This simulation models Earth's global carbon cycle across four major reservoirs: the atmosphere, the land biosphere (vegetation and soils), the surface ocean, and the deep ocean ā with fossil fuel extraction as an external source. Carbon fluxes are driven by photosynthesis (gross primary production moving CO2 from air to plants), ecosystem respiration and decomposition (returning carbon to the atmosphere), ocean gas exchange (CO2 dissolving at the sea surface proportional to the atmospheric excess above pre-industrial levels), and the thermohaline mixing that transfers dissolved carbon from the surface into the deep ocean on century timescales. The flux magnitudes are calibrated to IPCC AR6 estimates.
Use the Emissions slider to set fossil fuel and industrial CO2 output (current global rate ~9 GtC/yr), and Deforestation to simulate land-use change. Hit Net Zero to instantly zero both sources and watch the atmosphere slowly recover as natural sinks absorb the excess carbon. The info panel tracks simulated year, atmospheric carbon in GtC and approximate CO2 concentration in ppm, estimated warming relative to pre-industrial (using a 3 Ā°C climate sensitivity per CO2 doubling), and the net annual flux into the atmosphere. The animated particles show the direction and relative intensity of each flux pathway.
What is the carbon cycle?
The carbon cycle describes how carbon atoms continuously move between the atmosphere, living organisms, soils, oceans, and rocks. It is driven by biological processes like photosynthesis and respiration, physical processes like gas exchange at the ocean surface, and geological processes like volcanism and weathering over millions of years. Human activities have significantly accelerated the atmospheric component by burning fossil fuels.
How much carbon is in the atmosphere right now?
As of 2024 the atmosphere contains roughly 860 gigatonnes of carbon (GtC), corresponding to about 420 parts per million of CO2. Before industrialisation began around 1750 the atmospheric stock was approximately 590 GtC (~280 ppm). Human emissions have added more than 270 GtC to the air since then, though roughly half has been absorbed by land plants and the ocean.
What are the main natural carbon sinks?
The two dominant natural sinks are land vegetation plus soils, which currently absorb about 3 GtC per year through enhanced plant growth stimulated by higher CO2 (CO2 fertilisation), and the surface ocean, which absorbs around 2.5 GtC per year through physical dissolution of CO2 into seawater. Together these sinks offset about half of current annual human emissions of roughly 10 GtC per year.
CO2 dissolves into seawater according to Henry's Law: the higher the atmospheric partial pressure of CO2, the more gas enters the ocean surface layer. Once dissolved it reacts with water to form carbonic acid, which partly dissociates into bicarbonate and carbonate ions. The surface ocean carbon then slowly transfers to the deep ocean over decades via thermohaline circulation and sinking of organic particles. This process is becoming less efficient as the ocean warms and acidifies.
Higher atmospheric CO2 concentrations increase the rate of photosynthesis in many plant species by improving the efficiency of the enzyme rubisco. This CO2 fertilisation effect is observed in satellite data as a global greening trend. However, its benefit is limited by temperature, water, and nutrient availability. Current models estimate the land biosphere absorbs an extra 1ā2 GtC/yr above its pre-industrial uptake because of elevated CO2.
Net zero means the total amount of greenhouse gases emitted into the atmosphere equals the amount removed. For CO2 this typically requires cutting fossil fuel burning to near zero while enhancing or preserving natural sinks. Reaching net zero halts further accumulation of CO2 in the atmosphere but does not immediately reverse warming, because the CO2 already added will persist for centuries and the climate system has substantial inertia.
There is no single answer because carbon is exchanged continuously between reservoirs at different rates. About half of a CO2 pulse is absorbed by land and ocean within decades. The remaining fraction persists much longer: roughly 20% stays in the atmosphere for centuries, and about 5ā10% will persist for tens of thousands of years until geological weathering removes it. This long tail is why early action on emissions matters so much.
When CO2 dissolves in seawater it forms carbonic acid, lowering the pH of the ocean. Since industrialisation the average ocean pH has dropped from 8.2 to about 8.1 ā a 26% increase in acidity. This threatens organisms that build shells or skeletons from calcium carbonate, including corals, oysters, and many plankton species that underpin marine food webs. A more acidic ocean also becomes less efficient at absorbing additional CO2, creating a positive feedback on climate.
Fossil fuel CO2 emissions are calculated from national statistics on coal, oil, and natural gas production and trade, multiplied by the carbon content and combustion efficiency of each fuel type. Industrial processes such as cement production (which releases CO2 from limestone) are also included. The Global Carbon Project compiles these estimates annually; 2023 emissions reached a record 10.3 GtC/yr from fossil fuels alone.
Forests are a vital and cost-effective carbon sink, but they cannot fully offset current emission rates. Global forests store about 860 GtC and absorb a few GtC per year. Protecting existing forests from deforestation and degradation preserves existing stocks, while reforestation and afforestation can modestly increase uptake. However, forests are also vulnerable to drought, fire, and pest outbreaks that can flip them from sinks to sources, so they complement but cannot replace fossil fuel reductions.