Several human-driven and natural processes increase the rate of greenhouse gases entering the atmosphere. The largest single source is burning fossil fuels for electricity and heat, which alone accounts for 34% of global emissions. But agriculture, industrial manufacturing, waste decomposition, and even thawing permafrost all play significant roles. Atmospheric carbon dioxide recently reached 430 parts per million, and understanding the processes behind that number is the first step to making sense of climate change.
Fossil Fuel Combustion
Burning coal, oil, and natural gas releases carbon that was locked underground for millions of years. Fossil fuels still supply about 80% of the world’s total energy, and the combustion of those fuels is the dominant driver of rising CO2 levels. Coal is the worst offender relative to its share of the energy mix: it makes up 27% of global energy supply but is responsible for 44% of combustion-related CO2 emissions. Oil follows at 32% of emissions, and natural gas at 22%.
These emissions come from power plants generating electricity, engines in cars and trucks, furnaces heating buildings, and industrial boilers. In 2021, global CO2 from fuel combustion rebounded by nearly 6% after a brief dip during the Covid-19 pandemic, returning close to pre-pandemic levels. That rebound illustrates how tightly global economic activity is still linked to fossil fuel use.
Deforestation and Land-Use Change
Forests absorb CO2 as trees grow, storing carbon in their wood, leaves, and root systems. When forests are cleared, whether by burning or logging, that stored carbon is released back into the atmosphere. Tropical deforestation is responsible for the vast majority of these emissions. In 1980, an estimated 1.8 to 4.7 trillion grams of carbon were released from land-use changes, with nearly 80% coming from tropical forest clearing alone.
Before 1960, the annual release of carbon from deforestation and soil disturbance actually exceeded emissions from fossil fuels. Today, agriculture, forestry, and other land use together account for roughly 22% of global greenhouse gas emissions. That figure includes not just deforestation but also crop cultivation and livestock production. The loss of forests is a double hit: it adds CO2 to the atmosphere while simultaneously removing trees that would otherwise pull CO2 back out.
Livestock and Enteric Fermentation
Cattle, sheep, goats, and other ruminants produce methane as a natural byproduct of digestion. Their stomachs contain a dense community of microorganisms, including bacteria, fungi, and a group of single-celled organisms called methanogens. As these microbes break down plant material in the animal’s gut, they produce short-chain fatty acids that supply about 80% of the animal’s energy needs. The tradeoff is that the process also generates hydrogen, and methanogens convert that hydrogen and CO2 into methane, which the animal then belches out.
Methane is far more potent than CO2 as a greenhouse gas. Over a 100-year period, a ton of methane from biological sources traps about 27 times more heat than a ton of CO2. Methane from fossil fuel sources is slightly more potent, at about 30 times CO2, because the methane itself eventually breaks down into CO2 that persists in the atmosphere. With roughly a billion cattle on Earth, enteric fermentation is one of the largest single sources of methane emissions globally.
Synthetic Fertilizers and Nitrous Oxide
When nitrogen-based fertilizers are applied to cropland, soil bacteria convert some of that nitrogen into nitrous oxide (N2O) through two natural microbial processes: nitrification, where bacteria convert ammonia into nitrate, and denitrification, where other bacteria convert nitrate back into nitrogen gas. Both processes release nitrous oxide as a byproduct. Domestic wastewater treatment also generates N2O through the same pathways, breaking down urea, ammonia, and proteins.
Nitrous oxide is the most overlooked of the major greenhouse gases, yet it has a 100-year global warming potential of 273, meaning one ton of N2O traps as much heat as 273 tons of CO2. Natural soil and ocean bacteria produce some N2O on their own, but the widespread use of synthetic fertilizers has dramatically accelerated this process.
Industrial Gases With Extreme Warming Potential
Some of the most potent greenhouse gases have no natural sources at all. Fluorinated gases, including hydrofluorocarbons (HFCs), perfluorocarbons, and sulfur hexafluoride (SF6), are entirely human-made. They’re used as refrigerants, in semiconductor manufacturing, aluminum smelting, magnesium processing, and as insulating gases in electrical equipment.
Their warming potential dwarfs every other greenhouse gas. HFCs can trap up to 12,400 times more heat than CO2 over a century. SF6 is even worse: with a global warming potential of 23,500, it is the most potent greenhouse gas the Intergovernmental Panel on Climate Change has ever evaluated. These gases are emitted in far smaller quantities than CO2 or methane, but even tiny leaks from refrigeration systems or electrical switchgear have an outsized effect on atmospheric warming.
Landfill Decomposition
When organic waste like food scraps, paper, and yard trimmings end up in landfills, they decompose in an oxygen-free environment. Without oxygen, anaerobic bacteria break down the material and produce methane as a primary byproduct, much like the process inside a cow’s stomach. Between 2000 and 2017, landfills worldwide produced an estimated 60 to 69 million metric tons of methane per year. Gas production at individual landfill sites can be highly variable, spiking and dropping depending on the age of the waste, moisture levels, and temperature.
Landfill methane is one of the more manageable sources of emissions because the gas can be captured and either flared (burned off) or used to generate electricity. Still, many landfills around the world lack capture systems, allowing methane to escape directly into the atmosphere.
Permafrost Thaw and Natural Feedback Loops
Arctic and subarctic soils contain vast stores of organic matter that have been frozen for thousands of years, in some cases since the Pleistocene era. As global temperatures rise, this permafrost thaws, exposing that ancient organic material to microbial decomposition for the first time. Microorganisms break down the newly available carbon and release greenhouse gases, primarily CO2.
This creates a feedback loop: warming temperatures thaw permafrost, which releases greenhouse gases, which drive further warming, which thaws more permafrost. Research on Pleistocene permafrost deposits in Arctic Siberia found that CO2 dominated the gas released after thawing, while methane played a smaller role due to the absence of methane-producing microorganisms in those particular deposits. The overall concern is that this feedback loop could accelerate warming beyond what human emissions alone would cause, since the carbon stored in global permafrost is enormous and currently not accounted for in most emissions inventories.
How These Processes Compare by Sector
Globally, electricity and heat production is the single largest sector at 34% of emissions, followed by industry at 24% and agriculture, forestry, and land use at 22%. The remaining share comes from transportation, buildings, and other sources. These percentages reflect 2019 data and give a useful snapshot of where the biggest levers for reduction exist.
What makes the problem complex is that these processes operate on different timescales and involve different gases. CO2 from fossil fuels persists in the atmosphere for centuries. Methane is far more potent in the short term but breaks down after about a decade. Nitrous oxide lasts roughly 114 years. Fluorinated gases can persist for thousands of years. Reducing the rate of greenhouse gas accumulation requires addressing not just the largest sources but also the most potent ones, because a small reduction in SF6 or methane emissions can have an impact that far exceeds its volume.