The Carbon Cycle: Reservoirs, Feedbacks and Tipping Points

Four reservoirs, wildly different sizes

Carbon moves continuously between four great reservoirs. The lithosphere — carbonate rock and buried kerogen — holds an almost incomprehensible ~66,000,000 GtC, essentially locked away on any human timescale. The ocean holds ~38,000 GtC, mostly as dissolved bicarbonate. The terrestrial biosphere — soil, plants, and dead organic matter — holds ~2,600 GtC. And the atmosphere, the reservoir we actually breathe and that sets the greenhouse effect, holds only ~870 GtC. It's this smallest, fastest-cycling pool that a shift of a few gigatonnes per year can visibly move.

The biological pump and the ocean sink

On land, the biological pump runs on photosynthesis pulling CO2 out of the air and respiration/decay putting it back — global gross primary production is roughly 120 GtC/yr, a churn far larger than human emissions but very nearly balanced by the reverse flux. The ocean absorbs carbon two ways: the solubility pump (CO2 dissolving directly into cold water) and its own biological pump (phytoplankton, then sinking organic matter). Since 1850 the ocean has absorbed roughly 170 GtC of anthropogenic carbon — but at a cost: seawater pH has fallen from about 8.18 to 8.05, measurable ocean acidification that weakens shell- and reef-building organisms.

Reservoir            Approx. size (GtC)
Lithosphere (rock)     66,000,000
Ocean                      38,000
Terrestrial biosphere       2,600
Atmosphere                    870
Human emissions            ~10.5 GtC / year (net, all sources)

The slow cycle, human disruption, and feedbacks

Beneath the fast biological and ocean cycles runs a slow cycle — rock weathering draws down ~0.2–0.4 GtC/yr, volcanism releases ~0.1–0.3 GtC/yr back — operating on thousand- to million-year timescales. Human activity now dwarfs this slow balance: fossil fuel combustion, cement production and land-use change add roughly 10.5 GtC per year, pushing atmospheric CO2 from a pre-industrial 280 ppm to about 424 ppm today. Worse, several reservoirs risk becoming sources rather than sinks: permafrost holds 1,400–1,700 GtC that thaw could release, warmer soils respire faster under the Q10 rule, the ocean sink is weakening as it warms and acidifies, and the Amazon risks dieback past a 20–25% deforestation threshold — each a self-reinforcing feedback loop that accelerates warming independent of further human emissions (Armstrong McKay et al., 2022, estimate boreal permafrost alone could add 10–35 GtC by 2100). Proposed countermeasures span nature-based solutions (reforestation, soil carbon), direct air capture, enhanced rock weathering, and bioenergy with carbon capture and storage (BECCS).

Frequently asked questions

Try it live

Everything above runs in your browser — open Carbon Cycle, dial emissions and deforestation rates up or down, and watch the box-model reservoirs respond in real time. Nothing is installed, nothing is uploaded.