the last 20 years, more detailed data shows an average global temperature increase of around 0.5°C (0.9°F) in the ocean’s upper 500 meters (1,640 feet). These surface waters have warmed the most due to climate change, but, because ocean currents and other processes redistribute that heat, it also works its way into deeper waters.

Data for the deep ocean below 2,000 meters (6,500 feet) are sparse, but available measurements suggest even these cold, sunless waters have warmed roughly 0.01°C (0.018°F) over the last two decades. The deep Southern Ocean near Antarctica is warming particularly quickly, at a rate four times faster than the global average for the deep ocean.

More frequent measurements from more locations in the deep ocean will be essential to better track the pace of climate change, and to inform climate models designed to forecast impacts such as sea-level rise.   

How do scientists measure ocean warming?

Prior to the 1980s, scientists collected measurements of ocean temperature via ships, coastal instruments and stationary buoys. Since the 1980s, satellites and drifting buoys from the National Oceanic and Atmospheric Administration (NOAA) funded and Scripps-based Global Drifter Program have been able to measure sea-surface temperature across the entire planet. Sea surface temperature is a vital measurement when it comes to ocean heat because it is where the ocean and the atmosphere meet and interact. 

The Global Drifter Program’s drifting buoys also measure variables like ocean currents, atmospheric pressure, winds and wave heights. The buoys are designed, built and operated by the Lagrangian Drifter Laboratory at Scripps. The Global Drifter Program and satellite measurements work synergistically — with the drifting buoys providing direct measurements of temperature that represent the main benchmark for validating the accuracy of the satellite data and enabling truly global measurements that are essential to activities such as tracking long term climate change as well as weather forecasting and predicting extreme events such as atmospheric rivers and hurricanes. 

The ocean’s surface tells an essential part of the story but it must be paired with an understanding of what’s going on beneath the surface. Since 1999, thousands of robotic floats in a network known as Argo have given researchers the most complete view ever of heat in the upper ocean. Each Argo float is equipped with an array of sensors that collect oceanographic data, including temperature. Each float drifts in ocean currents, periodically descending to 2,000 meters (6,561 feet) and then returning to the surface to transmit its data via satellite. Argo currently has approximately 4,000 floats globally. 

Temperature readings from deeper than 2,000 meters still largely rely on a slow drip of data from ships and a handful of deepwater moorings fixed to the sea floor. These methods provide only tiny glimpses of what’s going on in a few select parts of the ocean. 

 So, to fill in this data gap, Argo scientists and engineers created robotic floats capable of descending to 6,000 meters (20,000 feet). These floats, called Deep Argo, are giving scientists a better understanding of how the deep ocean is responding to climate change. As of 2025, there are more than 200 active Deep Argo floats, but the team’s target is 1,200. The remaining 85% or so of the array has not been deployed due to a lack of funding, hampering the world’s ability to understand how heat is moving into and within the deep ocean. This understanding has important ramifications for predicting sea-level rise and could help improve the accuracy of models used to forecast the effects of climate change.

How does climate change cause ocean warming?

Since the Industrial Revolution, human activities such as burning fossil fuels have increased atmospheric concentrations of the greenhouse gas carbon dioxide by 50%, from roughly 280 parts per million (ppm) in 1750 to 430 ppm in 2025. Carbon dioxide, methane and nitrous oxide are called greenhouse gases because they absorb heat energy that would otherwise escape into space, like the glass roof of a greenhouse. As human activities emit more greenhouse gases, Earth’s atmosphere traps more of the sun’s heat, raising the planet’s average temperature. 

Being in direct contact with the warming atmosphere, the ocean has absorbed more than 90% of the excess heat from climate change since the 1970s. 

Three other factors contribute to the ocean’s ability to absorb heat: First, the volume of water in the ocean is massive (an estimated 321 million cubic miles of water); second, water has a higher heat capacity than air, meaning it can hold onto more heat than air by volume; and third, the water in the ocean circulates. This circulation folds heat from the surface into deeper waters. 

Where is the ocean warming the fastest?

The distribution of ocean heat varies from year to year. However, there are certain regions that, on average, are heating up faster than the rest of the ocean, such as parts of the Indian Ocean as well as the polar Arctic and Southern oceans. At Earth’s poles, increased ocean temperatures have contributed to the melting of glaciers, ice sheets and sea ice which have significant implications for sea level rise as well as, in the case of Arctic sea ice, geopolitics and global shipping. 

How do we know ocean warming is caused by climate change and not some natural phenomenon like El Niño?

The ocean warming trend scientists have observed is global in scale, steeper and longer lasting than the localized, short-term increase in sea-surface temperatures in the equatorial eastern Pacific that defines El Niño. 

Climate phenomena such as El Niño can temporarily exacerbate or mute (in the case of El Niño’s cooler sibling La Niña) the warming effect of climate change when they happen, but the long-term warming trend caused by climate change remains consistent. Put another way, the temporary spike in ocean surface temperatures caused by El Niño is like a wave on the back of the rising tide of climate change. For this reason, the coldest average sea-surface temperature from a La Niña year in the 2010s was still hotter than the warmest temperatures, including several El Niño years from the 1950s, 1960s, 1970s and 1980s.

What are the main impacts of ocean warming?

The impacts of ocean warming are numerous and are still in the process of being understood. Ocean warming is driving sea-level rise, intensifying extreme weather events such as tropical cyclones and hurricanes, accelerating the melting of Earth’s ice sheets, harming temperature-sensitive marine life such as corals and has the potential to alter fundamental patterns of ocean circulation that help regulate Earth’s climate. 

How does ocean warming impact sea-level rise? 

Ocean warming impacts sea levels in two main ways. First, a warmer ocean raises sea levels because water expands as it heats up — a phenomenon known as thermal expansion. About a third of the sea-level rise since 2004 is due to thermal expansion. Second, higher ocean temperatures are accelerating ice melt where ice sheets and glaciers meet the sea in Earth’s polar regions. As this ice melts, fresh water is added to the ocean, which increases sea level. It is important to note that the melting of floating sea ice does not contribute directly to sea-level rise. Rising sea levels are predicted to make coastal flooding in places like California more frequent and more severe.

How does ocean warming impact the weather?

A hotter ocean is thought to make tropical cyclones wetter and more powerful. This is because these storms, regionally known as hurricanes or typhoons, are fueled by ocean heat. The warmer the water is, the more energy is available for the storm. Warmer water also evaporates more easily, allowing the hurricane to suck up more moisture, making the storm dump more rain if it makes landfall. This effect is compounded by the fact that warmer air is also capable of holding more moisture than cold air. 

NOAA predicts hurricanes will deliver 10-15% more precipitation due to ocean warming. Recent hurricanes have caused severe flooding via prodigious rainfall totals: Harvey in 2017 delivered more than 60 inches in some locations, Florence in 2018 rained 35 inches and Imelda in 2019 tallied 44 inches. The Global Drifter Program is essential to forecasting, tracking and studying these powerful, life-threatening storms. The program’s drifting buoys are frequently launched directly into the path of hurricanes, deployed from U.S. Air Force “Hurricane Hunter” planes, to provide data that can generate life-saving warnings for U.S. coastal communities, ships and airplanes. 

Another way that ocean warming can impact Earth’s weather is by altering ocean circulation. The world as we know it requires large-scale ocean circulation, known as meridional overturning circulation, in which cold, dense seawater forms near the poles, sinks into the deep ocean because of its density and eventually rises back up to the surface where it warms, beginning the cycle again. These broad patterns maintain a turnover of heat, nutrients and carbon that underpins global climate, marine ecosystems and the ocean’s ability to mitigate climate change. 

For example, the Gulf Stream, a well-known ocean current that is part of meridional overturning circulation in the Atlantic, carries warm, extra-salty water from the tropics near Florida across the Atlantic Ocean towards Europe. This transfer of heat from the warm waters of the Gulf Stream is the primary reason Europe’s climate is warmer than parts of northeastern North America at a similar latitude.

But climate change is melting so much ice in Greenland that researchers worry it could significantly slow or even stop the density-driven ocean circulation in the Atlantic. Melting ice dumps freshwater into the ocean, making the North Atlantic less salty and, as a result, less dense. With enough meltwater, the worry is that North Atlantic waters might not get dense enough to sink, impeding or halting ocean circulation in ways that could make average temperatures in parts of Europe significantly colder. 

How is ocean warming impacting marine life?

One of the most basic ways that a warmer ocean impacts marine life is by lowering oxygen levels, making it harder for fish and other creatures such as corals to breathe underwater. The main reason this occurs is because as the temperature of water increases, it can’t hold as much oxygen. But it also happens because as the ocean’s upper layer gets hotter, it becomes less likely to mix with deeper waters. This formation of distinct layers in the ocean’s interior is called stratification and it has the effect of preventing oxygenated surface waters from delivering oxygen to deeper, darker parts of the ocean via mixing. This lack of oxygen can make it harder for species to make a living in the deep sea — or even survive. 

Because ocean species have different temperature preferences, those that are capable of moving in response to changing conditions have shifted their ranges in space and time. This can mean moving closer to the poles or swimming to greater depths to find cooler water, or it can mean expanding a species’ range into regions they previously couldn’t venture into. Such changes can have major implications for fisheries and marine ecology. 

Coral reefs and kelp forests are two key marine ecosystems that are negatively impacted by warming seas.

Coral reefs

Corals have become the unhappy poster children for the impacts of climate change on marine life. If average air temperatures (which, as discussed above, impact ocean temperatures) on Earth increase by 1.5°C (2.7°F) relative to preindustrial times, 70-90% of corals are predicted to die off. Earth is creeping closer to 1.5°C of warming and mass coral die-offs are becoming more common. If warming continues unchecked to 2°C (3.6°F) above preindustrial levels and beyond, the majority of our planet’s corals will be threatened. 

The reason that warmer waters are so bad for corals comes down to heat stress. Corals, which are actually colonies of tiny polyps embedded in a skeleton they secrete, evolved to thrive in warm tropical waters within a particular range of temperatures. If the water temperature pushes past the upper limit of the coral’s heat tolerance for extended periods of time, things start to go haywire. This has been occurring more frequently due to the higher incidence of marine heat waves under climate change.

When the water gets too hot for corals, they “bleach,” meaning the photosynthetic algae that live within their bodies jump ship. The algae are responsible for giving corals their color and produce most of their food. This is what turns bleached corals white, and losing this key food source makes corals much more likely to die. 

Kelp forests

 Just as ocean warming is bad news for corals, it has not been kind to kelp forests. These cool-water ecosystems are now under threat across much of the world including large portions of the U.S. West Coast and Tasmania. Kelp and the creatures that inhabit kelp forests typically thrive in cool, nutrient-rich waters. Higher than average ocean temperatures can directly kill temperature sensitive species, including kelps, and warming can reduce the delivery of nutrients, like nitrogen, to surface waters that many kelp need to thrive. Coastal upwelling is a form of ocean mixing that normally brings deep, nutrient-rich waters up to the surface, but warmer ocean conditions can reduce upwelling and weaken nutrient supply. This happens because as the ocean’s upper layers warm, especially when winds are weak, the thermocline (the boundary separating surface and deeper, nutrient-rich waters) is often pushed deeper. As the thermocline gets deeper, stronger or more sustained winds are required to drive upwelling. This can result in fewer nutrients being delivered to the surface waters, which is no help for kelp. 

Increased ocean heat has also had indirect impacts on kelp forest health and resilience. During a marine heatwave known as “the Blob” that began in 2014 and overlapped with the 2015-16 El Niño, sea stars along the west coast of North America started dying in droves due to a bacterial infection called sea star wasting disease. The loss of sea stars rippled through the ecosystem, allowing sea urchins, which eat kelp and are normally kept in check in part by predators like the sunflower seastar, to emerge from their hiding places and feed until many kelp forests were stripped bare. Kelp forests in much of Central and Northern California have still not yet recovered and have been replaced by “urchin barrens” which are less biodiverse and less productive. 

Though scientists have not established a direct causal link between ocean warming and this marine epidemic, recent research suggests that higher water temperatures exacerbate the disease’s impact on sea stars and can worsen outbreaks by enhancing growth of the bacteria behind the infection.

 Kelp is capable of recovering after fleeting disturbances such as short-lived heat waves or storms that can rip kelp off the seafloor, but when these stressors become more frequent and more severe, kelp forests don’t have enough time to recover and become less resilient as a result. Heat stress, fewer nutrients and marine epidemics like sea star wasting disease are some examples of how increasing ocean heat indirectly drives kelp’s decline. Scientists are actively working on ways to restore degraded kelp forests and coral reefs to restore the natural resilience of these iconic marine ecosystems.  

Is ocean warming making harmful algal blooms more frequent or intense?

In California, harmful algal blooms (HABs) can delay the opening of crab fishing season and kill marine mammals like sea lions and dolphins in droves. There are several different microorganisms responsible for HABs in California but one of the most prominent, a diatom named Pseudo-nitzschia, produces a neurotoxin called domoic acid that sickens birds, marine mammals and people when they consume contaminated fish or shellfish. Blooms of Pseudo-nitzschia appear to be becoming more common and severe in California — there have been significant domoic acid events four years in a row starting in 2022 — leading many to wonder whether climate change might be to blame. 

At this point, there is no clear evidence showing a direct causal link between warmer ocean temperatures and increased frequency or severity of marine HABs in California or elsewhere in the world. However, Scripps Oceanography researchers are studying the indirect ways that ocean warming might be altering ocean conditions in ways that may promote HABs, such as shifting patterns of ocean circulation and altering nutrient chemistry. 

Expert Reviewers

• Lynne Talley, Distinguished Professor of Oceanography at Scripps
• Sarah Purkey, Assistant Professor of Physical Oceanography at Scripps
• Nathalie Zilberman, Associate researcher in physical oceanography at Scripps
• Jennifer Smith, Professor of Marine Biology at Scripps
• Ed Parnell, Associate researcher in marine biology at Scripps
• Luca Centurioni, Director, Lagrangian Drifter Laboratory; Researcher in physical oceanography at Scripps
• Clarissa Anderson, Director, Southern California Coastal Ocean Observing System; Director, Cooperative Institute for Marine, Earth, and Atmospheric Systems; Researcher in biological oceanography at Scripps
• Mohammad Sedarat, PhD candidate at Scripps

Recent News Releases

• Ocean Warming Projected to Stall Expected Mangrove Recovery (Sept. 2, 2025)
• Meltwater Flowing Beneath Antarctic Glaciers May Be Accelerating Their Retreat (Oct. 27, 2023)

References

• EPA - Average Global Sea Surface Temperature 1880-2023
• NASA - Ocean Warming
• NOAA - Where does global warming go during La Niña?
• Intergovernmental Panel on Climate Change  “Global Warming of
• “Refined Estimates of Global Ocean Deep and Abyssal Decadal Warming Trends,” Johnson and Purkey, Geophysical Research Letters, 2024.
• “Drivers and distribution of global ocean heat uptake over the last half century,” Huguenin et al., Nature Communications, 2022.
• “Projections of an ice-free Arctic Ocean,” Jahn et al., Nature Reviews Earth & Environment, 2024.
• “Advancing ocean monitoring and knowledge for societal benefit: the urgency to expand Argo to OneArgo by 2030”, Thierry et al., Frontiers Marine Science, 2025.
• “Observing the full ocean volume using Deep Argo floats”, Zilberman et al., Frontiers Marine Science, 2023.
• “Deep ocean steric sea level change in the subtropical Northwest Atlantic Ocean”, Zilberman et al., Geophysical Research Letters, 2025.