What If? Lab
What if warmer seas changed ocean life?
The sea is a home for corals, fish and whales. What happens to them when the water gets too warm? Follow the story in Olly's lab!
When seawater gets too warm, corals can lose the tiny algae that feed them and turn white. Fish and shellfish move toward cooler water, and a long ocean heatwave can leave whole food webs short of food. Explore what scientists have seen, and what they still study about storms.
Ocean warming acts on marine life through heat stress (coral bleaching), shifting habitat (species moving poleward and deeper) and food-web disruption during marine heatwaves. NOAA confirmed a fourth global bleaching event in 2024. How warmer seas may change hurricanes is an active research question with projections, not certainties.
The big question
Corals, fish and whales all live in the sea. Each one likes water that is just the right warmth.
What if the sea got warmer? What would the sea animals do?
Sea creatures live where the water suits them. Corals in sunny tropical reefs, cod in the cool North Atlantic and krill near Antarctica all depend on water that stays within the range they need.
So what happens when the sea warms up? This thought experiment follows real observations: corals, fish on the move, ocean food webs, and a question scientists are still working on, about storms.
Marine organisms have thermal tolerances, and many live close to their upper limits. When seawater warms beyond them, the responses scale from individual stress to shifts in where species live and to restructured food webs.
This lab is a thought experiment built on observations: coral bleaching records from NOAA Coral Reef Watch, a U.S. indicator of species' distribution shifts, and NOAA's account of the North Pacific marine heatwave that began in 2014. It ends with tropical cyclones, where findings are projections with stated confidence.
Explore the idea
Make a prediction
The sea gets too warm for a coral. What happens to the coral?
When a coral reef's water stays too warm, what happens to the corals?
Prolonged heat stress on reef-building corals most often causes…
- They turn white
- They grow much faster
- They swim away
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Established fact
Too-warm water makes corals turn white.
When water is too warm for too long, corals push out their algae and bleach white (NOAA).
Thermal stress is the leading cause of coral bleaching, the expulsion of symbiotic zooxanthellae (NOAA).
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Established fact
Many fish are moving to cooler water.
Tracked fish and shellfish off the northeastern U.S. have shifted north about 110 miles on average since the early 1970s (NOAA Climate.gov).
A USGCRP indicator from NOAA data shows 140 species shifting poleward and deeper off the northeastern U.S. and in the eastern Bering Sea.
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An idea scientists discuss
Warmer seas might make the strongest storms stronger.
Scientists' models suggest warmer seas might make hurricanes stronger and rainier on average, but they are still studying it.
Projected increases in cyclone intensity and rain rates carry medium to high confidence; attribution of past Atlantic changes is not yet established (NOAA GFDL).
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Established fact
Sea heatwaves happen. One lasted years in the Pacific.
In 2014 a huge marine heatwave, nicknamed 'the Blob', formed off the U.S. West Coast, with water up to 7 °F warmer than normal.
The North Pacific marine heatwave that began in 2014 reached up to 7 °F above normal and exceeded the extent of the strongest El Niño patterns of the previous 30 years (NOAA Fisheries).
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Established fact
In 2016, warm water spread across much of the world's ocean.
During 2016, about 70 percent of the world's oceans had a marine heatwave of some kind.
A global evaluation found about 70 percent of the world's oceans affected by a marine heatwave to some degree in 2016 (NOAA Fisheries).
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An idea scientists discuss
Scientists think sea heatwaves might happen more often.
Studies suggest that in the North Pacific, marine heatwaves might become more common, cover larger areas and last longer as the climate warms.
Climate studies suggest North Pacific heatwaves may become more frequent, extensive and persistent; NOAA Fisheries' chief scientist called the Blob years a 'climate stress test'.
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Established fact
Hot water makes corals push out their algae and turn white.
When water is too warm, corals expel the algae in their tissues and turn white. Warmer water from climate change is the leading cause of bleaching.
NOAA identifies increased ocean temperature caused by climate change as the leading cause of coral bleaching.
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Established fact
From 2023 to 2025, very many reefs around the world felt too much heat.
From 2023 to 2025, heat stress strong enough to bleach corals reached about 84 percent of the world's reef area: the biggest global bleaching event ever recorded.
NOAA Coral Reef Watch: 1 Jan 2023 to 30 Sep 2025, bleaching-level heat stress over ~84.4% of global reef area, mass bleaching in at least 83 countries and territories; the fourth global event, confirmed 15 April 2024.
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Established fact
Corals can get better if the heat does not last too long.
If the bleaching is not severe, corals have been known to recover; if the stress continues, they eventually die.
Recovery is possible after mild bleaching; prolonged algae loss under continued stress leads to coral death (NOAA).
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Established fact
Many sea animals are moving north or deeper.
Off the northeastern U.S., tracked fish and shellfish have moved north about 110 miles on average since the early 1970s, and many have moved deeper.
USGCRP indicator from NOAA data: northeastern U.S. species +110 miles north since the early 1970s; eastern Bering Sea +19 miles since the early 1980s; both moving deeper.
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Established fact
When small fish move, bigger hunters may follow.
As smaller prey animals move to new waters, the predators that eat them may follow.
Relocation of smaller prey species can draw predator species after them (NOAA Climate.gov).
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An idea scientists discuss
Fishing towns might need to fish in new places.
If fish keep moving, people who fish might need to travel farther or catch different kinds of fish. Climate.gov notes that changing water temperatures threaten fisheries.
Continued shifts might move fishing grounds away from established ports; NOAA Climate.gov notes that temperature-driven habitat change threatens commercial and recreational fisheries.
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Established fact
Warm water brought fewer krill, so many animals had less to eat.
In the Blob, warm water brought leaner plankton and fewer krill, while fish needed more food than usual.
Warming accelerated fish metabolism as plankton became leaner and krill declined, reducing the lipids salmon and others need (NOAA Fisheries).
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Established fact
Some whales changed what they ate.
When krill and other deep-water food declined, some whales switched to eating anchovies and other fish in shallower water nearer the shore.
Whales off the West Coast responded to declining krill and deep-water forage by feeding on anchovy closer to shore (NOAA Fisheries).
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An idea scientists discuss
A sea heatwave might be a sneak peek of the future.
NOAA Fisheries' chief scientist sees the Blob years as a preview of conditions that might become common as the climate warms.
NOAA Fisheries' chief scientist, Cisco Werner, describes the Blob years as a preview of conditions likely to become routine with warming; how ecosystems will adapt is still being learned.
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Established fact
Hurricanes grow over warm sea water.
Hurricanes form over relatively warm sea surfaces.
Tropical cyclones form over warm sea surfaces, which is why their response to ocean warming is studied (NOAA GFDL).
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What the model shows
Computer models say storm rain might get heavier.
Climate models project that with 2 °C of warming, hurricanes might be 1 to 10 percent stronger on average and rain about 14 percent harder near the storm.
GFDL summary of model projections for 2 °C: intensity +1 to 10 percent, rain rates +14 percent (6 to 22 percent), larger share of Category 4–5 storms; total storm numbers may fall or change little.
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An idea scientists discuss
Scientists are still finding out about storms.
Scientists say it is too early to be sure that warming has already changed Atlantic hurricanes beyond their natural ups and downs.
It is premature to conclude with high confidence that greenhouse gases have changed past Atlantic hurricane activity beyond natural variability (NOAA GFDL).
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Established fact
Satellites measure how warm reefs are.
NOAA Coral Reef Watch uses satellite data to map heat stress on coral reefs around the world.
NOAA CRW's 5 km satellite products map maximum accumulated heat stress and bleaching alert levels globally.
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Established fact
Climate change is the biggest danger to coral reefs.
NOAA calls climate change the greatest global threat to coral reef ecosystems.
NOAA identifies climate change as the greatest global threat to coral reefs, through warming, more frequent bleaching and disease, and ocean acidification.
In our world
Corals look like rocks, but they are animals. Tiny plant-like algae live inside them.
The algae make food for the coral. They also give the coral its colors.
Reef-building corals are animals that live in partnership with tiny algae called zooxanthellae. The algae live inside the coral's tissues, are the coral's main source of food, and give it its color. The coral and its algae depend on each other to survive.
Out in the open sea, scientists track where fish and shellfish live year after year. Small animals like krill and plankton feed bigger ones, so changes at the bottom of the food web can reach the animals at the top.
Corals and their symbiotic zooxanthellae exchange nutrients: the algae are the coral's primary food source and its pigmentation (NOAA). The partnership is sensitive to temperature, light and nutrients.
Other species track temperature through their ranges. A U.S. Global Change Research Program indicator, built on NOAA survey data for 140 marine species off the northeastern U.S. and in the eastern Bering Sea, follows each year's average latitude and depth, letting scientists see how distributions change over decades.
When the water warms
When the water gets too hot, corals push their algae out. Then the corals turn white. This is called bleaching.
Some fish swim away to find cooler water.
When water stays too warm, stressed corals push out their algae and turn white. This is called coral bleaching. A bleached coral is not dead, and it can recover if the heat does not last too long, but it has lost its main food source. NOAA says warmer water caused by climate change is the leading cause of bleaching.
Meanwhile, fish move. Off the northeastern United States, the species scientists track have shifted north by an average of 110 miles since the early 1970s, and many have moved into deeper water.
NOAA Coral Reef Watch reports that from January 2023 to September 2025 bleaching-level heat stress reached about 84.4 percent of the world's reef area, with mass bleaching in at least 83 countries and territories: the fourth and largest global bleaching event, after 1998, 2010 and 2014–2017 (68.2 percent).
The distribution indicator shows northeastern U.S. species shifting north by an average of 110 miles since the early 1970s and eastern Bering Sea species by 19 miles since the early 1980s, with both groups moving deeper. As smaller prey relocate, predators may follow.
Why it happens
Too much heat upsets corals. They stop sharing well with their algae.
Warm water can also bring less food. Then fish must look harder to eat.
Heat changes how sea creatures' bodies work. Too much heat makes corals' partnership with their algae break down. During one long heatwave, warm water sped up fish's bodies, so they needed more food.
Warming can also change the food itself. During a long marine heatwave in the Pacific, warmer water brought leaner plankton and fewer krill, so fish needed more food just when the food web was offering less.
NOAA's account of the 'Blob' heatwave that began in 2014, with temperatures up to 7 °F above normal, describes the mechanism: warming raised fish metabolism, increasing their energy needs, while the warm water brought leaner plankton and krill declined, cutting the fat-rich food salmon and others rely on.
Corals bleach when stress breaks down the symbiosis; if the stress continues, the coral eventually dies. NOAA lists other bleaching triggers too, such as runoff, strong sunlight and extreme low tides, and notes that cold water can also bleach corals.
Watching the warming sea
Scientists use satellites to watch the sea's warmth. Their maps show where reefs are getting too hot.
They also study how warm seas may change big storms.
NOAA's Coral Reef Watch uses satellites to map heat stress and bleaching alerts on reefs around the world. When the Pacific's long heatwave struck, young sea lions went hungry while their mothers hunted longer, fishermen caught skinny cod, and whales moved closer to shore to eat anchovies when krill grew scarce.
Storms are an open question. Hurricanes form over warm sea surfaces, and computer models suggest that with more warming, the strongest storms might become a bigger share of all storms, with heavier rain. Scientists say it is too early to be sure warming has already changed Atlantic hurricanes.
NOAA GFDL summarizes current understanding: with 2 °C of global warming, models project tropical cyclone intensities rising on average by 1 to 10 percent and rainfall rates within about 100 km of storms by about 14 percent (6 to 22 percent), and a larger global proportion of Category 4 and 5 storms (medium to high confidence), while the total number of storms may fall or change little.
GFDL also states that it is premature to conclude with high confidence that greenhouse gases have already changed past Atlantic hurricane activity beyond natural variability. These are projections and assessments, not forecasts of any single storm.
Amazing facts
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Corals are animals, not plants or rocks.
Corals are animals that share their bodies with tiny algae; the algae are the coral's main food source.
Coral–zooxanthellae symbiosis supplies most of a reef-building coral's food and its color (NOAA).
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Cold water can make corals turn white too.
In January 2010, unusually cold water in the Florida Keys caused corals to bleach as well.
A January 2010 cold-water event, about 6.7 °C below typical temperatures, bleached corals in the Florida Keys and killed some (NOAA).
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In one hot year, many Caribbean reefs were hurt.
In 2005, a massive bleaching event cost the United States half of its Caribbean coral reefs in a single year.
NOAA reports the U.S. lost half its Caribbean coral reefs in one year during the 2005 bleaching event, with thermal stress exceeding the previous 20 years combined.
Words to know
- coral bleaching
- when stressed corals lose their algae and turn white
- algae
- tiny plant-like living things; some live inside corals
- symbiosis
- a partnership where two living things help each other
- marine heatwave
- a long stretch of unusually warm sea water
- plankton
- tiny plants and animals that drift in the water
- projection
- what a scientific model says might happen in the future
Discovery challenge
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Why do bleached corals turn white?
Why does a coral turn white when it bleaches?
What causes the loss of color in a bleached coral?
- It pushes out the algae living inside it
- It is covered in snow
- It turns into a rock
Show the answer
It pushes out the algae living inside it
It pushes out its colorful algae.
It expels the algae that gave it color and food.
Expulsion of its symbiotic zooxanthellae, which provide its pigmentation.
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Where are many fish moving as the sea warms?
Off the northeastern U.S., where have many fish and shellfish been moving?
The observed trend in tracked northeastern U.S. marine species is a shift…
- North and deeper, to cooler water
- South and shallower, to warmer water
- Onto land
Show the answer
North and deeper, to cooler water
North and deeper, where it is cooler.
North by about 110 miles on average since the early 1970s, and deeper.
…poleward (about 110 miles since the early 1970s) and into deeper water.
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What do scientists say about warm seas and big storms?
What do scientists say about warmer seas and hurricanes?
Which statement matches NOAA GFDL's assessment?
- Models suggest stronger storms, but they are still studying it
- Hurricanes will stop forming
- Warm water has no link to hurricanes
Show the answer
Models suggest stronger storms, but they are still studying it
Scientists are still finding out. Models give clues.
Models suggest stronger, rainier storms, but it is too early to be sure about past changes.
Projected intensity and rain increases (medium to high confidence); past Atlantic attribution premature.
Experiments to try
- Watch Try it What If? Lab What if the oceans became warmer? Water expands a little when it warms. The ocean is so deep and wide that this small stretch adds up: in our model, warming the top 700 meters by 2 °C raises the sea about 28 centimeters, with no ice melting at all.
- Watch Try it What If? Lab What if an ocean food web changed? Antarctic krill are small shrimp-like animals, but they feed a huge number of bigger ones. Take krill out of our model's Southern Ocean food web and all seven other species are affected: every animal in it loses all its food, from silverfish to orcas.
- Watch What If? Lab What if the sea were not salty? Rain and rivers wash tiny bits of salt out of rocks and carry them to the sea, where they build up. Imagine a sea with no salt: most sea creatures are built for salty water, sea ice would form differently, and deep ocean currents would lose one of their pushes.
- Watch Try it What If? Lab What if orcas left the Southern Ocean? Orcas are apex predators: they eat at the top of the food web. Take them out of our model's Southern Ocean food web and the seals, penguins and fish they eat could become more common, which then changes the food of others. Explore the knock-on effects, and the real orcas that make long trips north.
- Watch What If? Lab What if coral reefs could be regrown? Scientists grow broken pieces of coral in underwater nurseries, then plant them back on damaged reefs. Follow how coral gardens work, how scientists search for corals that cope with heat, and what happens when fish come back to a restored reef.
Explore next
- Watch Try it What If? Lab What if the oceans became warmer? Water expands a little when it warms. The ocean is so deep and wide that this small stretch adds up: in our model, warming the top 700 meters by 2 °C raises the sea about 28 centimeters, with no ice melting at all.
- Watch Try it What If? Lab What if an ocean food web changed? Antarctic krill are small shrimp-like animals, but they feed a huge number of bigger ones. Take krill out of our model's Southern Ocean food web and all seven other species are affected: every animal in it loses all its food, from silverfish to orcas.
- Watch What If? Lab What if the sea were not salty? Rain and rivers wash tiny bits of salt out of rocks and carry them to the sea, where they build up. Imagine a sea with no salt: most sea creatures are built for salty water, sea ice would form differently, and deep ocean currents would lose one of their pushes.
- Watch Try it What If? Lab What if orcas left the Southern Ocean? Orcas are apex predators: they eat at the top of the food web. Take them out of our model's Southern Ocean food web and the seals, penguins and fish they eat could become more common, which then changes the food of others. Explore the knock-on effects, and the real orcas that make long trips north.
- Watch What If? Lab What if coral reefs could be regrown? Scientists grow broken pieces of coral in underwater nurseries, then plant them back on damaged reefs. Follow how coral gardens work, how scientists search for corals that cope with heat, and what happens when fish come back to a restored reef.
- Watch Try it What If? Lab What if a coast were made of hard rock? Waves wear coasts away, but not every coast wears at the same speed. In our model, a bare sandy island facing ordinary waves keeps about 72 percent of its land after 50 model years. Make the same island of hard rock and it keeps about 99 percent. That is why hard rock is often left sticking out to sea as headlands.
- Watch Try it What If? Lab What if all the sea ice melted? Sea ice floats, so it is already pushing aside about as much water as it will make when it melts, like an ice cube in a glass. In our model, melting all the Arctic's springtime sea ice raises the sea by only about 2 millimeters. The same ice melting on land would raise it about 7 centimeters.
- Watch Try it What If? Lab What if icebergs were made of wood? An iceberg floats with about nine-tenths of it hidden under the water. Swap the ice for wood in our model and watch the block bob up much higher, then find out why how much shows depends on density.
For parents & teachers
What children will learn
- Corals depend on algae living inside them, and heat stress makes them bleach.
- Many fish and shellfish are moving north or deeper as seas warm.
- Marine heatwaves can leave food webs short of food.
- Scientific projections about storms carry stated confidence and are different from observations.
Recommended ages: 7 to 17 · About 15 minutes
Talk about it
- Why might a bleached coral be able to recover, and what would stop it?
- If fish move north, who else is affected: other animals, people, towns?
- What is the difference between something scientists have seen and something a model projects?
Try at home or in class
- Make a simple chart of a fish's 'comfortable' temperature and color in where it could live as the water warms.
- Look at a map of the U.S. East Coast and measure 110 miles on the scale to see how far some species have shifted.
Key vocabulary: coral bleaching, algae, symbiosis, marine heatwave, plankton, projection
Where these facts come from
- What is coral bleaching? , NOAA National Ocean Service
- How does climate change affect coral reefs? , NOAA National Ocean Service
- Current Global Bleaching: Status Update , NOAA Coral Reef Watch
- In search of cooler waters, marine species are shifting northward or diving deeper , NOAA Climate.gov
- Looking Back at The Blob, Chapter 2: Marine Heat Wave Intensifies, 'Completely Off the Chart' , NOAA Fisheries
- Global Warming and Hurricanes , NOAA Geophysical Fluid Dynamics Laboratory
Checked, not yet human-reviewed. Last updated October 27, 2026. Spotted a mistake? Tell us.