Carbon dioxide enters the atmosphere through both natural processes and human activities, but the balance between them has shifted dramatically. The global average CO2 concentration hit a record 422.7 parts per million in 2024, up from roughly 280 ppm before the Industrial Revolution. That increase is almost entirely driven by human sources, which now release more than 35 billion metric tons of CO2 per year.
Burning Fossil Fuels
The single largest way CO2 enters the atmosphere is through combustion. When a hydrocarbon fuel like coal, oil, or natural gas burns, its carbon atoms combine with oxygen to produce carbon dioxide and water. A propane grill illustrates the basic chemistry: one molecule of propane reacts with five molecules of oxygen to produce three molecules of CO2 and four molecules of water vapor. Scale that reaction up to every power plant, car engine, furnace, and industrial boiler on the planet, and the numbers become enormous.
Electricity and heat production alone account for 29.5% of global greenhouse gas emissions. Transportation adds another 13.3%. Together, these two sectors are responsible for nearly half of all the CO2 humans put into the air each year. The rest comes from manufacturing, agriculture-related energy use, and buildings.
Cement and Industrial Chemistry
Not all industrial CO2 comes from burning fuel. Some manufacturing processes release carbon dioxide through chemical reactions in the raw materials themselves. Cement production is the clearest example. To make cement, manufacturers grind limestone and heat it in a kiln at temperatures between 2,700 and 3,000 degrees Fahrenheit. That intense heat triggers a reaction called calcination: the limestone (calcium carbonate) breaks apart into calcium oxide and CO2 gas. This chemical release, separate from the fuel burned to heat the kiln, accounts for almost two-thirds of cement’s total CO2 emissions. Globally, cement production contributes about 3.4% of all greenhouse gas emissions.
Deforestation and Land Use Change
Trees and soil store vast amounts of carbon. When forests are cleared, burned, or degraded, that stored carbon escapes into the atmosphere as CO2. Between 2010 and 2019, land use changes released a net 5.9 billion metric tons of CO2 equivalent per year, with deforestation responsible for 45% of that total. Tropical forests are the biggest concern because they hold the most carbon per acre, and clearing them often involves burning, which releases CO2 immediately rather than over decades of slow decomposition.
Volcanoes and Geologic Sources
Volcanoes release CO2 from deep within the Earth, both during eruptions and through continuous venting from magma beneath the surface. Early estimates placed volcanic emissions at roughly 0.3 billion metric tons of CO2 per year. A more recent analysis that included subsurface magma sources revised that figure upward to about 0.6 billion metric tons. Either way, human emissions are more than 90 times greater than volcanic output. Volcanoes are a real source of atmospheric CO2, but they are not a meaningful driver of the modern increase.
Ocean-Atmosphere Gas Exchange
The ocean both absorbs and releases CO2 in a constant exchange at the water’s surface. This process is governed by the difference in CO2 concentration between the air and the water, along with an exchange coefficient that determines how fast gas molecules cross the boundary. When ocean water warms, it holds less dissolved gas, so CO2 escapes into the air. When water cools, it absorbs more. Upwelling zones, where deep water rich in dissolved carbon rises to the surface, are natural CO2 sources.
On balance, the ocean currently acts as a net carbon sink, absorbing more CO2 than it releases. But rising ocean temperatures and changes in circulation patterns could reduce that uptake over time, effectively letting more CO2 stay in the atmosphere.
Biological Respiration and Decomposition
Every living organism that uses oxygen produces CO2. Plants absorb carbon dioxide during photosynthesis, but they also release it when they respire at night or during periods of stress. Animals exhale it constantly. Microbes in soil break down dead organic matter and release CO2 as a byproduct. These biological fluxes are enormous in absolute terms, moving hundreds of billions of tons of carbon between the land, oceans, and atmosphere each year. Under stable conditions, the carbon absorbed by photosynthesis roughly balances the carbon released by respiration and decay. The problem arises when other sources, particularly fossil fuels, add carbon on top of that natural cycle.
Thawing Permafrost
Arctic permafrost contains organic material that has been frozen for thousands of years. As global temperatures rise, this ground thaws and microbes begin decomposing the newly accessible carbon, releasing CO2 and methane. Estimates from the Woodwell Climate Research Center project that permafrost thaw could release between 110 and more than 550 billion metric tons of CO2 by 2100. The upper end of that range is comparable to the total cumulative emissions of the United States at its current rate. Unlike fossil fuel emissions, permafrost thaw is a feedback loop: warming causes thaw, thaw releases greenhouse gases, and those gases cause more warming.
Why CO2 Accumulates
Carbon dioxide does not have a single, clean residence time in the atmosphere. Some molecules are absorbed by plants or oceans within a few years. Others persist for centuries. The IPCC describes the atmospheric lifetime of CO2 as ranging from 5 to 200 years, noting that no single number captures it because different removal processes work at different speeds. This means that even if emissions stopped tomorrow, a significant portion of the CO2 already in the atmosphere would remain there for generations.
The core issue is one of imbalance. Natural sources and sinks moved carbon in a roughly closed loop for millennia. Human activity, primarily fossil fuel combustion, now adds approximately 40 billion metric tons of CO2 per year on top of that loop. Natural sinks like forests and oceans absorb roughly half of it. The rest accumulates, pushing atmospheric concentrations higher year after year.