Quick questions

Find answers to popular queries and science questions about Earth's oceans and climate change.

Our oceans and marine life

To dive deeper into pioneering marine research and life beneath the waves, read our ocean stories.

Quick questions

How deep is the ocean?

The ocean has an average depth of approximately 3.7 kilometres (or 2.3 miles). A calculation from satellite measurements in 2010 put the average depth at 3,682 metres (12,080 feet). However, at the time only about 10% of Earth's seafloor had been mapped to high resolution, so this figure is only an estimate.

Ocean depth is divided into zones: littoral, bathyal, abyssal and hadal. The deepest part of the ocean, the hadal zone, is anywhere deeper than six kilometres.

Challenger Deep, in the Mariana Trench, is the deepest point in the ocean known so far, at approximately 11 kilometres - deeper than Mount Everest is tall. The trench is located in the western Pacific Ocean.

The extreme depth of the Mariana Trench and other oceanic trenches is caused by subduction - where on the boundary of two converging tectonic plates, one descends down into Earth's mantle, creating a deep trough.

Why is the sea salty?

Salt in the sea, or ocean salinity, is mainly caused by rain washing mineral ions from the land into water.

Carbon dioxide in the air dissolves into rainwater, making it slightly acidic. When rain falls, it weathers rocks, releasing mineral salts that separate into ions. These ions are carried with runoff water and ultimately reach the ocean.

Sodium and chloride, the main constituents of the type of salt used in cooking, make up over 90% of all the ions found in seawater. Around 3.5% of the weight of seawater comes from dissolved salts.

Some mineral ions are used by marine animals and plants, removing them from the water. The leftover minerals have built up in concentration over millions of years. Underwater volcanoes and hydrothermal vents on the seabed can also release salts into the ocean.

Isolated bodies of water can become extra salty, or hypersaline, through evaporation. The Dead Sea is an example of this. Its high salt content increases the water's density, which is why people float in the Dead Sea more easily than in the ocean.

What is a coral reef?

Coral reefs are made up of colonies of hundreds to thousands of tiny individual corals, called polyps. These marine invertebrate animals have hard exoskeletons made of calcium carbonate, and are sessile, meaning permanently fixed in one place. Polyps grow slowly, forming different shapes and sizes depending on their species.

Assisted by other animals with calcium carbonate skeletons and also coralline algae, corals form complex, three-dimensional reefs.

Coral reefs provide an important ecosystem for marine life, offering food and shelter among their crevices and branches for animals including fishes, molluscs, sea urchins and sponges.

Corals are found in all of Earth's oceans, from tropical to freezing temperatures, however they only build coral reefs in warm, shallow seas in the tropics. Among the biggest and best-known are the reef systems of the Great Barrier Reef of Australia, which is around 2,300 kilometres long. The most biologically diverse reefs in the world can be found in a region known as the Coral Triangle in Southeast Asia.

Watch a video about corals, the builders of the reef and explore more about coral reefs.

Why are coral reefs important?

Coral reefs provide an important ecosystem for life underwater, protect coastal areas by reducing the power of waves hitting the coast, and provide a crucial source of income for millions of people.

Coral reefs teem with diverse life. Thousands of species can be found living on one reef. The Great Barrier Reef contains over 400 coral species, 1,500 fish species, 4,000 mollusc species and six of the world's seven sea turtle species. The Coral Triangle - a coral-rich marine region in Southeast Asia that encompasses the waters between Indonesia, Malaysia, the Philippines and Papua New Guinea - is the most biologically diverse marine ecosystem on Earth. 

Coral reefs have an estimated global value of £6 trillion each year, due in part to their contribution to fishing and tourism industries and the coastal protection they provide.

More than 500 million people worldwide depend on reefs for food, jobs and coastal defence. The ridges in coral reefs act as barriers and can reduce wave energy by up to 97%, providing crucial protection from threats such as tsunamis. They help protect areas such as mangrove forests and seagrass beds that act as nurseries for marine animals, as well as human coastal populations.

Extracts from animals and plants living on reefs have been used to develop treatments for asthma, arthritis, cancer and heart disease.

Explore the risks facing coral reefs.

How do oysters make pearls?

Pearls are made by marine oysters and freshwater mussels as a natural defence against an irritant such as a parasite entering their shell or damage to their fragile body.

The oyster or mussel slowly secretes layers of aragonite and conchiolin, materials that also make up its shell. This creates a material called nacre, also known as mother-of-pearl, which encases the irritant and protects the mollusc from it.

When pearls are cultured commercially an irritant is manually inserted into a mollusc to promote the production of mother-of-pearl.

Nacre can form naturally around almost any irritant that gets inside the shell, creating some very unique and precious pearls.

Other bivalve molluscs and gastropods can produce pearls, but these aren't made of nacre.

What do killer whales eat?

Killer whales (also called orcas) are apex predators, meaning they are at the top of their food chain. They feed on fish and squid like other odontocetes (toothed whales) do, but will also target seals, sea birds and even whale species far bigger than themselves. Killer whales are also the only known predators of great white sharks.

Killer whales are the largest dolphin species. They are highly social and spend most of their lives swimming in large pods of family members. Hunting techniques are passed down through generations, so their diets depend on the region they inhabit and the pod's approach to hunting.

These highly intelligent cetaceans have been documented creating large waves to wash seals off ice floes, and even intentionally beaching themselves to catch prey on the shore.

Read more about what whales eat.

What do blue whales eat?

Blue whales eat krill - tiny, shrimp-like crustaceans that live throughout Earth's oceans. The huge whales can eat up to four tonnes of krill every day.

Blue whales lunge through large swarms of krill with their mouths open, taking in more food in one mouthful than any other animal on Earth. Krill make up the vast majority of a blue whale's diet.

The blue whale is a filter-feeder. Its throat has an expandable, pleated structure to engulf a volume of water and prey that is greater than the animal's own body weight. The water it takes in at the same time as its food is pushed out of the mouth by its enormous tongue, through strainer-like baleen plates which hang down from the upper jaw.

Watch how a blue whales lunges for its food.

How much plastic is in the ocean?

Much of the plastic that does not end up in landfill or go through other waste management pathways (such as recycling or incineration) is thought to end up in the ocean.

Between 4.8 and 12.7 million tonnes of plastic enter the ocean each year, according to figures published in the journal Science in 2015.

Plastic can enter the ocean as large, identifiable items or as microplastics - pieces under five millimetres in length. Both pose a threat to marine life. Large pieces degrade over time to become microplastics, but never fully disappear.

Plastic has accumulated in huge quantities throughout the ocean - even in deep-sea areas previously thought to be untouched by humans.

A 2014 study involving Museum researcher Dr Lucy Woodall found high levels of contamination in deep-sea sediments. It revealed that around four billion microscopic plastic fibres could be littering each square kilometre of deep-sea sediment around the world.

Find out how plastic is affecting marine life.

What causes ocean acidification?

Ocean acidification is mainly caused by carbon dioxide gas in the atmosphere dissolving into the ocean. This leads to a lowering of the water's pH, making the ocean more acidic.

Many factors contribute to rising carbon dioxide levels. Currently, the burning of fossil fuels such as coal, oil and gas for human industry is one of the major causes.

Deforestation results in fewer trees to absorb the gas. Also, when plants are cut down and burnt or left to rot, the carbon that makes up their organic tissue is released as carbon dioxide.

Some parts of the ocean are naturally acidic, such as at hydrothermal vent sites - underwater 'hot springs'.

In the past, ocean acidification occurred naturally but over much longer periods of time. It is occurring faster now than in the last 20 million years.

Find out how ocean acidification is causing problems for marine life.

How does ocean acidification affect marine life?

Ocean acidification can negatively affect marine life, causing organisms' shells and skeletons made from calcium carbonate to dissolve. The more acidic the ocean, the faster the shells dissolve. 

Animals that produce calcium carbonate structures have to spend extra energy either repairing their damaged shells or thickening them to survive. Using energy for this could impact the animals' abilities to grow and reproduce.

Animals able to survive and reproduce in more acidic waters are likely to become smaller, potentially affecting the food chain that relies on them.

Discover more about ocean acidification and its impacts.

How long can whales hold their breath?

The longest ever recorded dive by a whale was made by a Cuvier's beaked whale. It lasted 222 minutes and broke the record for diving mammals. Other whales can also hold their breath for a very long time. A sperm whale can spend around 90 minutes hunting underwater before it has to come back to the surface to breathe. In 1969, a male sperm whale was killed off the coast of South Africa after surfacing from a dive lasting 117 minutes.

Whales' lungs are particularly efficient at taking up oxygen when they breathe air in and out through their blowholes at the water's surface. Special adaptations help them hold their breath for a long time.

Discover the secrets of the deepest-diving whales.

How can whales hold their breath for so long?

Rather than keeping oxygen in their lungs like humans do, whales' bodies are specially adapted to store oxygen in their blood and muscles. They have extraordinarily high levels of the oxygen-storing proteins haemoglobin and myoglobin.

Whales also reduce their heart rate and stop the blood flow to certain parts of the body, temporarily shutting down organs such as their kidneys and liver while they hunt. This helps them use the oxygen they have in their bodies more slowly.

Furthermore, beaked whales (which can dive for a particularly long time) have a streamlined body shape. Their flippers fit in indentations in the body, enabling them to take on a torpedo-like shape. This helps them to swim, and often to glide, with minimal effort and extend their oxygen stores for as long as possible.

Read the secrets of the deepest-diving whales.

How deep can a whale dive?

Whales are accomplished divers. The deepest whale dive recorded so far was made by a Cuvier's beaked whale. A 2014 study used satellite-linked tags to follow the dives of eight beaked whales off the southern California coast. The deepest recorded dive was 2,992 metres, breaking the record for diving mammals.

Experts have suggested that this dive was unusually deep for this species. A more normal depth would be 2,000 metres.

Sperm whales also regularly dive 1,000 to 2,000 metres deep.

Read more about deep-diving whales.

Climate change

For the latest facts and news about one of the biggest challenges facing humanity, read our climate change explainers and stories.

Quick questions

Is climate change the same as global warming?

Not quite - climate change is the result of global warming. Global warming refers to how much the Earth's surface temperature is rising, and it is the effect of this warming on average weather conditions that is known as climate change.

While weather is what happens in a certain place at a certain time - such as whether it's snowing or raining in London one day, climate is the average weather a place experiences - such as how many snowy or rainy days are likely to occur in London in any given year.

Climate change alters these established patterns, leading to consequences: more frequent droughts and heatwaves, more devastating hurricanes, and more intense rainfall, to name a few. As well as causing more extreme weather events, climate change can also alter the timing of the seasons, disrupting the lives of plants and animals.

Dr Joeri Rogelj, Director of Research at the Grantham Institute, Imperial College London, says, 'Global warming and climate change have both occurred throughout Earth's history. But it's the speed at which the world is currently warming, and how fast the climate is changing, that is so concerning.'

The surface temperature of the planet has increased around 0.08°C per decade since 1880. However, the average rate of increase between 1981-2019 has been more than twice that rate. These changes are unquestionably the result of human actions.

Find out more about climate change and why it matters.

What is a carbon footprint?

Your carbon footprint is how much carbon is released into the atmosphere as a result of your everyday activities. Carbon emissions - in the form of carbon dioxide and methane - are what cause global warming and climate change.

Unless you drive a car that runs on petrol or diesel, you might not think that you emit any carbon. But if you get your energy from a supplier that doesn't use renewable sources, you will be creating carbon emissions every time you turn on the lights or your TV. The vast majority of homes in the UK are also heated using gas-fired boilers or have gas stoves for cooking, which release carbon dioxide.

Your food choices also impact your personal carbon footprint. For example, if you buy fruit that has been shipped from overseas, the carbon emitted on that journey will add to your footprint. Similarly, if you travel by flying or driving, you will contribute more carbon emissions than if you ride a bike or use public transport.

The USA has one of the highest average carbon footprints in the world, at a rate of about 16 tonnes of carbon released per person per year in 2019. In the same year, the figure for the UK was nearly 5.5 tonnes per person, with the global average sitting at around 4.7 tonnes. To avoid global temperatures rising by much more than 1.5°C, carbon emissions, and therefore carbon footprints, need to drop to zero by 2050.

Carbon footprints don't only apply to people - they can be calculated for companies, events, places and products. Alyssa Gilbert, Director of Policy and Translation at the Grantham Institute, Imperial College London, says, 'While we can all do more to reduce our own carbon footprint, we should also put pressure on companies and those in power that have an even bigger capacity to make changes and reduce carbon emissions.'

A variety of organisations have created carbon footprint calculators to help you estimate your personal impact on the planet. Check out this useful list of calculators.

Explore ways you can help the planet, including reducing your carbon footprint.

How does carbon dioxide increase Earth's temperature?

Carbon dioxide (CO2) is a greenhouse gas. This means that it causes an effect like the glass in a greenhouse, trapping heat and warming up the inside. This effect is important: without the CO2 that naturally exists in the atmosphere, Earth might be too cold to support human life. However, the atmosphere is very sensitive to changing levels of CO2. Even though this gas makes up less than 0.1% of the atmosphere, it can have a huge effect on how much heat the planet's surface retains.

When energy from the Sun reaches the top of our atmosphere, most of it passes through to Earth's surface, where it is absorbed. Some of this energy is re-emitted, heading back towards space. At this stage, it interacts with molecules of CO2 in a way that prevents some of it from escaping Earth's atmosphere. The trapped heat energy leads to increased average global surface air temperatures.

One reason carbon dioxide has such a big impact on global temperatures is that hotter air can hold more water vapour. Water vapour is itself a greenhouse gas, which further enhances the greenhouse effect.

While the presence of carbon dioxide in Earth's atmosphere is natural, the rising levels since the Industrial Revolution in the 1800s are due to human activities, primarily the burning of fossil fuels such as coal and oil.

Where does carbon dioxide come from?

Carbon dioxide (CO2) comes from both natural sources (including volcanoes, the breath of animals and plant decay) and human sources (primarily the burning of fossils fuels like coal, oil and natural gas to generate energy). Human activities have been the main cause of rising carbon dioxide levels in our atmosphere since the 1800s.

The amount of carbon dioxide in the atmosphere is determined by the carbon cycle - a system of 'sources' and 'sinks' of the gas that add and remove it, respectively. One part of the cycle involves rocks, starting with volcanoes, which belch CO2. This is countered by 'weathering', a process where atmospheric CO2 mixes with rainwater to make an acid that reacts with rocks, locking the CO2 away.

The emergence of life on our planet added a new layer to the carbon cycle. As plants grow, they take CO2 out of the atmosphere, and when they die, it is released again. Animals that consume the plants also store the CO2 for a while, before they too die and decompose.

Some dead plants don't decompose and instead become layers of coal, oil and other organic-rich sediments such as peat. Eventually, these layers would naturally burn or be recycled through volcanoes, returning the CO2 to the atmosphere over many thousands (if not millions) of years.

However, humans have been digging up these layers and burning them at a rate the planet has never seen before, releasing vast amounts of CO2 in a geological blink of an eye. Estimates show that by burning these fossil fuels, humans have essentially taken millions of years of carbon uptake by plants and returned it to the atmosphere in less than 300 years.

Natural sinks of carbon are unable to keep up with this rate of change. This causes CO2 to build up in the atmosphere, which rose from a concentration of around 280 parts per million (ppm) in 1750 to more than 415ppm in 2021.

What does carbon neutral mean and what is net zero?

A person, company or country is carbon neutral if they balance the carbon dioxide they release into the atmosphere through their everyday activities with the amount they absorb or remove from the atmosphere. This is also called net zero carbon emissions or net zero carbon, because overall no carbon dioxide is added to the atmosphere.

The definition of net zero emissions is sometimes expanded to include other gases such as methane, nitrous oxide and hydrofluorocarbons. This is sometimes referred to as net zero greenhouse gas emissions or simply net zero. These other gases contribute about 24% of global greenhouse gas emissions and carbon dioxide the remaining 76%.

There are two main ways to achieve net zero: reducing emissions and removing carbon dioxide from the atmosphere, through technologies that actively take in carbon dioxide or by enhancing natural removal methods - by planting trees, for example. These methods can be used in combination.

Net zero greenhouse gas emissions are seen as key targets for reaching the Paris Agreement goal of keeping the rise in global temperature well below 2°C and preferably below 1.5°C above pre-industrial levels. Many countries have pledged to reach net zero emissions by 2050, including the UK, which has enshrined in law a target to slash greenhouse gas emissions by 78% by 2035 compared to 1990 levels.

The UK's plan for reaching net zero includes increasing energy efficiency, using more renewable energy to produce electricity for heating and transport, and making use of hydrogen to replace fossil fuels.

What is carbon capture and storage?

Carbon capture and storage is a process that prevents carbon dioxide from entering the atmosphere when it is emitted from sources such as coal-fired power plants. A related term is carbon dioxide removal, which refers to methods for taking carbon dioxide back out of the atmosphere, either by using technologies or enhancing natural processes - by expanding forests and wetlands, for example.

Both carbon capture and storage and carbon dioxide removal often involve a step that locks away the carbon dioxide for a long time, making sure it can't re-enter the atmosphere. This is called carbon sequestration. The carbon dioxide can be 'stored' by being used in products like building materials, or it can be pumped underground where it reacts with certain kinds of rocks, locking it inside.

Alyssa Gilbert, Director of Policy and Translation at the Grantham Institute, Imperial College London, says, 'Many countries are working to reduce their carbon emissions, and these reductions are the central pillar to tackling climate change. But there are some areas where reducing emissions is particularly difficult - in some types of heavy industry, for example. This means we will need to remove some carbon dioxide from the atmosphere to limit climate change. However, many carbon removal technologies and methods are not well developed yet. The slower we reduce our emissions, the more we will need to rely on these methods that may not be available to pick up the slack.'

Is methane a greenhouse gas?

Methane is a more powerful greenhouse gas than carbon dioxide, but there is far less of it in the atmosphere and it does not stay there as long. Methane is more than 25 times as potent as carbon dioxide at trapping heat in the atmosphere over the course of a century, but it has an 'atmospheric lifetime' of around 12 years, whereas carbon dioxide molecules hang around for hundreds of years.

This means that if humans stopped adding any methane to the atmosphere tomorrow, within several decades all trace of the extra methane and its climate influence would be gone, whereas the same is not true for carbon dioxide. However, methane is still an important greenhouse gas because there are many human-caused sources of it. Methane today is responsible for about 0.5°C of total warming.

Methane is released during the extraction and transport of fossil fuels including coal, oil and natural gas. It is also released by rice fields, the decay of food waste in rubbish dumps, and even cows - meaning the rise in beef consumption worldwide has increased methane emissions.

There are also natural sources of methane that are being released faster due to global warming itself. These include the melting of permafrost, the layer of previously permanently frozen ice within soil in polar and sub-polar regions. These methane emissions could in turn accelerate warming, leading to the release of more methane, and so on.

Dr Joeri Rogelj, Director of Research at the Grantham Institute, Imperial College London, says, 'Methane does not persist for long in the atmosphere but is nevertheless a powerful greenhouse gas. If we make rapid cuts to methane emissions now, together with efforts to drastically reduce our carbon dioxide emissions, it will ensure that we limit global warming as much as possible.'

At the COP26 climate summit, more than 90 nations agreed to cut their methane emissions by 30% by 2030 compared to 2020.

Is nuclear energy renewable?

By many definitions, nuclear energy is not renewable. But in terms of climate change, nuclear energy production does not release greenhouse gases, so it is a low-carbon fuel.

'Renewable' energy refers to energy from sources that are constantly replenished - like the water for hydroelectric dams that is topped up by the rain, or the sunlight that reappears every day for solar panels. Because nuclear power uses up radioactive fuel, it is not renewable in the same way.

Nuclear energy, however, is the second-largest source of low-carbon electricity in the world behind hydropower. Some researchers say it is essential for helping countries including the UK reach targets of producing all their energy without releasing greenhouse gases. This is because there is not yet enough renewable energy capacity to provide for all our electricity needs. Renewable power is also intermittent - for instance, wind turbines don't produce power when the wind doesn't blow (although large batteries to store this energy are improving all the time).

Nuclear energy is not without its issues. Most notably, it produces radioactive waste that must be transported safely to long-term storage, where it will not be disturbed for tens of thousands of years until the material is no longer a danger to human health or the environment. These challenges are ongoing.

How does climate change affect the ocean?

Climate change warms the ocean, causing knock-on effects such as thermal expansion - which leads to a rise in sea level - and changes in ocean currents. The melting of ice both on land and in the sea also affects the ocean, causing more sea-level rise and reducing the salinity of the ocean, respectively. Greater concentrations of carbon dioxide in the atmosphere also mean that more of it dissolves in the ocean, leading to acidification.

Each of these changes affect marine wildlife. Warming ocean temperatures can lead to coral bleaching - the sudden die-off of large parts of coral reefs - as well as cause animals such as fish to seek cooler waters, shifting their habitats north. This can have a knock-on impact on human communities that rely on those fish for food.

Acidification can weaken sea animals' shells and external skeletons. This includes coral exoskeletons - a further reason coral reefs are under threat worldwide.

When ice on land - ice sheets and glaciers - melts, it adds water volume to the ocean. This increases sea levels globally, which can inundate low-lying land and important coastal environments such as mangrove forests and wetlands.

When sea ice melts, it doesn't add volume to the ocean, but it does add freshwater, locally decreasing the saltiness of the sea. Saltiness and temperature are the drivers of ocean currents that move heat and nutrients around the world. Melting sea ice and rising temperatures can disrupt these currents, not only affecting wildlife that depends on them but also, potentially, local climates.

For example, the UK is comparatively mild because of the heat brought by an ocean current from the Gulf of Mexico. Melting Arctic sea ice is weakening this current, meaning the UK could face more extreme weather as a result of climate change.

What is climate justice?

Climate justice recognises that climate change will not affect everyone in the same way, and that this will lead to inequalities between places, people and even generations. It moves climate change conversations beyond the science and the physical impacts, to questions of politics and ethics, such as who should bear responsibility for paying for the damage caused by climate change, or how much developed countries should help the developing world increase their energy use in a sustainable way.

The impacts of climate change are likely to be felt most by those communities that contributed least to the problem, such as developing countries, indigenous peoples and future generations. For example, a study published in 2021 found that children born today across the globe will on average face seven times more scorching heatwaves, 2.6 times more droughts, 2.8 times as many river floods, almost three times as many crop failures, and twice the number of wildfires during their lives than their grandparents.

In recent years, various actions to increase awareness or tackle these issues have arisen. For example, in 2019 The Hague Court of Appeal ordered the Dutch government to reduce the country's greenhouse gas emissions by at least 25% by the end of 2020 compared to 1990. This placed the Netherlands under a legal obligation to take measures to protect its citizens from the consequences of climate change.

Movements like Fridays for Future and Extinction Rebellion have also gained in popularity as people use their voices to call for climate justice.