Ocean Warming: A Cascade of Disasters

How much is the ocean warming? How fast is it hotting up? What is thermal inertia? How long can oceans continue to store Earth’s excess heat and CO2? What are the effects of warming seas? We explain the answers to these questions, and more.
Ocean Temperature Satellite Reading
Scientists use satellite imagery to measure sea temperature and salinity. Photo: NASA

The world’s seas and oceans cover 71 percent of the planet’s surface. They occupy an area of around 360 million square kilometers (139.5 million sq mi) and have an average ocean depth of 3.6 kms (2.25 miles). In total, they contain about 1.3 billion cubic kilometers (328 million cubic miles) of water.

Numerous studies demonstrate that the only logical explanation for ocean warming is man-made global warming – nothing else is consistent with the data.

In particular, the simultaneous warming of the upper layers of all seven seas cannot be accounted for by natural climate variability alone. Natural climate swings can only explain at most about ten percent of ocean warming. The rest is due to human activity: that is, greenhouse gas emissions from our power plants, our cars and our factories. 1

This should come as no surprise. After all, 20 of the 21 hottest years in NASA’s 135-year history of record-keeping, have occurred since 2001. 2

The latest research indicates that, since 1970, oceans have been absorbing more than 90 percent of the thermal energy created by global warming, while only 4 percent has been absorbed by the atmosphere and the land. 3

The southern hemisphere in particular has experienced intense ocean warming over the past decade and a half, notably in the mid-latitude regions of the Pacific and Indian Oceans. And oceanographers believe that the situation will only deteriorate over the next decade as greenhouse gases accumulate in the atmosphere.

This infusion of heat has led to rising temperatures throughout the water column. This has triggered a cascade of disasters across the oceans.

Unfortunately, if the marine environment suffers because of ocean warming, humans are likely to suffer the most.

Pacific islanders are carefully monitoring the rise in sea levels – 30-40 percent of which is due to thermal expansion, caused by ocean warming – while coastal nations are already beginning to feel the effects on their shoreline infrastructure as well as their mangrove forests and local biodiversity of wildlife.

Ocean warming also impacts fish stocks, which is particularly bad news. Fish provides 1.5 billion people with about 20 percent of their intake of animal protein, and 3 billion people with roughly 15 percent of their animal protein. 4

Ocean Heat Content

More than 90 percent of all heat energy that reaches the surface of the Earth is soaked up by the ocean. Initially, this thermal energy is absorbed at the surface and near-surface, but eventually it spreads into the depths, thanks to thermohaline circulation deep-water currents. These currents – a vital component of Earth’s climate system – distribute warm and cold water around the world, thus keeping temperatures as even as possible.

The total amount of heat taken in by the oceans is called “ocean heat content,” and by measuring water temperature, we can gauge the amount of heat in the water at any particular time and place.

Ocean temperature — especially sea surface temperature (SST) – plays a key role in climate variability. For example, tropical weather cycles like the El Nino-Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD) and the travelling Madden-Julian Oscillation (MJO), are heavily influenced by areas of warm humid air rising from the sea in different parts of the Pacific and Indian oceans.

Ocean Warming and Thermal Inertia

Because of their huge mass, oceans can absorb extremely large amounts of solar heat without an immediate rise in temperature. 5 The near-surface water takes only decades to heat up in response to greenhouse gas emissions, but the deep ocean is likely to take centuries. This resistance to a change in temperature is known as thermal inertia. For more, see: How Do Oceans Influence Climate?

In effect, oceanic thermal inertia protects the terrestrial world from the effects of climate change, but this protection can only last so long. No one is suggesting that the ocean is about to stop absorbing the heat from our CO2 emissions, but many scientists are wondering how long can it can go on sheltering us from the effects of global warming, before it suffers irreparable damage to its ecosystems and chemistry.

The point is, although heat taken in by the ocean can be moved around by currents, winds and tides, it doesn’t disappear. It always reappears in some form. Either, it melts ice or evaporates water, or it reheats the atmosphere. So, it can warm the planet decades, or even centuries, after it was initially absorbed. 6

Whatever happens, sooner or later, any heat stored in the ocean will eventually be released. And the more heat that goes into the ocean, the more that comes out. In this sense, thermal inertia can lure us into a false sense of security. It can tempt us into thinking we’re coping quite well with global warming until we realize that we’ve only seen a tiny fraction of it. Because 90 percent of it is still in the ocean, waiting to be released.

How Much is the Ocean Warming?

One of the most recent studies, conducted by an international team of 14 scientists, shows that seawater temperatures are now at record levels. 3

The study found that the global rate of ocean warming is the equivalent of five Hiroshima atomic bombs being dropped into the water every second.

Lijing Cheng, the study’s lead author and an associate professor at the International Center for Climate and Environmental Sciences at the Chinese Academy of Sciences, said: “the amount of heat we have put into the world’s oceans in the past 25 years is equivalent to 3.6 billion Hiroshima atom-bomb explosions.”

The American Meteorological Society (AMS) is equally gloomy. In its State of the Climate report (2019) the AMS warns that global upper ocean heat content is currently at an all-time high. This reflects the growing accumulation of heat in the top 700 meters (2,300 feet) of the ocean.

In addition, the AMS points out that heat content is also at a record high in the deeper layer, from 700 meters to a depth of 2000 meters (6,500 feet). 7

Over the past quarter of a century (1993-2019) ocean heat-gain rates were 0.36 to 0.41 watts per square meter for the upper ocean layer (0–700 m), and 0.14 to 0.32 for the mid-layer (700–2,000 m). For ocean water between 2000 and 6000 meters, the increase was 0.06 watts per square meter for the period from June 1992 to July 2011.

The IPCC’s Fifth Assessment Report (2013) states that warming in the upper ocean (0-700 m) accounts for about 63 percent of the total thermal energy generated by climate change from 1971 to 2010, while warming from 700 meters down to the ocean floor adds another 30 percent. The panel also warns, in its 1,300-page Special Report on the Ocean and Cryosphere in a Changing Climate (2019), that acidification has increased, and that ocean deoxygenation now extends to a depth of 1,000 meters (3,300 feet).

How Fast is the Ocean Warming?

According to Cheng’s study, the oceans experienced consistent warming between 1955 and 1986, but since then, the pace has accelerated rapidly over the last few decades. Between 1987-2019, ocean warming was four and a half times greater than during the previous 30-year period. 3 The IPCC are more cautious. They say simply that the rate of ocean warming has more than doubled since 1993. 8

Whatever the precise rate of increase, the evidence shows clearly that the oceans are hotting up faster than ever before. The past decade has been the warmest on record for ocean temperatures, globally. What’s more, the hottest five years ever have all come in the last five. 9

How Long Can Oceans Continue to Store Earth’s Excess Heat and CO2?

As we have seen, the ocean is a great buffer. It’s like a rich parent that pays 90 percent of all the bills run up by a profligate child. And just like the child, who thinks the parent has endless patience and a bottomless wallet, we can fall into the trap of thinking that the ocean has an endless capacity for absorbing our excess heat and CO2.

Unfortunately, there are signs that suggest the oceans’ ability to soak up the effects of climate change may be declining. As a result, it could start emitting heat and reducing its absorption of CO2, which would lead quickly to an increase in Earth’s temperature around the world.

At present, the ocean absorbs 31 percent of humanity’s CO2 emissions, with most ending up in the Southern Hemisphere oceans, where winds bury it deeply. 10 But as waters get warmer they store less CO2. And with increasing atmospheric variability from weather cycles like the Southern Annular Mode and the Madden-Julian Oscillation, winds could abate or reverse. This could lead to a major surge of heat-trapping CO2 into the atmosphere — not unlike the release of ocean carbon that triggered the last Ice Age.

Sea surface temperature (SST) is a key metric. Because the ocean is so vast, and because water requires more energy to heat up than air, the SST has only warmed by about 0.55 degrees Celsius (1-degree Fahrenheit) over the past century.

But now the rate of surface warming is speeding up. It’s already about 24 percent faster than it was a few decades ago, and the pace is quickening. 11

This means the ocean is releasing more heat into the air through evaporation and, because warm water absorbs less CO2, it’s sequestering less heat-trapping gas. It’s a double-whammy of extra warming.

Map of average sea surface temperature
The graph shows how the average surface temperature of the oceans has changed since 1880. The average temperature for the period 1971-2000 is used as the baseline. Image Credit: NOAA, 2016

Effects of Ocean Warming

The marine ecosystem is showing obvious signs of stress because of all the heat it contains.

Ocean warming is thought to be the reason for an estimated 40 percent reduction in phytoplankton population since 1950. 12

Warming seas are also a major contributor to the melting of the Antarctic Ice Sheet, notably the Thwaites and Pine Island glaciers on the west coast of Antarctica, as easterly winds slacken.

Marine heatwaves are increasing in both frequency and intensity, with calamitous effects on mangroves and corals.

In addition, the ocean has swallowed enormous amounts of man-made carbon dioxide since the 1980s, which has led to a decline in the ocean’s pH level. This ocean acidification is threatening coral reefs throughout the tropics and sub-tropics.

Also, blue carbon ecosystems are being degraded by a combination of extreme weather events and intensifying weather cycles, both of which are influenced by climate change, especially in the tropics.

Meanwhile, ocean deoxygenation (hypoxia) is spreading in the open sea, posing a danger to millions of marine animals and thus to the marine food web. This is because warmer water increases ocean stratification, which prevents the spread of oxygen-rich surface water to lower depths. Low-oxygen zones are predicted to double in size by the end of the century.

The extra thermal energy released by higher SSTs is also leading to more powerful storms. In the Pacific and Indian oceans, for example, tropical cyclones are projected to increase in intensity, with more rainfall and higher storm surges. 13

In addition, due to warmer-than-average sea surface temperatures in the tropical Atlantic Ocean and Caribbean Sea (and several other factors), the Atlantic Ocean is producing more hurricanes than ever and their maximum sustained winds are getting stronger. 14

According to the US National Oceanic and Atmospheric Administration (NOAA), the 2020 Atlantic hurricane season shattered records and caused devastation across North America. It featured 30 named storms, 12 of which made landfall, exceeding the record of 28 named storms in 2005. Thirteen of the storms grew into hurricanes, the second-highest number on record, second only to the 15 in 2005. Six of these hurricanes were major events. 15 By comparison, an average Atlantic hurricane season (June-Nov) has 12 named storms, six hurricanes and three major hurricanes.

For an explanation of the 7 major impacts of climate change on the marine world, see our article: Effects of Global Warming on Oceans.


  1. “Human-induced global ocean warming on multidecadal timescales.” Gleckler, P., et al. Nature Clim Change 2, 524–529 (2012). []
  2. NASA. []
  3. “2018 Continues Record Global Ocean Warming.” Cheng, L., Zhu, J., Abraham, J. et al. Adv. Atmos. Sci. 36, 249–252 (2019). [][][]
  4. How much fish is consumed worldwide? Greenfacts.org []
  5. “Bad News Good News.” NASA Earth Observatory. April 24, 2006. []
  6. “Climate Change: Ocean Heat Content.” []
  7. “State of the Climate” (2019) American Meteorological Society (AMS) []
  8. IPCC. Special Report on the Ocean and Cryosphere in a Changing Climate (2019) []
  9. “Oceans.” Ocean temperatures hit record high as rate of heating accelerates. []
  10. “Global Ocean Absorbing More Carbon.” []
  11. “How fast are the oceans warming?” Lijing Cheng et al. Science 363(6423):128-129. January 2019. []
  12. Ocean greenery under warming stress.” Nature.com []
  13. Tropical cyclones and climate change”. Walsh, K. J. et al. (Dec 2019). Tropical Cyclone Research and Review. 8 (4): 240–250. []
  14. “Global increase in major tropical cyclone exceedance probability over the past four decades.” James P. Kossin, et al. PNAS, 2020 []
  15. “Record-breaking Atlantic hurricane season draws to an end.” []
Share on facebook
Share on twitter
Share on linkedin
Share on whatsapp
Share on email