Question 1

What is a carbon sink, and how much CO₂ do the oceans absorb?

Accepted Answer

A **carbon sink** is any reservoir absorbing more carbon than it releases. The two dominant natural sinks are the **ocean** and **land vegetation and soils**, and together they absorb roughly **half** of human CO₂ emissions each year.

**The ocean sink** takes up around a quarter of annual emissions. It works through two mechanisms:

**The solubility pump.** CO₂ dissolves in seawater, more readily in cold water. Cold, dense water sinking at high latitudes carries dissolved carbon into the deep ocean, where it can remain for centuries.

**The biological pump.** Phytoplankton take up CO₂ through photosynthesis. When they die, a fraction sinks as particulate matter, transporting carbon to depth.

**The land sink** absorbs a further quarter or so, through forest growth, soil carbon and the **CO₂ fertilisation effect** — plants growing somewhat faster in a higher-CO₂ atmosphere.

**Why this matters enormously.** Without these sinks, atmospheric CO₂ would be rising roughly twice as fast. They are doing an unpaid service of extraordinary scale.

**The costs and the concerns:**

**Ocean acidification** is the direct chemical consequence. Dissolved CO₂ forms carbonic acid, and ocean pH has fallen measurably since industrialisation — a roughly 30% increase in hydrogen ion concentration. This impairs organisms building calcium carbonate shells and skeletons: corals, molluscs, and some plankton at the base of marine food webs. It is sometimes called the other CO₂ problem, and it proceeds regardless of temperature.

**Sinks may weaken.** Warmer water holds less dissolved gas. Forests stressed by drought, fire and pests absorb less, and can become **net sources** — parts of the Amazon have been measured emitting more than they absorb in some years. Thawing permafrost releases stored carbon.

**Saturation.** The fraction of emissions absorbed is not guaranteed to hold.

**Blue carbon** — mangroves, saltmarsh and seagrass — stores carbon very densely per hectare.

Question 2

What is carbon capture and storage, and does it work?

Accepted Answer

Capturing carbon dioxide and putting it underground permanently. The technology demonstrably works; the contested questions are cost, scale, and whether its availability delays cutting emissions in

Question 3

What is blue carbon?

Accepted Answer

Carbon captured and stored by coastal and marine ecosystems — principally saltmarsh, mangroves and seagrass — which sequester carbon at rates far exceeding most terrestrial habitats, per unit area.

Question 4

What is a carbon offset and do they actually work?

Accepted Answer

A carbon offset is a credit representing one tonne of CO₂ either removed from the atmosphere or prevented from being emitted somewhere else, bought to compensate for emissions you produced. The types

Question 5

Why do peat bogs matter so much?

Accepted Answer

Because they store an extraordinary quantity of carbon in a very small area, and damaged peatland releases it — turning one of the most effective natural carbon stores into an emissions source. What

Question 6

What is ocean acidification?

Accepted Answer

The fall in ocean pH caused by seawater absorbing carbon dioxide from the atmosphere — sometimes called "the other carbon problem", because it is a direct chemical consequence of emissions that has

Question 7

What is eutrophication and why do rivers turn green?

Accepted Answer

Eutrophication is the over-enrichment of water with nutrients — principally nitrogen and phosphorus — which triggers explosive algal growth and can ultimately strip the water of oxygen. The sequence:

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