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 heating 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 heating from each greenhouse gas, and shown in chart 1. Two thirds of total radiative forcing in 2018 relative to year 1750 was due to carbon dioxide (CO2).2
Radiative forcing of greenhouse gases is partially reduced by that from cooling aerosols, and global heating is therefore caused by the net amount.5
Chart 2 shows the annual change of radiative forcing by each greenhouse gas.
Chart 3 shows the same data as chart 2, but by share. The share of annual change caused by CO2 has been greater than 70% for every year since 1993, and reached 90% or more in 2003, 2005 and 2013.
Our long term warming commitment, is almost solely determined by cumulative CO2 emissions.
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.6
The temperature response described above is shown in chart 4.
Nitrous oxide (N2O)8 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. As of the end of 2018, N2O emissions had caused 6.4% of total radiative forcing, whereas CO2 had caused 66%. Furthermore, chart 3 shows the rate of change of CO2 is much greater.
After 500 years, about a third of a CO2 emission pulse remains in the atmosphere.9
As shown below, 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. A third has been emitted in the past 22 years and a quarter in the last 15 years. In 2018 alone, 2% was emitted (assuming land-use change emissions in 2018 equalled that in 2017).10 11 12
Another way to represent cumulative emission is shown below.
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.”14
Consequently, the atmospheric concentration of CO2 has increased, as shown in chart 7(a). The steep change of growth began in 1955. Chart 7(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.
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.17
Our CO2 emissions originate from three sectors: energy (i.e. fossil fuel combustion), cement manufacture and land-use change. Chart 8(b) shows that fossil fuel emissions obviously dominate.
In 2017 and 2018, 83% of CO2 emissions originated from fossil fuels and flaring (the burning of waste gases).18 19 CO2 emissions in 2018 from all sectors are shown below, followed by fossil fuel CO2 emissions from 1959 to 2018.
Global emissions from fossil fuels and cement are projected to reach a historic high in 2019 of 36.8 GtCO2,20 as shown in chart 10(a) below.
Global fossil CO₂ emissions have risen steadily over the last decades & show no sign of peaking.Global Carbon Project (2019) Global Carbon Budget.20
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 +0.55% relative to 2018, as shown below.
Our CO2 emissions: (i) are trapping two thirds of the energy causing global heating; (ii) are the only rapidly increasing contributor; (iii) almost solely determine our long term heating 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.
- p. 5, Hansen, 2018, Climate Change in a Nutshell http://www.columbia.edu/~jeh1/mailings/2018/20181206_Nutshell.pdf, accessed 18 December 2018
- 2.044 W/m2 / 3.010 W/m2, https://www.esrl.noaa.gov/gmd/aggi/aggi.html
- 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
- p. 13, Climate Change in a Nutshell: The Gathering Storm, 18 Dec 2018. http://www.columbia.edu/~jeh1/mailings/2018/20181206_Nutshell.pdf
- Matthews, H.D., Gillett, N.P., Stott, P.A. and Zickfeld, K., 2009. The proportionality of global warming to cumulative carbon emissions. Nature, 459(7248), p.829. http://indiaenvironmentportal.org.in/files/The%20proportionality%20of%20global%20warming.pdf.
- Reprinted from 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/.
- 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.
- Value for 2018: projection for fossil fuel and flaring emissions = 37.1GtCO2 from http://www.globalcarbonproject.org/carbonbudget/18/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%.
- Emissions from fossil fuel combustion and cement production: Boden, T. A., Marland, G., and Andres, R. J.: Global, Regional, and National Fossil-Fuel CO2 Emissions, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A., doi 10.3334/CDIAC/00001_V2017, 2017. Data available at https://www.icos-cp.eu/GCP/2018, download labelled ‘2018 Global Budget v1.0’.
- Value for 2018 is projection made in http://www.globalcarbonproject.org/carbonbudget/18/presentation.htm.
- What percentage of all global fossil fuel CO₂ emissions since 1751 have occurred in my lifetime? @neilrkaye, Climate data scientist at UK Met Office.
- 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
- Data from 1850 to 2010 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 2011 to 2018 inclusive: Ed Dlugokencky and Pieter Tans, NOAA/ESRL, https://www.esrl.noaa.gov/gmd/ccgg/trends/gl_data.html
- Ed Dlugokencky and Pieter Tans, NOAA/ESRL, https://www.esrl.noaa.gov/gmd/ccgg/trends/gl_data.html
- 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
- 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’.
- 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%.