Query: The clearest of these is simple accounting - humans are emitting CO2 at a rate twice as fast as the atmospheric increase (natural sinks are absorbing the other half).

Relevant passages:
Carbon sink <=SEP=> A carbon sink is a natural or artificial reservoir that accumulates and stores some carbon-containing chemical compound for an indefinite period . The process by which carbon sinks remove carbon dioxide from the atmosphere is known as carbon sequestration . Public awareness of the significance of CO2 sinks has grown since passage of the Kyoto Protocol , which promotes their use as a form of carbon offset . There are also different strategies used to enhance this process .

Carbon dioxide <=SEP=> Carbon dioxide ( chemical formula ) is a colorless gas with a density about 60 % higher than that of air ( 1.225 g/L ) that is odorless at normally encountered concentrations . Carbon dioxide consists of a carbon atom covalently double bonded to two oxygen atoms . It occurs naturally in Earth 's atmosphere as a trace gas at a concentration of about 0.04 percent ( 400 ppm ) by volume . Natural sources include volcanoes , hot springs and geysers , and it is freed from carbonate rocks by dissolution in water and acids . Because carbon dioxide is soluble in water , it occurs naturally in groundwater , rivers and lakes , ice caps , glaciers and seawater . It is present in deposits of petroleum and natural gas .   As the source of available carbon in the carbon cycle , atmospheric carbon dioxide is the primary carbon source for life on Earth and its concentration in Earth 's pre-industrial atmosphere since late in the Precambrian has been regulated by photosynthetic organisms and geological phenomena . Plants , algae and cyanobacteria use light energy to photosynthesize carbohydrate from carbon dioxide and water , with oxygen produced as a waste product .   Carbon dioxide is produced by all aerobic organisms when they metabolize carbohydrates and lipids to produce energy by respiration . It is returned to water via the gills of fish and to the air via the lungs of air-breathing land animals , including humans . Carbon dioxide is produced during the processes of decay of organic materials and the fermentation of sugars in bread , beer and winemaking . It is produced by combustion of wood and other organic materials and fossil fuels such as coal , peat , petroleum and natural gas .   It is a versatile industrial material , used , for example , as an inert gas in welding and fire extinguishers , as a pressurizing gas in air guns and oil recovery , as a chemical feedstock and in liquid form as a solvent in decaffeination of coffee and supercritical drying . It is added to drinking water and carbonated beverages including beer and sparkling wine to add effervescence . The frozen solid form of , known as `` dry ice '' is used as a refrigerant and as an abrasive in dry-ice blasting .   Carbon dioxide is the most significant long-lived greenhouse gas in Earth 's atmosphere . Since the Industrial Revolution anthropogenic emissions - primarily from use of fossil fuels and deforestation - have rapidly increased its concentration in the atmosphere , leading to global warming . Carbon dioxide also causes ocean acidification because it dissolves in water to form carbonic acid .

Greenhouse gas <=SEP=> A greenhouse gas ( abbrev . GHG ) is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range . This process is the fundamental cause of the greenhouse effect . The primary greenhouse gases in Earth 's atmosphere are water vapor , carbon dioxide , methane , nitrous oxide , and ozone . Without greenhouse gases , the average temperature of Earth 's surface would be about -18 ° C , rather than the present average of 15 ° C . In the Solar System , the atmospheres of Venus , Mars and Titan also contain gases that cause a greenhouse effect .   Human activities since the beginning of the Industrial Revolution ( taken as the year 1750 ) have produced a 40 % increase in the atmospheric concentration of carbon dioxide , from 280 ppm in 1750 to 406 ppm in early 2017 . This increase has occurred despite the uptake of a large portion of the emissions by various natural `` sinks '' involved in the carbon cycle . Anthropogenic carbon dioxide emissions ( i.e. , emissions produced by human activities ) come from combustion of fossil fuels , principally coal , oil , and natural gas , along with deforestation , soil erosion and animal agriculture .   It has been estimated that if greenhouse gas emissions continue at the present rate , Earth 's surface temperature could exceed historical values as early as 2047 , with potentially harmful effects on ecosystems , biodiversity and the livelihoods of people worldwide . Recent estimates suggest that on the current emissions trajectory the Earth could pass a threshold of 2 ° C global warming , which the United Nations ' IPCC designated as the upper limit to avoid `` dangerous '' global warming , by 2036 .

Fossil fuel <=SEP=> Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms , containing energy originating in ancient photosynthesis . The age of the organisms and their resulting fossil fuels is typically millions of years , and sometimes exceeds 650 million years . Fossil fuels contain high percentages of carbon and include petroleum , coal , and natural gas . Other commonly used derivatives include kerosene and propane . Fossil fuels range from volatile materials with low carbon to hydrogen ratios like methane , to liquids like petroleum , to nonvolatile materials composed of almost pure carbon , like anthracite coal . Methane can be found in hydrocarbon fields either alone , associated with oil , or in the form of methane clathrates .   The theory that fossil fuels formed from the fossilized remains of dead plants by exposure to heat and pressure in the Earth 's crust over millions of years was first introduced by Georgius Agricola in 1556 and later by Mikhail Lomonosov in the 18th century .   The Energy Information Administration estimates that in 2007 the primary sources of energy consisted of petroleum 36.0 % , coal 27.4 % , natural gas 23.0 % , amounting to an 86.4 % share for fossil fuels in primary energy consumption in the world . Non-fossil sources in 2006 included nuclear 8.5 % , hydroelectric 6.3 % , and others ( geothermal , solar , tidal , wind , wood , waste ) amounting to 0.9 % . World energy consumption was growing about 2.3 % per year .   Although fossil fuels are continually being formed via natural processes , they are generally considered to be non-renewable resources because they take millions of years to form and the known viable reserves are being depleted much faster than new ones are being made .   The use of fossil fuels raises serious environmental concerns . The burning of fossil fuels produces around 21.3 billion tonnes ( 21.3 gigatonnes ) of carbon dioxide ( CO2 ) per year . It is estimated that natural processes can only absorb about half of that amount , so there is a net increase of 10.65 billion tonnes of atmospheric carbon dioxide per year . Carbon dioxide is a greenhouse gas that increases radiative forcing and contributes to global warming . A global movement towards the generation of renewable energy is underway to help reduce global greenhouse gas emissions .