Are the Increases in Atmospheric Carbon Dioxide and Other Greenhouse Gases During the Industrial Era Caused by Human Activities?

Yes, the increases in atmospheric carbon dioxide and other greenhouse gases during the industrial era are caused by human activities. In fact, the observed increase in atmospheric carbon dioxide concentrations does not reveal the full extent of human emissions in that it accounts for only 55% of the carbon dioxide released by human activity since 1959. In all cases, atmospheric concentrations of greenhouse gases, and their increases, are determined by the balance between sources (emissions of the gas from human activities and natural systems) and sinks (the removal of the gas from the atmosphere by conversion to a different chemical compound). Fossil fuel combustion (plus a smaller contribution from cement manufacture) is responsible for more than 75% of human-caused carbon dioxide emissions. Land use change (primarily deforestation) is responsible for the remainder. For methane, another important greenhouse gas, emissions generated by human activities exceeded natural emissions over the last 25 years. For nitrous oxide, emissions generated by human activities are equal to natural emissions in the atmosphere. Most of the long-lived halogen-containing gases (such as chlorofluorocarbons) are manufactured by humans, and were not present in the atmosphere before the industrial era. On average, present-day tropospheric ozone has increased 38% since pre-industrial times, and the increase results from atmospheric reactions of short-lived pollutants emitted by human activity. The recent rate of change is dramatic and unprecedented; increases in carbon dioxide never exceeded 30 ppm in 1 kyr – yet now carbon dioxide has risen by 30 ppm in just the last 17 years.

Carbon Dioxide

Emissions of carbon dioxide from fossil fuel combustion, with contributions from cement manufacture, are responsible for more than 75% of the increase in atmospheric carbon dioxide concentration since pre-industrial times. The remainder of the increase comes from land use changes dominated by deforestation (and associated biomass burning) with contributions from changing agricultural practices. All these increases are caused by human activity.

Natural processes such as photosynthesis, respiration, decay and sea surface gas exchange lead to massive exchanges, sources and sinks of carbon dioxide between the land and atmosphere, and the ocean and atmosphere. Were it not for the natural sinks taking up nearly half the anthropogenic carbon dioxide over the past 15 years, atmospheric concentrations would have grown even more dramatically.

The increase in atmospheric carbon dioxide concentration is known to be caused by human activities because the character of carbon dioxide in the atmosphere, in particular the ratio of its heavy to light condition of fossil fuel carbon. In addition, the ratio of oxygen to nitrogen in the atmosphere has declined as carbon dioxide has increased; this is as expected because oxygen is depleted when fossil fuels are burned. A heavy form of carbon, the carbon-13 isotope, is less abundant in vegetation and in fossil fuels that were formed from past vegetation, and is more abundant in carbon in the oceans and in volcanic or geothermal emissions. The relative amount of the carbon-13 isotope in the atmosphere has been declining, showing that the added carbon comes from fossil fuels and vegetation.

Halogen-Containing Gases

Human activities are responsible for the bulk of long-lived atmospheric halogen-containing gas concentrations. Before industrialization, there were only a few naturally occurring halogen-containing gases, for example, methyl bromine and methyl chloride. The development of new techniques for chemical synthesis resulted in a proliferation of chemically manufactured halogen-containing gases during the last 50 years of the 20^th century. Concentrations of several important halogen-containing gases, including CFCs, are now stabilizing or decreasing at the Earth’s surface as a result of the Montreal Protocol on Substances that Deplete the Ozone Layer and its Amendments. Concentrations of HCFCs, production of which is to be phased out by 2030, and of the Kyoto Protocol gases HFCs and PFCs, are currently increasing.

Methane

Methane sources to the atmosphere generated by human activities exceed methane sources from natural systems. Between 1960 and 1999, methane concentrations grew an average of at least six times faster than over any 40-year period of the two millennia before 1800, despite a near-zero growth rate since 1980. The human activities that produce methane include energy production from coal and natural gas, waste disposal in landfills, raising ruminant animals (e.g., cattle and sheep), rice agriculture and biomass burning. Once emitted, methane remains in the atmosphere for approximately 8.4 years before removal, mainly by chemical oxidation in the troposphere.

Nitrous Oxide

Nitrous oxide sources to the atmosphere from human activities are approximately equal to nitrous oxide sources from natural systems. Between 1960 and 1999, nitrous oxide concentrations grew an average of at least two times faster than over any 40-year period of the two millennia before 1800. Human activities that emit nitrous oxide include transformation of fertilizer nitrogen into nitrous oxide and its subsequent emission from agricultural soils, biomass burning, raising cattle and some industrial activities, including nylon manufacture. Once emitted, nitrous oxide remains in the atmosphere for approximately 114 years before removal, mainly by destruction in the stratosphere.

Tropospheric Ozone

Tropospheric ozone is produced by photochemical reactions in the atmosphere involving forerunner chemicals such as carbon monoxide, methane, volatile organic compounds and nitrogen oxides. These chemicals are emitted by natural biological processes and by human activities including land use change and fuel combustion. The increase of 38% (20-50%) in tropospheric ozone since the pre-industrial era is human-caused.

It is very likely that the increase in the combined radiative forcing from carbon dioxide, methane, and nitrous oxide was at least six times faster between 1960 and 1999 than over any 40-year period during the two millennia prior to the year 1800.