Greenhouse Gas Emissions

Updated June 2022

To help explain how the increase of anthropogenic (of human origin) greenhouse gases (GHGs) heat the planet, physicist John Tyndall used a dam as an analogy1; if water continually flows into a dam and the dam’s wall is made higher, the dam holds more water than before, until it again overflows. In the same manner, the sun’s radiation continually enters Earth’s atmosphere, humanity has increased the atmospheric concentration of greenhouse gases, this will cause the atmosphere to hold more heat until again the same amount of energy arriving from the sun is released to space.

The temperature impact of each of our GHG emissions is shown below. Most recent values are shown on the left (2019 relative to 1750), and over time since 1750 on the right, indicating that carbon dioxide (CO2) is the only rapidly changing contributor.


Chart 1. Contributions to warming in 2019 relative to 1750. Reference: IPCC, AR6, WG1, chapter 7, The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity2.
Chart 2. Contributions to warming over time, from 1750 to 2019. Reference: IPCC, AR6, WG1, chapter 7, The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity3.

The total human forced global surface air temperature change from 1750 to 2019 is calculated to be 1.29 [1.00 to 1.65]°C (high confidence).

IPCC, AR6, WG1, chapter 7, The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity2.

Note that while the aim of the “Paris Agreement”4 is to hold the global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels, it does not define “pre-industrial”. The IPCC’s “Special Report on Global Warming of 1.5˚C” states –

This IPCC Special Report on Global Warming of 1.5°C uses the reference period 1850–1900 to represent pre-industrial temperature. This is the earliest period with near-global observations and is the reference period used as an approximation of pre-industrial temperatures in the IPCC Fifth Assessment Report.


IPCC, FAQ Chapter 1, FAQ 1.25.

Carbon dioxide (CO2) accounts for 80% of warming6, methane (CH₄) 22%, and nitrous oxide (N₂O) 10%, all with a high level of confidence2. Cooling by aerosols accounts for -0.5˚C, with medium confidence2.

CO2 is a long-lived GHG, as shown by chart 3. Earth’s long term warming commitment is therefore almost solely determined by cumulative CO2 emissions (N2O7 is also a long-lived greenhouse gas that contributes to the warming commitment, but has a much smaller contribution as detailed above).

Chart 4 shows the temperature response to sustained GHG emissions (note that different colours apply to gases in each chart). On a century–long timescale, a pulse emission of CO2 causes constant warming, and continuous CO2 emissions accumulate, causing ever increasing warming. Emissions due to CH₄ and other short-lived greenhouse gases stabilise, as shown in chart 4.

Chart 3. Temperature response to a 1 year pulse of our emissions from 2008. Reference: IPCC AR5 WG18.
Chart 4. Temperature response to sustained emissions, using 2011 as an example. Carbon dioxide (red), methane (blue), organic and black carbon (black), nitrous oxide (green) and HFC-152a (pink). Reference: “New use of global warming potentials to compare cumulative and short-lived climate pollutants”9.

The warming impact of the cumulative pollutants, CO2 and nitrous oxide, increases steadily as long as these emissions persist, whereas sustained emissions of methane and organic and black carbon aerosols cause temperatures to warm rapidly at first, and then stabilize. A permanent reduction of 50–75% in these SLCPs (short lived climate pollutants) could reduce global temperatures by over 0.5˚C by mid-century, comparable to the impact on these timescales of similar-magnitude reductions of CO2 emissions and, it has been argued, at much lower cost. Stabilizing global temperatures, however, requires net emissions of cumulative pollutants, predominantly CO2, to be reduced to zero.

Allen, M., Fuglestvedt, J., Shine, K. et al. New use of global warming potentials to compare cumulative and short-lived climate pollutants.10.

After 500 years, about a third of a CO2 emission pulse remains in the atmosphere11.

Anthropogenic CO2 emissions originate from three sectors: energy, cement and land-use change. Chart 5 shows that emissions from the world energy system (i.e. fossil fuels) dominate. These rebounded strongly in 2021, as shown in chart 6 (this isn’t shown in chart 5 because data for land-use change in 2021 isn’t available at time of publishing).

Chart 5. Left: Annual world anthropogenic CO2 emissions in units of billions of tonnes (GtCO2), from 1850 to 2020. Right: Same as left but showing separate contributors from 1959 to 2020. Reference: Author’s own work using data from the Global Carbon Project published in April 202212.

Global fossil CO2 emissions in 2021 are set to rebound close to their pre-COVID levels after an unprecedented drop in 2020. Emissions from coal and gas use are set to grow more in 2021 than they fell in 2020, but emissions from oil use remain below 2019 levels.

Global Carbon Budget Summary Highlights13.
Chart 6. Anthropogenic annual CO2 emissions, 1959-2021 (1959-2020 for Flaring and Land-use change). Data: Global Carbon Project published in April 202212. Values for 2021 are projected14.
Chart 7. World anthropogenic CO₂ sources and sinks in 2020. Note that the share of CO₂ from cement shown is that emitted by clinker production, the main constituent of cement. This share is often quoted as about 8% but that refers to the cement industry, and therefore includes emissions from the industry’s energy consumption. In the above chart this is already included in the fossil fuel segments. Reference: Author’s own work using data from the Global Carbon Project published in April 202215.

Chart 8 shows fossil fuel CO₂ emissions categorised by country, and by economic sector. The red outer bar in the left chart shows that fossil fuel CO₂ emissions from countries emitting less than a 5% share account for nearly half of total emissions (i.e. 46%). The blue outer bar in the right chart shows 13 sectors across only 6 countries also account for about half global fossil fuel CO₂ emissions (49%, numbered 1 to 13).

Chart 8. World fossil fuel CO₂ emissions in 2018 by country (left), and economic sector (right). Reference: Author’s own work using IEA(2020)16.

Current emission trends are so rapid that, if maintained, the quantity of CO2 to be emitted between 2010 and 2030 will be equivalent to that emitted between 1750 and 1970 –

It has taken society nearly 220 years (from 1750 to 1970) to emit the first trillion tons of CO2 and only another 40 years (1970–2010) to emit the next trillion tons. The third trillion tons, under current emission trends, would be emitted by 2030 and the fourth trillion tons before 2050.

Xu, Yangyang, and Veerabhadran Ramanathan, “Well below 2 C: Mitigation strategies for avoiding dangerous to catastrophic climate changes”17.

The consequential increase of atmospheric CO2 concentration is shown in chart 9. Rapid growth began in 1955. The rightmost chart shows that for every year since 2001, the atmospheric concentration of CO2 has increased by more than 1.5ppm. The largest annual increase of 3.0ppm occurred in 2015.

Chart 9. Left: Annual global mean CO2 atmospheric concentration in units of parts per million (ppm). Reference: Author’s own work using data from IPCC and NOAA ESRL18. Middle: Same as left over the period 1960 – 2021, showing values19. Right: Annual global mean CO2 atmospheric concentration growth rate. Reference: Author’s own work using data from NOAA ESRL19. Values rounded for clarity.

We conclude that, given currently available records, the present anthropogenic carbon release rate is unprecedented during the past 66 million years. 

Zeebe, Ridgwell and Zachos, 2016, Anthropogenic carbon release rate unprecedented during the past 66 million years.20

Summary

Sustained emissions of short lived climate pollutants (mainly CH₄) cause global warming to increase rapidly at first, and then stabilise. A permanent 50–75% reduction could reduce warming by over 0.5˚C by 2050.

Sustained emissions of long lived climate pollutants (mainly CO2) cause global warming to increase steadily as long as these emissions persist.

Our CO2 emissions:

  • account for 80% of global warming since 1750;
  • are the only rapidly increasing contributor;
  • almost solely determine Earth’s long term warming commitment;
  • continue to grow with no peak in sight, at a rate unprecedented in the past 66 million years;
  • and must be reduced to zero to stabilise warming.

Almost all anthropogenic CO2 is emitted by the world energy system.

Footnotes
  1. p. 5, Hansen, 2018, Climate Change in a Nutshell http://www.columbia.edu/~jeh1/mailings/2018/20181206_Nutshell.pdf, accessed 18 December 2018()
  2. p. 961, Forster, P., T. Storelvmo, K. Armour, W. Collins, J.-L. Dufresne, D. Frame, D.J. Lunt, T. Mauritsen, M.D. Palmer, M. Watanabe, M. Wild, and H. Zhang, 2021: The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 923–1054, doi:10.1017/9781009157896.009.()()()()
  3. p. 962, Forster, P., T. Storelvmo, K. Armour, W. Collins, J.-L. Dufresne, D. Frame, D.J. Lunt, T. Mauritsen, M.D. Palmer, M. Watanabe, M. Wild, and H. Zhang, 2021: The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 923–1054, doi:10.1017/9781009157896.009.()
  4. Article 2.1(a) https://unfccc.int/files/meetings/paris_nov_2015/application/pdf/paris_agreement_english_.pdf()
  5. IPCC, 2018: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. In Press. https://www.ipcc.ch/sr15/faq/faq-chapter-1/()
  6. 1.01˚C/1.27˚C()
  7. https://en.wikipedia.org/wiki/Nitrous_oxide()
  8. Figure 8.33, page 719, IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp. https://www.ipcc.ch/report/ar5/wg1/.()
  9. Allen, M., Fuglestvedt, J., Shine, K. et al. New use of global warming potentials to compare cumulative and short-lived climate pollutants. Nature Climate Change 6, 773–776 (2016), https://www.nature.com/articles/nclimate2998, http://sequoiaforestkeeper.org/pdfs/attachments/Allen_et_al_on_SLCP_GWP_2016.pdf()
  10. Allen, M., Fuglestvedt, J., Shine, K. et al. New use of global warming potentials to compare cumulative and short-lived climate pollutants. Nature Clim Change 6, 773–776 (2016), https://www.nature.com/articles/nclimate2998, http://sequoiaforestkeeper.org/pdfs/attachments/Allen_et_al_on_SLCP_GWP_2016.pdf()
  11. Fig. 1 of Aamaas, B., Peters, G. P., and Fuglestvedt, J. S.: Simple emission metrics for climate impacts, Earth Syst. Dynam., 4, 145-170, 2013. https://doi.org/10.5194/esd-4-145-2013.()
  12. Data: Global Carbon Project. (2021). Supplemental data of Global Carbon Budget 2021 (Version 1.0) [Data set]. Global Carbon Project. https://doi.org/10.18160/gcp-2021()()
  13. https://www.globalcarbonproject.org/carbonbudget/21/highlights.htm()
  14. https://4c-carbon.eu/resources/carbon-outlooks()
  15. Global Carbon Project. (2021). Supplemental data of Global Carbon Budget 2021 (Version 1.0) [Data set]. Global Carbon Project. https://doi.org/10.18160/gcp-2021()
  16. https://www.iea.org/reports/co2-emissions-from-fuel-combustion-overview()
  17. Xu, Yangyang, and Veerabhadran Ramanathan. “Well below 2 C: Mitigation strategies for avoiding dangerous to catastrophic climate changes.” Proceedings of the National Academy of Sciences 114, no. 39 (2017): 10315-10323. https://www.pnas.org/content/pnas/114/39/10315.full.pdf()
  18. Data from 1850 to 1980 inclusive: p. 1404, Table AII.1.2 IPCC, 2013: Annex II: Climate System Scenario Tables [Prather, M., G. Flato, P. Friedlingstein, C. Jones, J.-F. Lamarque, H. Liao and P. Rasch (eds.)]. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. https://www.ipcc.ch/site/assets/uploads/2017/09/WG1AR5_AnnexII_FINAL.pdf. Data from 1981 to 2019 inclusive: Ed Dlugokencky and Pieter Tans, NOAA/ESRL, https://www.esrl.noaa.gov/gmd/ccgg/trends/gl_data.html()
  19. Ed Dlugokencky and Pieter Tans, NOAA/ESRL, https://www.esrl.noaa.gov/gmd/ccgg/trends/gl_data.html()()
  20. Zeebe, Richard E., Andy Ridgwell, and James C. Zachos. “Anthropogenic carbon release rate unprecedented during the past 66 million years.” Nature Geoscience 9, no. 4 (2016): 325. https://www.nature.com/articles/ngeo2681()