Climate Change: Greenhouse Gases

Carbon Dioxide

Global Temperature Change from 1850-2018. Graphics available for the globe and most countries. Ed Hawkins
Record high 414.71 ppm carbon dioxide reached June 13, 2019
No precedent for this level of carbon dioxide in over 800,000 years of ice core data

Click to access mlo_full_record.pdf

View of Mauna Kea and Mauna Loa from Haleakala
Photo by Ralph Howland on Unsplash
The Keeling Curve–the iconic documentation of carbon dioxide in the Earth’s atmosphere from 1958

The atmospheric abundance of CO2 has increased by an average of 1.83 ppm per year over the past 40 years (1979-2018). The CO2 increase is accelerating — while it averaged about 1.6 ppm per year in the 1980s and 1.5 ppm per year in the 1990s, the growth rate increased to 2.3 ppm per year during the last decade (2009-2018). The annual CO2 increase from 1 Jan 2018 to 1 Jan 2019 was 2.5 ± 0.1 ppm (see, which is slightly higher than the average of the previous decade, and much higher than the two decades before that.

NOAA Carbon Dioxide Radiative Forcing

The relentless rise of carbon  dioxide

Total Global CO2 Emissions

Photo by Martin Adams on Unsplash

Carbon dioxide emissions grew by 2.0% in 2018, the fastest growth for seven years; China (2.2% increase), India (7% increase) and the U.S.(2.6% increase) were responsible for around 69% of global emissions uptick (

Total global carbon dioxide emissions in 2017: 41.2±2.8 GtCO2; 53% increase over 1990 (Source: Global Carbon Project)

  • Carbon dioxide emissions attributable to land-use change are highly uncertain, with no clear trend in the last decade
  • Land-use change was the dominant source of annual CO2 emissions until around 1950. In 1960 land-use change emissions accounted for 43% of emissions; between 2008-2017 they averaged 13%. In total, CO2 from land-use change accounts for 31% of cumulative emissions between 1870-2017.
  • Fossil CO2 emissions now dominate global changes: Coal (32%), Oil (25%), Gas (10%), Others (2%) account for 69% of cumulative emissions between 1870-2017.

Fate of anthropogenic CO2 emissions (2008-2017)

Source: Global Carbon Project Carbon Budget 2018 GCP Presentation
Source :  Figures from the Global Carbon Budget 2018 

Nitrous Oxide (N2O) (EPA N2O)

Shanghai, China  Photo by Holger Link on Unsplash

Radiative Forcing:  298 times that of CO2 over a 100 year period Residence time: 114 years


  • Anthropogenic (40%)
  • Agriculture (soil management—fertilizers, manure, burning of agricultural residues)
  • Fuel Combustion
  • Industry
  • Wastewater treatment

Growth of nitrous oxide

Red line is a linear fit to the global mean data demonstrating a fairly constant annual growth rate greater than 0.75 ppb/year

Global history of nitrous oxide as a function of latitude (y-axis) and time (x-axis).

Short-Lived Climate Pollutants

Black Carbon: 6600Gg (2015)

  • Radiative Forcing:    Black Carbon (BC) has a strong influence on radiative forcing, affecting the climate globally and regionally, and is responsible for a significant proportion of the global forcing to date.
    • Bond (2013)*  estimated that black carbon, with a total climate forcing of +1.1 W m-2, is the second most important human emission in terms of its climate-forcing in the present-day atmosphere; only carbon dioxide is estimated to have a greater forcing. For comparison, the radiative forcings including indirect effects from emissions of the two most significant long-lived greenhouse gases, carbon dioxide (CO2) and methane (CH4), in 2005 were +1.56 and +0.86 W m-2, respectively.
    • BC deposited on the cryosphere leads to enhanced melting rates and can affect the intensity and distribution of precipitation. Climate and Clean Air Coalition
    • Residence Time:  4–12 days

! ! Global emission estimates have uncertainties of about a factor of 2 (i.e., -50% to +100%) or even by a factor of 3 (See Climate and Clean Air Coalition)


  • Domestic biomass combustion (especially in traditional cookstoves)
  • Open-burning of municipal solid waste
  • Crop residue open-burning in the field
  • Traditional brick kilns
  • Forest fires and savanna burning
  • Traditional coke ovens
  • Charcoal making
  • Flaring from oil gas extraction and processing
  • Transportation (diesel engines in on-and off-road vehicles, ships, generators)

*Bond, T. C., Doherty, S. J., Fahey, D. W., Forster, P. M., Berntsen, T., DeAngelo, B. J., Flanner, M. G., Ghan, S., Kärcher, B., Koch, D., Kinne, S., Kondo, Y., Quinn, P. K., Sarofim, M. C., Schultz, M. G., Schul, M., Venkataraman, C., Zhang, H., Zhang, S., Bellouin, N., Guttikunda, S. K., Hopke, P. K., Jacobson, M. Z., Kaiser, J. W., Klimont, Z., Lohmann, U., Schwarz, J. P., Shindell, D., Storelvmo, T., Warren, S. G., and Zender, C. S.: Bounding the role of black carbon in the climate system: A scientific assessment, J. Geophys. Res.-Atmos., 118, 5380– 5552,, 2013.

EPA Black Carbon

Global Methane: 1856.2 ppb (February 2018)

Source: NOAA methane trends

  • Radiative Forcing:  84 times stronger than CO2 per unit of mass in a 20 year period; 28 times greater over a 100 year period
  • Residence Time:  12 years
  • Key precursor to tropospheric ozone formation (see “Particulates and Ozone”)
  • Anthropogenic Sources (60% of global total emissions)
  • Livestock enteric fermentation (see note below chart)
  • Livestock manure
  • Rice cultivation
  • Miscellaneous agricultural sources
  • Waste treatment
  • Waste water treatment

Blue Arrow:  After a 7 year near-zero growth, methane emissions surged starting in 2007, and accelerated again in 2014.  Over half of all anthropogenic methane emissions are attributable to agriculture.  The recent surge in methane may be due in part (~50%) to an increase in ruminant emissions as well as a possible spike in tropical wetland emissions (Rising methane:  a new challenge). Rising methane emissions were not accounted for in IPCC projections of atmospheric temperature change; decreasing methane emissions were assumed.

Stratospheric Ozone Depletion

The abundances of the majority of Ozone Depleting Substances (ODSs) that were originally controlled under the Montreal Protocol are now declining, as their emissions are smaller than the rate at which they are destroyed. In contrast, the abundances of most of the replacement compounds, HCFCs and hydrofluorocarbons are increasing. (United Nations Scientific Assessment of Ozone Depletion 2018)

Linkages between stratospheric ozone depletion, UV radiation, and climate change, including environmental effects and potential consequences for human well-being, food and water security, and the sustainability of ecosystems (solid lines), with important feedback effects driven by human action (double-arrow solid lines) and other processes (dashed lines).

Source: Environmental Effects and Interactions of Stratospheric Ozone Depletion


  The ozone hole is not technically a “hole” where no ozone is present, but is actually a region of exceptionally depleted ozone in the stratosphere over the Antarctic that happens at the beginning of Southern Hemisphere spring (August–October). Satellite instruments provide us with daily images of ozone over the Antarctic region. The ozone hole image shows the very low values (blue and purple colored area) centered over Antarctica on 4 October 2004. From the historical record we know that total column ozone values of less than 220 Dobson Units were not observed prior to 1979. From an aircraft field mission over Antarctica we also know that a total column ozone level of less than 220 Dobson Units is a result of catalyzed ozone loss from chlorine and bromine compounds. For these reasons, we use 220 Dobson Units as the boundary of the region representing ozone loss. Using the daily snapshots of total column ozone, we can calculate the area on the Earth that is enclosed by a line with values of 220 Dobson Units (the white line in the figure).                     Source:  NASA Ozone Watch 

Antarctic Situation June 24 2019 Source: UK Met Ozone

  • The 2018 ozone hole:  The polar vortex began to form in early May and reached its maximum area in late September at around 34 million square kilometres.  It was a little larger than the decadal mean in size, and was generally of average or above average stability.  The ozone hole grew rapidly and by its maximum in late September was above the average size for the decade at 24.8 million square kilometres.  The area with ozone hole values had declined to zero by the end of November, later than in the last two years, but sooner than the decadal average.    NASA observations show that a minimum ozone amount of 102 DU was reached on October 11 and 12.  Although this is a low value it is not as low as around 1990 to 2000.  Ozone depletion would have been much worse this year without the protection of the Montreal Protocol.
  • Although the amount of ozone destroying substances in the atmosphere is going down, the inter-annual variation in the size and depth of the ozone hole is largely controlled by the meteorological conditions in the stratosphere. 
  • A simple extrapolation of the trend in minimum values gives the final year with ozone hole levels as 2073, though the error bars on this estimate are very large.  Models suggest that recovery may be more rapid after 2010. 
  • It is still too soon to say that we have had the worst ever ozone hole, particularly as there has been no major volcanic eruption in the Southern Hemisphere since 1992.  There has also been little cooling of the lower stratosphere since the mid 1990s. 

Chlorofluorocarbons (CFC’s)

Observations reported in Nature in May 2018 showed that the rate of decline of CFC-11, an ozone depleting substances in the atmosphere, which is also a greenhouse gas, had become slower than predicted.  This suggested that either something unusual was taking place in the atmosphere or that there were additional man-made emissions.  The paper suggested that the most likely reason was illegal manufacture and release from somewhere in eastern Asia.  Investigation by the EIA has found that production of polyurethene foam in China can explain the observed changes.  They encourage the Chinese government to take immediate action.  This became news again in May 2019 when another paper was published in Nature.

Source:  UK Met Ozone

Hydrofluorocarbons & Hydrochlorofluorocarbons (HCFCs) Source: HFC’s

HCFCs are growing at a rate of 8-15%/year

Radiative ForcingThe most abundant HFC is 1,430 times stronger than carbon dioxide per unit of mass

Source:  UK Met Ozone

            CFC’s—refrigerants; decompose in the upper atmosphere and catalyze the destruction of ozone.  Phased out according to the provisions of the 1990 Montreal Protocols of 1987, 1990.

            HFC’s and HCFC’s—Substitutes for CFC’s.  HCFC to be phased out by 2040 because they destroy ozone.  HFC’s targeted for emission reductions in the  Kyoto Protocol

Residence Time:

            CFC’s—100 years (CFC-12)

            HCFC’s—12 years

            HFC’s –13 years (e.g., HFC-134a); lifetime weighted by usage—15 years

Ozone Depleting Gas Index (ODGI) Source: NOAA ODGI

The ODGI is estimated directly from observations at Earth’s surface of the most abundant long-lived, chlorine and bromine containing gases regulated by the Montreal Protocol (15 individual chemicals). These ongoing, surface-based observations provide a direct measure of the total number of chlorine and bromine atoms in the lower atmosphere, or troposphere, contained in chemicals with lifetimes longer than approximately 0.5 yr. Because the lower atmosphere is quite well-mixed, these observations also provide an accurate estimate of the composition of air entering the stratosphere. The threat to stratospheric ozone from ODSs, however, is derived only after considering additional factors: the time it takes for air to be transported from the troposphere to different regions of the stratosphere, air mixing processes during that transport, and the rate at which ODSs photolytically degrade and liberate reactive forms of chlorine and bromine in the stratosphere.

ODGI “100” is defined as 100 at the peak in ozone depleting halogen abundance as determined by NOAA observations, and zero for the 1980 level, which corresponds to when recovery of the ozone layer might be expected based on observations in the past.
  • 2018 Antarctic ODGI 79; i.e., we have progressed 21% towards the 1980 benchmark
  • 2018 Mid-latitude ODGI 55; i.e., we have progressed 45% towards the 1980 benchmark

Particulates and Ozone

Atmospheric composition is determined by the mixture of emissions to the atmosphere, transport within the atmosphere, and UV-B radiation. The key interactions determining the composition includes (1) transport of ozone from the stratosphere, (2) emission of a wide range of substances from the ground, (3) transformation of material through the action of UV radiation (and particularly UV-B), and (4) mixing of the pollutants in the atmosphere. The resultant O3 and aerosols, in turn, have impacts on human health (5) and plants (6).
Environmental Effects and Interactions of Stratospheric Ozone Depletion (Chapter 6)

Particulate Matter (PM2.5) Air Quality

Mumbai, India            Photo by Abhay Singh on Unsplash

Particulate matter contains microscopic solids or liquid droplets that are so small that they can be inhaled and cause serious health problems. Some particles less than 10 micrometers in diameter can get deep into your lungs and some may even get into your bloodstream. Of these, particles less than 2.5 micrometers in diameter, also known as fine particles or PM2.5, pose the greatest risk to health.( EPA PM2.5) These particles are small enough to work their way deep into the lungs and into the bloodstream, where they trigger heart attack, stroke, lung cancer and asthma (Berkeley Earth Air Pollution)

Get the Airvisual 2018 World Air Quality Report Here

  • Air pollution kills more people worldwide each year than does AIDS, malaria, diabetes or tuberculosis.
  • For the United States and Europe, air pollution is equivalent in detrimental health effects to smoking 0.4 to 1.6 cigarettes per day.
  • In China the numbers are far worse; on bad days the health effects of air pollution are comparable to the harm done smoking three packs per day (60 cigarettes) by every man, woman, and child.
  • Air pollution is arguably the greatest environmental catastrophe in the world today. (Berkeley Earth Air Pollution)
2018 Ten Most Polluted Countries (Airvisual)

Unhealthy: The following groups should avoid prolonged or heavy exertion.

  • People with heart or lung disease
  • Children and older adults

Unhealthy for Sensitive Groups: The following groups should reduce prolonged or heavy exertion

  • People with heart or lung disease
  • Children and older adults

Changes (1990 to 2015) in the aerosol column optical depth at 550 nm, computed as the mean of six global models Environmental Effects and Interactions of Stratospheric Ozone Depletion (Chapter 6)

Carbon Dioxide Radiative Forcing

The annual greenhouse gas index (AGGI) for 2018 was 1.43—a 43% increase in radiative forcing since 1990

Radiative forcing, relative to 1750 due to carbon dioxide alone since 1979. The percent change from January 1, 1990 in this forcing is shown on the right axis.
Source: NOAA Carbon Dioxide Radiative Forcing

Radiative forcing, relative to 1750, of all the long-lived greenhouse gases. The NOAA Annual Greenhouse Gas Index (AGGI), which is indexed to 1 for the year 1990, is shown on the right axis.  1990 is the year of the Montreal Protocol, which regulated ozone depleting gases.  The annual greenhouse gas index (AGGI) for 2018 was 1.43—a 43% increase in radiative forcing since 1990
NOAA Annual Greenhouse Gas Index

Pre-1978 changes in the CO2-equivalent abundance and AGGI based on the ongoing measurements of all greenhouse gases
NOAA Annual Greenhouse Gas Index

(Left vertical axis) The heating imbalance in watts per square meter relative to the year 1750 caused by all major human-produced greenhouse gases: carbon dioxide, methane, nitrous oxide, chlorofluorocarbons 11 and 12, and a group of 15 other minor contributors. Source: NOAA graph, based on data from NOAA ESRL.
Today’s atmosphere absorbs about 3 extra watts of incoming solar energy over each square meter of Earth’s surface. According to NOAA’s Annual Greenhouse Gas Index (right axis) the combined heating influence of all major greenhouse gases has increased by 41% relative to 1990.  Annual Greenhouse Gas Index

Uncertainties in Emissions Inventories Source: Climate and Clean Air Coalition

“Studies of emission inventories show that, of the major pollutants, the lowest uncertainties are associated with CO2 and SO2,which depend primarily on the quality of fossil fuel statistical data and fuel properties. Studies estimate globally an 8% uncertainty (90% confidence interval) for emissions of CO2 (Andres et al., 2012; IPCC, 2014) and 8–14% uncertainty for SO2, for a roughly 5–95% confidence interval (Smith et al., 2011). However, uncertainty for certain sectors can be much larger, for example 50% for global estimates of CO2 emissions from the combined Agriculture, Forestry and Other Land Use sector (IPCC, 2014). Similarly, uncertainty can be larger in certain regions (e.g. China) due to uncertainties in the level of coal consumption, emission factor for coal and the actual implementation and efficiency of control technology (Guan et al., 2012; Liu et al., 2015; Olivier et al., 2015; Xu et al., 2009; Zhang et al., 2012). Uncertainties for global inventories of GHGs other than CO2 are much higher, being estimated by IPCC (2014) at ±20% for CH4 and ±60% for N2O (both expressed as the 90% confidence interval). Again, uncertainties for some sectors are much higher, for example, for CH4 emissions from rice paddy fields, livestock enteric fermentation and landfill.

Emissions of PM, including BC and primary OC, are more uncertain, as these pollutants usually form under poor combustion conditions in small, inefficient installations burning poor-quality fuels, which are difficult to account for, resulting in large emission variability (Bond et al., 2004; Klimont et al., 2017; Hoesly et al., 2018). Considering local data and knowledge about emission sources and their emission factors could significantly reduce these uncertainties (Zhang et al., 2009). Inconsistencies in measurements of PM emissions (e.g. in-stack or directly after stack for industry; laboratory versus real- world measurements for cookstoves) in different countries contribute to overall global inventory uncertainties. Uncertainty can also be large for activity data of relevance to PM emissions – such as poor-quality fuels (e.g. biomass) in cook stoves or brick kilns (Klimont et al 2017) or even size and composition of local vehicle fleets.

Bond et al. (2004) estimated total uncertainties of about a factor of 2 (i.e. -50% to +100%) in their global estimates of BC and OC emissions for 1996 from contained combustion (excluding open-burning of vegetation and crop residues). More recent work (Bond et al., 2013) estimated larger uncertainties for a global BC inventory for the year 2000; of around a factor of 3 for energy-related emissions and >3 when open-burning is included. Advances in emission characterization for small residential, industrial, and mobile sources will be required to reduce the scale of these uncertainties. Uncertainties in national scale BC emission estimates are likely to be less than for the global inventories described above. For example, emission uncertainties are considered to be in the order of 1.5 to 2-fold for national BC inventories recently prepared by EU countries for CLRTAP.”   (Emphasis added)

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