IB ESS HLTopic 4 — Water SystemsPaper 1 & 2HL only~10 min read
Oceans as a Carbon Sink
The sea has quietly absorbed a huge share of the carbon dioxide we have released, and it is the main reason the climate has not warmed faster. But that service comes with a bill: the same chemistry that stores the carbon is slowly making the ocean more acidic. This page covers both sides.
📘 What you need to know
The oceans are a carbon sink — they take in more carbon dioxide than they release, slowing the rise in the atmosphere.
Short-term sequestration is CO2 simply dissolving in seawater.
Long-term sequestration is carbon locked into organisms, then into seabed sediments, then eventually into rock and fossil fuels.
Dissolved CO2 reacts with water to form carbonic acid, which lowers pH: this is ocean acidification.
Acidification makes it harder for corals, molluscs and some plankton to build calcium carbonate shells and skeletons.
The sink is not unlimited — warmer, more saturated water absorbs CO2 less easily, which is a positive feedback on warming.
What a carbon sink actually is
A sink is any store that takes in more of something than it gives back. Oceans qualify because CO2 dissolves at the surface all the time, and because living things in the sea keep pulling carbon down with them when they die.
Without that, the CO2 we have released by burning fossil fuels would all be sitting in the atmosphere and warming would be considerably worse. The ocean has not stopped climate change; it has slowed it down and bought us time.
Say it as a comparison. A strong answer does not just say the ocean absorbs carbon — it says the ocean absorbs more than it releases, so it is a net sink, and that this reduces the rate of atmospheric CO2 increase.
Short-term and long-term storage
The syllabus wants you to split ocean carbon storage into two timescales. They work in completely different ways.
Short-term: it just dissolves
CO2 in the air is in contact with the sea surface, and gases dissolve in water. Cold water dissolves more, which is why the coldest seas take up the most. This dissolved carbon can come straight back out again if conditions change, so it is only parked, not locked away.
Long-term: biology and burial
Phytoplankton use dissolved CO2 for photosynthesis, so carbon becomes part of living tissue.
Many marine organisms also combine dissolved carbon with calcium to build calcium carbonate shells and skeletons.
When those organisms die, tissue and shells sink to the seabed. Some decomposes on the way; some is buried in sediment before it can rot.
Buried for long enough, under heat and pressure, that carbon becomes rock such as limestone, or fossil fuels such as coal, oil and gas.
This is genuinely long-term storage: millions of years. And it is exactly the carbon we now dig up and burn in a couple of centuries.
The vertical scale is really a time scale. The deeper carbon travels, the longer it stays out of the atmosphere — which is why burning buried carbon has such an outsized effect.
Ocean acidification: the price of the sink
Dissolved CO2 does not just sit there. It reacts with seawater to make carbonic acid, which releases hydrogen ions. More hydrogen ions means a lower pH. Surface ocean pH has fallen from roughly 8.2 before industrialisation to about 8.1 today.
That looks tiny. It is not, because pH is a logarithmic scale: a drop of 0.1 means about a quarter more hydrogen ions in the water.
Step four is the one students miss. Falling pH does not only add acid — it removes the carbonate ions that shell-building organisms need.
Why shell builders suffer
Corals, molluscs, sea urchins and some plankton build hard parts from calcium carbonate.
Extra hydrogen ions in the water react with carbonate ions, so there are fewer left for organisms to use.
Shells and skeletons grow more slowly, become thinner, and in very acidic conditions can start to dissolve.
Weakened corals mean weaker reefs, which are the habitat for a quarter of all marine species — so the damage spreads through the whole food web.
Some plankton with carbonate shells sit at the base of that food web, so losses there travel upwards to fish, seabirds and mammals.
Do not say the ocean is becoming acidic. At pH 8.1 it is still alkaline. The correct phrasing is that it is becoming less alkaline, or that it is acidifying. Examiners notice the difference.
Can the sink fill up?
Yes, in a practical sense. Three things weaken the ocean’s ability to keep absorbing carbon:
Warming. Gases dissolve less readily in warm water, so a hotter ocean takes up less CO2.
Saturation. The more carbon already dissolved in surface water, the slower it absorbs more.
Stronger stratification. Warming makes surface water float more firmly on top of deep water, so carbon-loaded surface water mixes downwards more slowly. (That is the next page.)
Each of those is a positive feedback: warming reduces uptake, so more CO2 stays in the air, so warming increases. Being able to name that loop is often worth an extra mark.
Worked examples
WORKED EXAMPLE 1
What a 0.1 drop in pH really means
Surface ocean pH has fallen from about 8.2 to about 8.1. Show why this is a much bigger change than it looks.
Step 1: remember pH is logarithmic
Each whole pH unit is a ten times change in hydrogen ion concentration
Step 2: find the factor for 0.1 units100.1 = 1.26Step 3: turn it into a percentage(1.26 − 1) × 100 = 26%About 26% more hydrogen ionsand that has happened in roughly 250 years, far faster than natural change
WORKED EXAMPLE 2
Short-term or long-term sequestration?
Classify each and justify briefly: (a) CO2 dissolving into cold surface water off Iceland, (b) plankton shells settling into seabed mud, (c) an oil deposit forming from buried marine remains.
(a) Short-term
Only dissolved — it can return to the air if the water warms or mixes
(b) Long-term
Carbon is now solid carbonate in sediment, out of circulation for thousands of years
(c) Long-term
Heat and pressure turn buried carbon into fossil fuel — millions of years
Ask how easily the carbon can get back outeasy return means short-term, buried and chemically locked means long-term
WORKED EXAMPLE 3
Explain how rising atmospheric CO2 threatens coral reefs [4]
Write a chained answer.
Link 1
More CO2 in the air means more dissolves into the oceanLink 2
It forms carbonic acid, releasing H⁺ ions, so pH fallsLink 3
Those ions use up carbonate ions, so less is available for calcium carbonate
Link 4
Corals build skeletons more slowly and existing ones weaken, so reef structure degradesReef habitat is lost, and reef species with itif you have room, add that warming also causes bleaching — two stresses, same cause
💡 Exam tip
Split your answer by timescale whenever sequestration comes up. Short-term is dissolving; long-term is burial.
Use the word net: a sink absorbs more than it releases.
Give pH figures (about 8.2 to about 8.1). Numbers make an answer look informed.
Name the affected groups precisely: corals, molluscs, echinoderms and calcareous plankton, not just “sea creatures”.
Mention the positive feedback when discussing whether the sink will keep working.
Link across topics: this connects to the carbon cycle, to climate change and to the stratification page that follows.
⚠ Common mix-up
Saying the ocean is acidic. It is alkaline and becoming less so.
Treating a 0.1 pH change as trivial. Logarithmic scale — always convert it.
Confusing acidification with warming. Bleaching is caused by heat; shell problems are caused by chemistry. Both come from more CO2.
Thinking the sink is infinite. Warmer, more saturated water absorbs less.
Mixing up sequestration and emission. Sequestration takes carbon out of the atmosphere; burning puts it back.
Forgetting the biological route. Carbon does not only dissolve — organisms actively carry it downwards.
Up next: Why Water Forms Layers — how temperature and salinity split lakes and oceans into stacked layers that barely mix, and why that decides where oxygen and nutrients end up.
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