The Ocean Is Soaking Up Our Carbon Dioxide
― But Then What Happens?
You've probably heard that "the ocean absorbs carbon dioxide." It's true: the ocean has taken in a substantial share of the CO2 humans have released. But that's not the end of the story. Once absorbed, that CO2 changes form in seawater — and creates a whole new problem.
You may have heard in the news that "the ocean absorbs part of the carbon dioxide humans release." It's tempting to think, "great, the ocean's got it handled."
But absorbed CO2 doesn't simply vanish. Inside seawater, it undergoes chemical changes that are said to gradually alter the nature of the ocean itself.
So what does that absorbed CO2 actually become, once it's in the sea?
CO2 in the air dissolves into seawater at the ocean's surface. It's the same ordinary process as any gas dissolving into a liquid.
Dissolved CO2 reacts with water to form carbonic acid, which then releases hydrogen ions. This is what gradually pushes seawater toward the acidic side.
Absorption isn't the end of the story — let's look at the chemistry that follows.
Seawater "dissolves" carbon dioxide
Part of the CO2 in the air dissolves into seawater at the ocean's surface. It's the reverse of what happens when you open a bottle of soda and gas fizzes out. Gases dissolve into liquids more readily as the surrounding gas pressure rises. As atmospheric CO2 concentration climbs, the amount dissolving into the ocean tends to rise too.
Even this simple "dissolving" alone means the ocean cuts the amount of CO2 in the air. In fact, a substantial share of the CO2 humans have emitted by burning oil and coal is reported to have been absorbed by the ocean so far.
Once dissolved, CO2 turns into "carbonic acid"
Here's the real subject of this article. CO2 dissolved in seawater doesn't stay in that form. First, it combines with surrounding water molecules to become carbonic acid. Then, part of that carbonic acid splits into a hydrogen ion and a bicarbonate ion. A small fraction of that goes further, releasing another hydrogen ion to become a carbonate ion.
Every time this chain of reactions happens, hydrogen ions build up in the seawater. More hydrogen ions means the seawater is moving closer to being acidic. This phenomenon is called ocean acidification.
Beyond absorption, it keeps changing the very nature of seawater.
Acidification also affects shellfish and corals
Shellfish, corals, and some plankton use carbonate ions in seawater as the raw material for building their shells or skeletons (calcium carbonate). But as seawater moves toward being more acidic, some of that carbonate combines with hydrogen ions to become bicarbonate, making it harder to use as shell- or skeleton-building material.
As a result, these organisms may find it harder to build shells and skeletons, and existing shells may become more prone to dissolving, according to reports. Behind the ocean's "upside" of absorbing CO2, a new burden is emerging for marine ecosystems.
The amount of CO2 the ocean can absorb is thought to have limits. And the absorption itself is creating a new problem: ocean acidification. Rather than assuming "we're fine because the ocean absorbs it," it's important to understand what happens after absorption too.
Something you can check for yourself
- Pour water into a cup and add a little red cabbage juice (or a BTB solution, or any indicator you have that shows acidity/alkalinity)
- Using a straw, slowly blow your breath into the liquid (careful not to accidentally suck it up)
- Watch the liquid's color change over time
You can watch the CO2 in your breath dissolve into the water and push it toward the acidic side as a color change — a tiny, everyday version of what's happening in the ocean.
Summary
The ocean keeps absorbing part of the CO2 in the atmosphere, but the absorbed CO2 doesn't simply disappear — it changes form in seawater into carbonic acid, hydrogen ions, and carbonate ions. This shift drives "ocean acidification," pushing seawater toward the acidic side, which is thought to affect shell- and skeleton-building organisms such as shellfish and corals.
The ocean absorbing CO2 isn't a solution.
You could say it's just moving the problem from the air to the sea.
As for why CO2, at just 0.04% of the air, can affect temperature at all — Why Can Carbon Dioxide at Just 0.04% of the Air Change the Temperature? works it out by actually counting the amount above your head. And for how many trees it takes a forest to absorb one person's CO2 output, see How Many Trees Does It Take to Absorb One Person's CO2?.
Want to know more? ― Terms, numbers, and how this connects to textbooksWe label how advanced each part is, from junior-high science to active research topics
- JHSCovered in junior high school science
- HSCovered in high school "Earth Science Basics" / "Chemistry Basics"
- HS+High school "Chemistry," or advanced/column content in textbooks
- UnivContent not taught in high school — a university specialist subject (ocean chemistry)
- ResearchNot yet settled even at university level — something researchers are actively investigating
JHSTerms: vocabulary for the ocean and CO2
- Dissolution: when a gas or solid dissolves into a liquid.
- Carbonic acid: a weak acid formed when CO2 dissolves in water.
- Hydrogen ion: a positively charged particle tied to acid strength. More of them means stronger acidity.
- Ocean acidification: the process by which seawater moves toward being acidic as it absorbs CO2.
HSChecking the numbers: how much CO2 has the ocean absorbed?
Using representative statistics, we can estimate how much of the CO2 emitted by human activity the ocean has absorbed.
| Total CO2 emitted by humans since the Industrial Revolution | A representative estimate: roughly 240 billion tonnes (2.4 trillion tonnes) |
| Share of that absorbed by the ocean | A representative estimate: roughly 28% |
| Current annual human CO2 emissions | A representative estimate: roughly 40 billion tonnes (0.04 trillion tonnes) |
These figures vary depending on the survey and estimation method and the year. Treat them purely as representative values for getting a sense of scale.
| Convert 28% to a decimal for division | 28 ÷ 100 = 0.28 |
| Amount of CO2 absorbed by the ocean | 2400 × 0.28 = 672 |
| Amount absorbed by the ocean | ~67.2 billion tonnes |
Let's work out how many years' worth of current annual emissions this amount represents.
| How many years of current annual emissions | 672 ÷ 40 ≒ 16.8 |
| Years of annual emissions | ~16.8 years |
The calculation shows that the CO2 the ocean has absorbed so far amounts to roughly 17 years' worth of humanity's current annual emissions. That gives a sense of just how large the ocean's role has been.
※ The total emissions, absorption share, and annual emissions are all representative estimates, and vary by source and year of calculation.
HS+The reaction doesn't stop at one step
The reactions described in the main text actually occur as a reversible chemical equilibrium made up of multiple stages. The chain CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3⁻ ⇌ 2H+ + CO3²⁻ runs back and forth, staying in balance. In seawater, the bicarbonate ion (HCO3⁻) form is said to make up the largest share.
UnivThe ocean's "buffering capacity" has limits
In ocean chemistry, a concept called the Revelle factor is used as a measure of how much CO2 seawater can absorb while staying relatively stable. As the balance of the carbonate system in seawater shifts, this capacity to absorb is thought to gradually decline. In other words, the more CO2 the ocean absorbs, the harder it becomes to absorb the next increment.
ResearchWhat's still unclear
- How far the ocean's capacity to absorb CO2 will decline as sea temperatures rise is something climate models still have real uncertainty about. How the property that warmer water dissolves less gas will affect future absorption is an important open question.
- How much ocean acidification will affect various marine organisms — shellfish, corals, plankton — varies a great deal by species and remains under active study. Some organisms may be able to adapt to some degree to the changing environment, but a full picture is still some way off.
- Building more accurate models of the entire ocean carbon cycle to predict future absorption is also a major theme in climate science.
The relationship between the ocean and CO2 goes well beyond a simple "it absorbs it for us" — it remains a complex, actively researched topic.
Connections to textbooks (by level)
| Level | Subject/Unit | Where in this article |
|---|---|---|
| JHS | Science — properties of aqueous solutions | Basic terms: dissolution, carbonic acid, hydrogen ions |
| HS | Earth Science Basics / Chemistry Basics | Estimating the total from the absorption share |
| HS+ | Chemistry — acid-base equilibrium | The carbonate system's chemical equilibrium |
| Univ | Ocean chemistry | The Revelle factor and declining ocean buffering |
| Research | Climate science / marine ecology (ongoing) | Predicting future absorption, assessing impacts on marine life |
- Japan Meteorological Agency (気象庁), explanatory material on "ocean CO2 absorption."
- IPCC (Intergovernmental Panel on Climate Change) assessment reports, sections on the carbon cycle and ocean uptake.
- Ocean chemistry textbooks, sections on carbonate-system chemical equilibrium and the Revelle factor.
- Ministry of Education, Culture, Sports, Science and Technology (文部科学省) and Japan Meteorological Agency (気象庁), general-audience explanatory material on "ocean acidification."
- Review literature in marine ecology on the effects of ocean acidification on calcifying organisms.
※ Figures such as emission amounts and absorption shares are representative estimates and vary by source and calculation year. For the latest figures, please check official sources such as the Japan Meteorological Agency or the IPCC.
※This article is a general-audience science explainer. For the latest data and policy developments on climate change and ocean acidification, please check official sources such as the Japan Meteorological Agency or the IPCC.