This chapter discusses the biogeochemical cycles of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). These greenhouse gases have increased significantly since pre-industrial times, driving climate change. Together, they account for 80% of the total radiative forcing from well-mixed greenhouse gases. The increase in these gases is primarily due to anthropogenic emissions from fossil fuel use and land use changes, particularly agriculture. The observed changes in atmospheric concentrations result from the balance between emissions and natural processes that remove these gases. The physical climate system and these biogeochemical cycles are interconnected. Since the Industrial Era, CO₂ concentrations have increased by 40% from 278 ppm in 1750 to 390.5 ppm in 2011. CH₄ has increased by 150% from 722 ppb to 1803 ppb, and N₂O by 20% from 271 ppb to 324.2 ppb. These concentrations exceed any level measured in the past 800,000 years. The increase in CO₂ emissions from fossil fuels and land use changes is the dominant cause of the observed increase in atmospheric CO₂. About half of the emissions remain in the atmosphere, while the rest is removed by natural sinks. CO₂ emissions from fossil fuels and cement production increased faster between 2000–2011 than between 1990–1999. Anthropogenic CO₂ emissions from land use changes were 0.9 ± 0.8 PgC yr⁻¹, representing about 10% of total emissions. Atmospheric CO₂ concentration increased at an average rate of 2.0 ± 0.1 ppm yr⁻¹ during 2002–2011. The combined natural land and ocean sinks removed 55% of total anthropogenic emissions annually from 1958–2011. Atmospheric CH₄ concentrations have shown renewed growth since 2007, with natural wetlands and agriculture being major sources. Anthropogenic emissions account for 50–65% of total emissions. N₂O emissions have increased steadily, with anthropogenic emissions from agriculture, fossil fuels, and biomass burning contributing significantly. Natural N₂O emissions from soils and oceans are also substantial. Before the Industrial Era, atmospheric CO₂ concentrations varied between 180 ppm during glacial periods and 300 ppm during interglacial periods. The human-caused increase in CO₂ since pre-industrial times is much greater than natural variations. Future projections indicate that ocean carbon uptake will continue under all RCP scenarios, with higher uptake corresponding to higher concentration pathways. Land carbon uptake is more uncertain, with some models projecting a net loss due to climate andThis chapter discusses the biogeochemical cycles of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). These greenhouse gases have increased significantly since pre-industrial times, driving climate change. Together, they account for 80% of the total radiative forcing from well-mixed greenhouse gases. The increase in these gases is primarily due to anthropogenic emissions from fossil fuel use and land use changes, particularly agriculture. The observed changes in atmospheric concentrations result from the balance between emissions and natural processes that remove these gases. The physical climate system and these biogeochemical cycles are interconnected. Since the Industrial Era, CO₂ concentrations have increased by 40% from 278 ppm in 1750 to 390.5 ppm in 2011. CH₄ has increased by 150% from 722 ppb to 1803 ppb, and N₂O by 20% from 271 ppb to 324.2 ppb. These concentrations exceed any level measured in the past 800,000 years. The increase in CO₂ emissions from fossil fuels and land use changes is the dominant cause of the observed increase in atmospheric CO₂. About half of the emissions remain in the atmosphere, while the rest is removed by natural sinks. CO₂ emissions from fossil fuels and cement production increased faster between 2000–2011 than between 1990–1999. Anthropogenic CO₂ emissions from land use changes were 0.9 ± 0.8 PgC yr⁻¹, representing about 10% of total emissions. Atmospheric CO₂ concentration increased at an average rate of 2.0 ± 0.1 ppm yr⁻¹ during 2002–2011. The combined natural land and ocean sinks removed 55% of total anthropogenic emissions annually from 1958–2011. Atmospheric CH₄ concentrations have shown renewed growth since 2007, with natural wetlands and agriculture being major sources. Anthropogenic emissions account for 50–65% of total emissions. N₂O emissions have increased steadily, with anthropogenic emissions from agriculture, fossil fuels, and biomass burning contributing significantly. Natural N₂O emissions from soils and oceans are also substantial. Before the Industrial Era, atmospheric CO₂ concentrations varied between 180 ppm during glacial periods and 300 ppm during interglacial periods. The human-caused increase in CO₂ since pre-industrial times is much greater than natural variations. Future projections indicate that ocean carbon uptake will continue under all RCP scenarios, with higher uptake corresponding to higher concentration pathways. Land carbon uptake is more uncertain, with some models projecting a net loss due to climate and