Ocean Warming

What is ocean warming?

Rising ocean temperatures are driving unprecedented changes in global marine ecosystems, sea levels, and weather patterns. As heat transforms the ocean, threats to food supplies, economies, and weather multiply, putting human and environmental health at risk.

The ocean is a vital part of Earth’s climate system: It helps moderate climate and slow the impacts of global warming. Covering 70 percent of the planet, the ocean soaks up heat trapped in the atmosphere by carbon dioxide and other greenhouse gases.

Increasing ocean heat is closely linked to increases in atmospheric greenhouse gas concentrations, making the ocean an excellent indicator of how much Earth is warming. Since 1971, the ocean has absorbed 90 percent of the excess energy added to Earth's climate by burning fossil fuels and other human activities.

The ocean’s surface layer, home to most marine life, takes most of this heat. As a result, the top 700 meters (2,300 feet) of the global ocean has warmed about 1.5°F since 1901. And scientists recently found that the rate of warming in the top 6,500 feet of the ocean over the past few decades was about 40 percent higher than previously estimated.

Water can hold more heat than land or air, so it warms more slowly, but this rate is still alarming. The ocean won’t hold this heat forever. It will eventually be released, feeding back into the climate system and causing further warming.

Why is ocean warming important?

Even the seemingly slight increase of 1.5°F in ocean temperature represents an enormous amount of heat, one large enough to transform marine biodiversity, change ocean chemistry, raise sea levels, and fuel extreme weather. Earth is already experiencing the consequences of these changes, impacting property, lives, and livelihoods—and not just in coastal areas.

Rising ocean temperatures are connected to some weather extremes and can lead to more intense hurricanes, heavier rainfall, and snowstorms. Warmer sea surface temperatures influence weather patterns and shift precipitation, causing some regions to experience intense rainstorms and flooding and exacerbating drought conditions and wildfire risks in others.

Warmer ocean waters also contribute to rising sea levels, and this thermal expansion could be the biggest driver of sea level rise over long time scales. Because water becomes less dense and expands as it heats up, it occupies more space and causes sea levels to creep up. Polar ice sheets thin and erode in response, pushing levels even higher. Sea level rise has different rates and magnitudes in different places; scientists estimate sea level could rise at least 15 inches by 2100 in some areas, potentially displacing millions worldwide.

Marine life will suffer the most due to unprecedented ocean warming. Fish species now migrate north to find cooler temperatures and food sources, impacting communities and economies that depend on fishing. Rising ocean temperatures contribute to oxygen-depleted ocean dead zones in large coastal and open ocean areas, rendering them largely uninhabitable by marine life.

Hotter oceans can hold more carbon dioxide, which causes seawater to become more acidic. This phenomenon, known as ocean acidification, harms corals, clams, snails, and many other marine organisms, mainly by dissolving their calcium carbonate shells and skeletons.

Warmer ocean waters also encourage frequent, lengthy marine heatwaves, where temperatures can jump several degrees or more above average. These prolonged periods of heat wreak havoc on marine health and increase the risk of massive bleaching events on Australia’s Great Barrier Reef and other sensitive coral ecosystems.

MIT-WHOI Joint Program graduate students Alice Alpert (left) and Liz Drenkard (center), and U.S. Fish and Wildlife Service representative Kelsie Ernsberger (right), snorkel above a large coral at an atoll in the central equatorial Pacific in 2012. Alpert, Drenkard, and WHOI scientist emeritus George Lohmann, with the conservation organization Pangaea Exploration, sailed 2000 miles to these remote coral islands. They sampled seawater and coral and deployed instruments, part of a larger study by WHOI scientists Anne Cohen, Kris Karnauskas, and Delia Oppo aimed at gaining a better understanding of climate change and potential impacts of ocean warming and acidification on coral reef communities. (Photo by Chip Young, NOAA © Woods Hole Oceanographic Institution)

Is the ocean warming the same everywhere?

The ocean is warming unevenly, with rates varying according to the depth and region of the world.

Scientists use measurements from satellites, ships, and an array of tools and technology combined with historical records to understand how and where ocean temperatures are rising.

Their work reveals temperatures are rising throughout the ocean, but the surface down to about 10 feet is warming the most. Regional fast-warming hot spots include parts of the North Atlantic and Indian Ocean, and the Arctic and Southern Oceans, where hotter ocean temperatures contribute to ice sheets and glaciers melting.

As polar ice melts into the sea, the freshwater changes the density of seawater. Cold, salty water usually sinks, but when it’s fresher and warmer, it changes ocean circulation patterns and heat distribution throughout the ocean.

The powerful currents that move and distribute heat throughout the ocean also drive uneven warming. When those currents carry warm water masses to cooler latitudes and deeper levels, those areas begin heating up.

How does ocean warming impact the weather?

Rising ocean temperatures are making some extreme weather events worse by supercharging storms and altering global weather patterns.

Warm surface waters provide energy for hurricanes and other tropical cyclones, increasing their frequency and severity. The 2020 Atlantic hurricane season, which produced a record 30 named storms, may foreshadow future hurricane activity. The high winds, immense rainfall, and flooding associated with Hurricane Harvey in 2017 and Hurricane Florence in 2018 will likely become more common and damaging as the ocean heats up.

Weather patterns are changing due to warmer ocean temperatures because the ocean is key to the movement of water around the globe. Due to its enormous size, the ocean helps power weather by driving the planet’s evaporation and precipitation cycles.

Higher sea surface temperatures increase evaporation and add additional moisture to the atmosphere over the oceans. This extra water vapor boosts the precipitation dumped by rainstorms and blizzards on coastal areas and inland locations.

While wetter areas will experience more precipitation, dry regions of the world will likely become drier due to the changes ocean warming causes to the water cycle. These arid conditions will cause areas like the southwestern United States to experience prolonged droughts and increased wildfire risks.

There may also be a link between warming Arctic waters and the polar vortex—icy blasts of cold air—over the United States and Europe. Scientists are examining whether the lack of sea ice due to warmer ocean temperatures weakens the jet stream, which usually keeps Arctic air in check and enables that air to plunge south instead.

(© CNN)

How does ocean warming threaten marine life?

The majority of ocean life thrives in the ocean’s surface layer, the part that’s warming the fastest. Sensitive to even tiny temperature changes, marine organisms are quickly trying to adapt to a hotter ocean. Three threats may fundamentally change the sea and the life within it: ocean acidification, coral bleaching, and marine heatwaves.

Spurred by ocean warming, these hazards are altering critical ocean systems from primary productivity to the distribution of species. The impacts on fisheries and aquaculture threaten food security for millions and the global economies that depend on them.