Greenhouse gas emissions

Rapidly increasing CO2 emissions, mainly from our energy systems, almost solely determine Earth’s long term warming commitment. These emissions continue to grow with no peak in sight, at a rate unprecedented in the past 66 million years.

To help explain how greenhouse gases heat the planet, physicist John Tyndall used a dam as an analogy:1 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 energy continually enters Earth’s atmosphere and because humanity has increased the concentration of greenhouse gases, the atmosphere now traps more heat. This warming will continue until the atmosphere again returns to space the same amount of energy arriving from the sun.

The quantity of energy trapped by greenhouse gases is known as radiative forcing, with units of Watts per square metre (W/m2). This is a measure of the contribution to global warming from each greenhouse gas, and radiative forcing due to anthropogenic emissions shown in chart 1. Two thirds of total radiative forcing in 2019, relative to year 1750, was due to carbon dioxide (CO2).2

Chart 1(a). Annual radiative forcing of anthropogenic greenhouse gases relative to year 1750. Data: NOAA ESRL.3 4 Chart 1(b). Stacked version of (a). The values shown in the charts exclude modelled climate feedbacks as explained by the IPCC: ‘Forcing can also be attributed to emissions rather than to the resulting concentration changes. Carbon dioxide is the largest single contributor to historical RF from either the perspective of changes in the atmospheric concentration of CO₂ or the impact of changes in net emissions of CO₂. The relative importance of other forcing agents can vary markedly with the perspective chosen, however. In particular, CH4 emissions have a much larger forcing (about 1.0 W/m² over the Industrial Era) than CH4 concentration increases (about 0.5W/m²) due to several indirect effects through atmospheric chemistry.'5

Radiative forcing of greenhouse gases is partially reduced by that from cooling aerosols, and global warming is caused by the net amount.6

Chart 2 shows the annual change of radiative forcing by each greenhouse gas.

Chart 2. Annual change of radiative forcing by greenhouse gas, 1980-2019. Data: NOAA ESRL3

Chart 3 shows the same data as chart 2, but by share. The share of annual change caused by civilisation’s CO2 has been greater than 70% for every year since 1993, and reached 90% or more in 2003, 2005 and 2013.

Chart 3. Annual change of radiative forcing by greenhouse gas, as share of total annual change, 1980-2019. Data: NOAA ESRL.3

Our long term warming commitment is almost solely determined by cumulative CO2 emissions (nitrous oxide (N2O)7 is also a long-lived greenhouse gas that contributes to our warming commitment, but as shown in chart 1(a) above, it’s contribution is much smaller than that from CO2).

Climate–carbon modelling experiments have shown that: (1) the warming per unit CO2 emitted does not depend on the background CO2 concentration; (2) the total allowable emissions for climate stabilisation do not depend on the timing of those emissions; and (3) the temperature response to a pulse of CO2 is approximately constant on timescales of decades to centuries.

Matthews, 2009, The proportionality of global warming to cumulative carbon emissions.8

Chart 4 shows the temperature response to a 1 year pulse of emissions, using 2008 emissions as an example. Chart 5 shows the temperature response to sustained emissions, using 2011 emissions as an example.

Chart 4. Temperature response to a 1 year pulse of our emissions from 2008.9
Chart 5. Temperature response to sustained 2011 emissions. Carbon dioxide (red), methane (blue), organic and black carbon (black), nitrous oxide (green) and HFC-152a (pink).10

Allen et al. (2016) explains below that to stabilise global temperatures, net emissions of the long lived greenhouse gases (GHGs or ‘climate pollutants’) must be reduced to zero. These GHGs determine our warming commitment because their impact is cumulative.

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 atmosphere.11

Half of cumulative CO2 (i.e all emitted from preindustrial year 1750 to the end of 2018) has been emitted in just the previous 37 years, as shown below. A third has been emitted in the past 22 years and a quarter in the last 15 years. In 2018 alone, 2% was emitted.12 13 14

Chart 6. Proportion of total CO2 historically emitted over the period 1750 – 2019.14 15

Another way to represent cumulative emission is shown below.

Chart 7. What percentage of all global fossil fuel CO₂ emissions since 1751 have occurred in my lifetime? Credit: @neilrkaye16

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

Consequently, the atmospheric concentration of CO2 has increased, as shown in chart 8(a). The steep change of growth began in 1955. Chart 8(b) shows that for every year since 2001, the atmospheric concentration of CO2 has increased by more than 1.5ppm, as indicated by the columns that exceed the black line. The largest annual increase was in 2015.

Chart 8(a). Annual global mean CO2 concentration in units of parts per million (ppm). Data: IPCC and NOAA ESRL.18 Chart 8(b). Annual global mean CO2 growth rate. Data: NOAA ESRL.19 For clarity, values shown are rounded to one decimal place.

Our emissions of CO2 originate from three sectors: energy (i.e. fossil fuel combustion), cement manufacture and land-use change. Charts 9 and 10 show that fossil fuel emissions obviously dominate.

Chart 9(a). Annual global CO2 emissions from all sources (fossil fuels, cement and land use) in units of billions of tons of carbon dioxide (GtCO2), from 1850 to 2018. Chart 9(b). As per (a) but showing seperate contributors. Data: Global Carbon Project (2019).20

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.21
Chart 10. World CO2 emissions, 1959-2019 (1959-2018 for Land use Change). Data: Global Carbon Project (2019).20 Projected values shown for year 2019, shown in the Global Carbon Project’s Budget 2019 presentation.22

CO2 emissions in 2018 from all sources are shown below. In 2017 and 2018, 83% of CO2 emissions originated from fossil fuels and flaring (the burning of waste gases).20 23

Chart 11. World anthropogenic CO₂ emission by source in 2018.
Data: Global Carbon Project (2019).20
Fossil fuel share = 35% + 29.6% +17.8% + 0.8% = 83.2%.
The share of CO₂ from cement is emitted by clinker production, the main constituent of cement. This share is often quoted as 8% but that refers to the cement industry, and therefore includes emissions from the industry’s energy consumption.

Global emissions from fossil fuels reached a historic high in 2019 of 34.2GtCO2, as shown in chart 12 below.

Chart 12 World Fossil Fuel and Flaring CO2 emissions, 1965-2019. Data: BP(2020).24 25

Global fossil CO₂ emissions have risen steadily over the last decades & show no sign of peaking.

Global Carbon Project (2019) Global Carbon Budget.22

Although the recent acceleration of global emissions from fossil fuels and cement has ceased (based on the projected 2019 value of CO₂ emissions), the annual change is still an increase of +0.5% relative to 2018, as shown below.

Chart 13. Annual change of world CO2 emissions from fossil fuel and cement, 2000-2019. Percentage values are current year emissions relative to those from previous year. Data: Global Carbon Project (2019).20 Projected values used for year 2019, shown in the Global Carbon Project’s Budget 2019 presentation.22

Summary

Our CO2 emissions: (i) are trapping two thirds of the energy causing global warming; (ii) are the only rapidly increasing contributor; (iii) almost solely determine our long term warming commitment; and (iv) continue to grow with no peak in sight. Half of all CO2 emitted since preindustrial times has been emitted in the past 37 years, and almost all by the world’s energy sector. The present anthropogenic carbon release rate is unprecedented during the past 66 million years. 

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. 2.076 W/m2 / 3.1410 W/m2, https://www.esrl.noaa.gov/gmd/aggi/aggi.html()
  3. https://www.esrl.noaa.gov/gmd/aggi/aggi.html()()()
  4. 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()
  5. p. 56, Technical Summary, IPCC AR5, WG1, Stocker, T.F., D. Qin, G.-K. Plattner, L.V. Alexander, S.K. Allen, N.L. Bindoff, F.-M. Bréon, J.A. Church, U. Cubasch, S. Emori, P. Forster, P. Friedlingstein, N. Gillett, J.M. Gregory, D.L. Hartmann, E. Jansen, B. Kirtman, R. Knutti, K. Krishna Kumar, P. Lemke, J. Marotzke, V. Masson-Delmotte, G.A. Meehl, I.I. Mokhov, S. Piao, V. Ramaswamy, D. Randall, M. Rhein, M. Rojas, C. Sabine, D. Shindell, L.D. Talley, D.G. Vaughan and S.-P. Xie, 2013: Technical Sum- mary. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assess- ment 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/report/ar5/wg1/()
  6. p. 13, Climate Change in a Nutshell: The Gathering Storm, 18 Dec 2018. http://www.columbia.edu/~jeh1/mailings/2018/20181206_Nutshell.pdf()
  7. https://en.wikipedia.org/wiki/Nitrous_oxide()
  8. Matthews, H.D., Gillett, N.P., Stott, P.A. and Zickfeld, K., 2009. The proportionality of global warming to cumulative carbon emissions. Nature459(7248), p.829. http://indiaenvironmentportal.org.in/files/The%20proportionality%20of%20global%20warming.pdf.()
  9. 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/.()
  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. Value for 2019: projection for fossil fuel and flaring emissions = 36.8GtCO2 from https://www.globalcarbonproject.org/carbonbudget/19/presentation.htm. Land-use change emissions in 2017 = 1.39GtC = 1.39 * 44 / 12 GtCO2 = 5.1GtCO2. Total = 37.1 + 5.1 = 42.2 GtCO2. Cumulative CO2 from 1750 to 2018 = 2,355.75 GtCO2. Proportion emitted in 2018 = 42.2 / 2,355.75 = 1.8%.()
  13. Global Carbon Project. (2019). Supplemental data of Global Carbon Budget 2019 (Version 1.0) [Data set]. Global Carbon Project. https://doi.org/10.18160/gcp-2019. Download available at https://www.icos-cp.eu/GCP/2019, labelled ‘2019 Global Budget v1.0’.()
  14. Value for 2019 is projection made in https://www.globalcarbonproject.org/carbonbudget/19/presentation.htm()()
  15. Emissions from fossil fuel combustion and cement production: Global Carbon Project. (2019). Supplemental data of Global Carbon Budget 2019 (Version 1.0) [Data set]. Global Carbon Project. https://doi.org/10.18160/gcp-2019. Download available at https://www.icos-cp.eu/GCP/2019, labelled ‘2019 Global Budget v1.0’.()
  16. What percentage of all global fossil fuel CO₂ emissions since 1751 have occurred in my lifetime? @neilrkaye,Climate data scientist at UK Met Office.()
  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. Global Carbon Project. (2019). Supplemental data of Global Carbon Budget 2019 (Version 1.0) [Data set]. Global Carbon Project, https://www.icos-cp.eu/GCP/2019, download labelled ‘2019 Global Budget v1.0’.()()()()()
  21. 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()
  22. https://www.globalcarbonproject.org/carbonbudget/19/files/GCP_CarbonBudget_2019.pdf()()()
  23. Calculations: (1) 2017: Total emissions = fossil and cement emissions + land-use change emissions = 35.84 + 5.39 = 41.23 GtCO₂. Fossil fuel plus flaring emissions = (14.49 + 12.28 + 7.11 + 0.34) / 41.23 = 83%. (2) 2018: Total emissions = fossil and cement emissions + land-use change emissions = 36.60 + 5.53 = 42.13 GtCO₂. Fossil fuel plus flaring emissions = (14.69 + 12.43 + 7.49 + 0.34)/42.13 = 83%.()
  24. https://www.bp.com/en/global/corporate/energy-economics/statistical-review-of-world-energy.html()
  25. BP does not fully account for biofuels, and these may not be carbon-neutral as explained at https://www.worldenergydata.org/biofuels/()