Global Climate Change Explorer: Oceans and Water

The Heart of our Climate System

Oceans are the heart of our planet’s weather and climate systems. Covering more than 70% of the Earth’s surface, oceans absorb huge amounts of solar energy. Currents carry this heat around the globe, regulating climate in the same way your blood and circulatory system regulates your body’s temperature.

In the past, natural, long-term oscillations in the oceans’ capacity to store and transport heat have led to global temperature shifts, including past Ice Ages. Scientists are now observing changes in ocean currents probably related to the changing climate. The Atlantic current that includes the Gulf Stream, for example, had slowed down 15% in the last hundred years. Such oceanic changes will in turn influence our future weather and climate. Scientists are working hard to understand this powerful interplay between heat in the atmosphere and in the oceans.

Surface currents carry heat from the equator toward the poles. There, the water cools, sinks, and flows via deep ocean currents back toward the equator. The currents bring warmer-than-expected climates to places like Britain and cause cool summers along the U.S. West Coast.

Oceans and the Water Cycle

In addition to storing and transporting heat, oceans are fundamental to how water moves around the globe. These immense surfaces of water are constantly evaporating, creating the rainclouds and storm systems that power the world’s weather. Oceanic temperature conditions affect weather in distant parts of the world, leading to droughts in some regions and torrential rain in others. Warmer ocean waters spawn stronger hurricanes and other tropical storms, and more evaporation leads to more intense rainfall. As the oceans change due to climate alterations, rain and snowfall patterns will shift in response.

Hurricanes Maria and Jose, September 2017. Source: NASA

Ocean Acidification

The ocean has moderated the effects of excess carbon dioxide in the atmosphere by absorbing large amounts of the gas. This is now changing the chemistry of the ocean, increasing its acidity. Such changes affect marine life, interfering with some animals’ ability to form shells.

Overall, the world’s oceans have kept the planet from experiencing the full effects of a changing climate due to their ability to absorb heat and carbon dioxide. But this protection comes with its own costs, which scientists are still calculating.

DATASET 1
Today’s Sea Surface Temperatures

This map shows the general pattern of warm water along the equator growing progressively colder toward the North Pole and South Pole.

This map shows water temperatures today at the surface of the sea. The strip along the bottom gives the temperature scale, from 0°F (ice, shown as white) to 90°F (red). Source: Space Science and Engineering Center: University of Wisconsin-Madison

Tracking Sea Surface Temperatures

Researchers study sea surface temperatures (SST) to track annual shifts in ocean currents and temperatures. Such changes are related to seasonal weather patterns such as monsoons and droughts. These SST data are also crucial to understanding periodic events like El Niño and long-term shifts due to climate change.

Sea surface temperatures are measured by satellites using microwave sensors. Microwaves are a form of electromagnetic radiation that can be used as an indicator of temperature. The ocean’s surface gives off microwave energy; the frequencies and intensities of the energy, measured by satellite, reveal the temperature of the water.

In addition, a network of buoys and shore stations measure temperatures directly. The Exploratorium is part of the network—you can check the temperature data from our buoy.

More Sites About Ocean Temperature

• Hurricane and Tropical Storm Watch: National Weather Service U.S. hurricane and tropical storm tracking and warnings
• Microwave Remote Sensing: an explanation of how satellites collect SST and other measurements

DATASET 2
Changing Ocean Temperatures

Sea surface temperature (SST) is affected by many factors, including local weather, currents, and seasonal changes. To track changes, ocean scientists look at the difference between the actual temperature and the average temperature for that day of the year, based on data from the last several decades. The difference, called the temperature anomaly, indicates the trend—is it warmer or cooler than normal? This map shows those anomalies.

This map shows the difference between the expected and actual sea surface temperatures for the current week. Yellow, orange, and red spots indicate sea surface temperatures warmer than average, while green and blue spots indicate temperatures cooler than average. Source: National Oceanic and Atmospheric Administration (NOAA)

Why Measure Ocean Temperatures?

Slight differences between the average and actual sea surface temperatures are normal, but more severe anomalies can affect weather worldwide. For example, El Niño, a condition of warmer-than-average waters in the tropical Pacific, leads to unusual storm patterns in North America and unusually dry weather in Australia. Studying sea surface temperature anomalies can provide an early warning system for weather events.

During an El Niño year, the shaded areas experience significantly altered weather patterns. This map shows the effects in December through February. Source: National Weather Service

Tropical Storms

Warming air and ocean temperatures affect the formation of hurricanes and other storms. Hurricanes form when warm ocean water evaporates and rises from the ocean. The warmer the water, the more energy goes into the storm, so climate researchers expect the intensity of such storms to increase with rising ocean temperatures.

Marine Life in a Warming Sea

Sea surface temperatures also affect marine life. Fish generally stay in regions where the water temperature is comfortable for them and supports their favored prey. Shifting ocean temperatures can cause them to shift their ranges, disrupting interactions between species and affecting fisheries.

At the lower end of the ocean food chain, the warming ocean has mixed impacts on phytoplankton, the single-celled algae that feed the ocean’s ecosystems: some species explode in harmful algae blooms while others become scarce. In addition, the overall timing and location of plankton growth is changing.

Coral Bleaching

Unusually warm water can devastate coral reefs, which can’t migrate to stay in favorable temperatures. Higher temperatures are the main factor in coral bleaching. Corals live in partnership with special kinds of algae, which live within their cells and contribute energy from photosynthesis to the coral. The beautiful colors of coral are due to these algae. If the water gets too warm, the coral expel the algae. This deprives the coral of both energy and color—a condition called “bleaching.” Coral can recover from short-term bleaching, but if the warm water persists, the coral will eventually die.

South Pacific coral shown before and after bleaching. Source: XL Catlin Seaview Survey

More Sites About Ocean Temperature

• Weather impacts of El Niño and La Niña (ENSO)
• NASA page on hurricanes and tropical storms
• What Is Coral Bleaching?
• Hotspot’s Sea Surface Temperature Charts: Explains the connection between deep-water fishing and sea surface temperatures.

Dataset 3
Drought in the United States

Each color on this map represents a different level of drought. Scientists determine drought based on a combination of factors: they measure precipitation levels, soil moisture, stream flows, vegetation health, and more, and combine these with local moisture reports into an overall drought rating.

This map shows current drought conditions in the United States. The white areas are not experiencing drought. Yellow areas are drier than normal, while the darkest areas are in exceptional drought. This map is updated weekly. Source: National Weather Service Climate Prediction Center

Rainfall Changes to Come

Climate scientists have predicted that climate change will bring increasing overall aridity, or dryness, and historical records over the last 50 years bear this out. As rain and snow decrease, warmer temperatures increase evaporation from the soil. Together, these changes lead to longer and more severe droughts.

Increasing aridity will affect water supplies and change our ability to grow crops in many locations. Forests are also affected, becoming more vulnerable to disease and wildfire.

Left: Men gather water in Mali amid drought and sandstorm. Source: Velio Coviello (CC BY-SA 3.0). Right: Forest fire in Australia. Source: Quarrie Photography | Jeff Walsh | Cass Hodge (CC BY-NC-ND 2.0)

Wetter or Drier?

Like many climate effects, drought levels vary from place to place, and not every place will experience the same drying effects. In some regions, such as Australia, predictions include a paradox: the future could bring both more intense rainfall—leading to floods—and more drought between the rains. In some areas, the type of precipitation is changing, shifting from snow to rain, for example—which has implications for water storage and use.

Flooding in Haiti. Source: UN Photo/Logan Abassi (CC BY-NC-ND 2.0)

In general, precipitation patterns are likely to become less predictable and more extreme, with wet regions (the tropics and higher latitudes both north and south) becoming wetter and dry regions (the middle latitudes) drier.

By tracking drought through maps like this one, researchers are trying to understand and predict regional differences in climate change.

More Sites About Drought

• National Drought Portal: drought and wildfire information for the United States
• 7 Reasons We’re Facing a Global Water Crisis
• From Not Enough to Too Much: National Geographic article explains the world’s water crisis
• How Drought Affects Your Forest
• Climate change forecast: More intense deluges and downpours Down Under

Dataset 4
Ocean Heat Content

This graph shows how the ocean temperatures below the surface have varied from the average of the last half-century. On this graph, the zero line represents the average global heat content of the ocean (measured in the top half mile, 1955–2006). The bars show how each year’s temperature varied from that average, either lower or higher. Source: NOAA

Global Warming and Ocean Warming

The ocean is on average more than two miles deep. Measuring water temperature at the surface, where it’s warmed by the sun and air, gives only part of the picture.

When climate scientists calculated how much the levels of carbon dioxide in the atmosphere should heat up the air, their results didn’t match observed warming: air temperatures should have been higher than they were. Where was the missing heat? It turns out that the ocean has absorbed it, like a bucket of water absorbs heat when left out in the Sun. To find it, researchers had to look not just at the surface, but half a mile down into the sea.

Water temperature at the surface of the ocean is an important driver of weather, but the temperature farther down has long-term effects on the planet. Water holds onto heat energy—that bucket of water warmed by the Sun stays warm long after the evening air has cooled down. The ocean, by absorbing excess heat from the air, has saved us from worse warming on land, but the heat is now affecting oceanic systems.

Effects of a Warming Ocean

The rising heat content of the ocean is responsible for most of the sea level rise that coastal communities are experiencing. This is because water expands as it heats up, so a set amount of water fills more of the world’s ocean basins.

Warm water also affects ice in the polar regions, melting and thinning ice shelves and sea ice (floating ice). In the last several decades, the extent of this floating ice in the Arctic Ocean has steadily dropped—a change visible in satellite views of the region. What was once an extreme is now a norm, and climate scientists predict that by mid-century, the Arctic will be completely free of ice during the summer.

This visualization shows the extent of Arctic sea ice between 1984 and 2016. White shows older, thicker ice, while the blue-gray color is first-year ice. Source: NASA

Because of the heat-holding properties of water, the ocean will hold onto the increased heat for a long time, moderating the immediate effects of climate change on land but extending the impacts farther into the future.

Tracking Ocean Temperatures

To determine ocean temperatures, researchers look at data from two main sources. Orbiting satellites record the height of the ocean’s surface using radar. Knowing the sea level, they can determine the water temperature, because water expands a certain amount with every degree of temperature increase. In addition, a fleet of more than 3,500 floating sensors deployed across the world’s oceans measure temperatures directly. Having data from more than one source lets researchers cross-check their results, giving them a clearer, more robust understanding of ocean temperature.

These dots mark the locations of 3,781 Argo floats (as of 2018) deployed by 25 different countries across the world’s oceans. The floats measure ocean temperature at different depths. Source: Global Ocean Observing System

More Sites About Ocean Heat Content

• Ocean Heat Content: a good overview article
• Earth’s Big Heat Bucket: more about ocean heat content and why it matters
• Warm Water and Poisoned Clams: Exploratorium article exploring the consequences of one local ocean warming episode
• How Will Climate Change Impact Plankton? Smithsonian article on the effect of higher ocean temperatures on plankton
• How the Argo float array works

Dataset 5
Ocean Acidification

Growing levels of CO₂ in the atmosphere are affecting ocean chemistry. This graph shows changes in ocean pH (acidity) over a 28-year period from a monitoring station in Hawaii.

Ocean pH levels at this station rise and fall seasonally (orange line), but overall they show a trend of falling pH, or growing acidity (white line). Source: Climate Central, based on data from Dore 2009

How CO₂ Affects the Sea

At the surface of the ocean, there is a constant interchange between the air and the water. Water molecules evaporate and enter the air, and molecules from the air dissolve into the water—molecules including oxygen and carbon dioxide. Once in the water, carbon dioxide reacts with seawater to produce carbonic acid, CH₂CO₃. Much of the CO₂ pumped into our atmosphere by burning fossil fuels has been absorbed into the ocean, causing its chemistry to change: the water is moving from the alkaline end of the pH scale toward the acidic end. Scientists estimate that since 1900, surface waters have seen a 0.1 drop in pH, which translates into a 30% increase in acidity.

This animation is a computer model of changing ocean chemistry. It shows our best understanding of acidification from 1800, projected into 2100. Source: International Pacific Research Center

Marine Life and the New Chemistry

The ocean’s changing chemistry has important effects on many marine species. Marine animals with calcium-based shells or structures—including shellfish, corals, and some plankton—need carbonate, a molecule made of carbon and hydrogen, to make their shells. When CO₂ mixes with seawater to form carbonic acid, it uses up carbonate, reducing the availability of the molecule in the water. With less of this critical building block, marine animals can’t build or maintain strong shells.

This ocean butterfly, or pteropod, is just one form of ocean life that is vulnerable to changing ocean chemistry. Source: R. Giesecke (CC BY 2.5)

Scientists are just beginning to learn how other marine species might be affected by this change in chemistry; they’re studying reproductive rates, sensory systems, and food-chain effects, looking at many species of fish and invertebrates.

To learn more about ocean chemistry and its effects on marine life, try the Shell Shifts and Ocean Acidification in a Cup Science Snacks.

More Sites About Ocean Acidification

• What Is Ocean Acidification?: NOAA article describing ocean acidification
• A Primer on pH: NOAA article covering basic acid-base chemistry
• Three Tools to Solve the Mystery of Ocean Acidification
• Ocean Acidification: Smithsonian overview of ocean acidification, including effects on marine life

Background: Oceans & Water