Climate commitment

Climate commitment describes the fact that Earth's climate reacts with a delay to influencing factors ("climate forcings") such as the growth and the greater presence of greenhouse gases. Climate commitment studies attempt to assess the amount of future global warming that is "committed" under the assumption of some constant or some evolving level of forcing. The constant level often used for illustrative purposes is that due to CO2 doubling or quadrupling relative to the pre-industrial level; or the present level of forcing.

The ongoing buildup of long-lived greenhouse gases in Earth's atmosphere, whose warming influence has nearly doubled since 1979, shows mankind's influence on the global climate.[1][2][3]

Definition

Climate commitment is the "unavoidable future climate change resulting from inertia in the geophysical and socio-economic systems".[4] Different types of climate change commitment are discussed in the literature. These include the "constant composition commitment"; the "constant emissions commitment" and the "zero emissions commitment".[4]: 2222 

Basic idea
The accumulation of excess heat in the ocean, at ever greater depths, measures global warming that has already become "irreversible" in the near term[5]

If a perturbation — such as an increase in greenhouse gases or solar activity — is applied to Earth's climate system the response will not be immediate, principally because of the large heat capacity (i.e., thermal inertia) of the oceans.[6]

As an analogue, consider the heating of a thin metal plate (by the sun or by a flame): the plate will warm relatively quickly. If a thick metal block is heated instead, it will take much longer for the entire block to reach equilibrium with the imposed heating because of its higher heat capacity.

Land only stores heat in the top few meters. Ocean water, by contrast, can move vertically and store heat within the ocean's depth (convection). This is why the land surface is observed to warm more than the oceans. It also explains the very large difference in response between

  • "equilibrium" climate prediction runs in which only a shallow ocean is used and it is assumed that the climate has come to equilibrium and
  • "transient" climate prediction runs in which a full ocean is used and the climate is out of balance.

The "commitment" can apply to variables other than temperature: because of the long mixing time for heat into the deep ocean, a given surface warming commits to centuries of sea level rise from thermal expansion of the ocean. Also once a certain threshold is crossed, it is likely that a slow melting of the Greenland ice sheet will commit us to a sea level rise of 5m over millennia.

Models
Main article: global climate model

Recent models forecast that even in the unlikely event of greenhouse gases stabilizing at present levels, the Earth would warm by an additional 0.5°C by 2100, a similar rise in temperature to that seen during the 20th century. In 2050, as much as 64% of that commitment would be due to past natural forcings. Over time, their contribution compared to the human influence will diminish. Overall, the warming commitment at 2005 greenhouse gas levels could exceed 1°C.[7] As ocean waters expand in response to this warming, global sea levels would mount by about 10 centimeters during that time. These models do not take into account ice cap and glacier melting; including those climate feedback effects would give a 1–1.5°C estimated temperature increase.[8]

History

The concept has been discussed as far back as 1995 in the IPCC TAR and in the SAR.

See also

References
  • Wetherald, Richard T.; Stouffer, Ronald J.; Dixon, Keith W. (2001). "Committed warming and its implications for climate change" (PDF). Geophysical Research Letters. 28 (8): 1535–8. Bibcode:2001GeoRL..28.1535W. doi:10.1029/2000gl011786.
  • Meehl, Gerald A.; Washington, Warren M.; Collins, William D.; Arblaster, Julie M.; Hu, Aixue; Buja, Lawrence E.; Strand, Warren G.; Teng, Haiyan (17 March 2005). "How Much More Global Warming and Sea Level Rise?" (PDF). Science. 307 (5716): 1769–72. Bibcode:2005Sci...307.1769M. doi:10.1126/science.1106663. PMID 15774757.
  1. "The NOAA Annual Greenhouse Gas Index (AGGI)". NOAA.gov. National Oceanic and Atmospheric Administration (NOAA). Spring 2023. Archived from the original on 24 May 2023.
  2. "Annual Greenhouse Gas Index". U.S. Global Change Research Program. Archived from the original on 21 April 2021.
  3. "The NOAA Annual Greenhouse Gas Index (AGGI) - An Introduction". NOAA Global Monitoring Laboratory/Earth System Research Laboratories. Retrieved 2 March 2023.
  4. IPCC, 2021: Annex VII: Glossary [Matthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C.  Méndez, S. Semenov, A. Reisinger (eds.)]. 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. 2215–2256, doi:10.1017/9781009157896.022.
  5. von Schuckmann, K.; Cheng, L.; Palmer, M. D.; Hansen, J.; et al. (7 September 2020). "Heat stored in the Earth system: where does the energy go?". Earth System Science Data. 12 (3): 2013-2041. Bibcode:2020ESSD...12.2013V. doi:10.5194/essd-12-2013-2020.
  6. Abraham, John; Cheng, Lijing; Mann, Michael E.; Trenberth, Kevin; von Schuckmann, Karina (1 July 2022). "The ocean response to climate change guides both adaptation and mitigation efforts". Atmospheric and Oceanic Science Letters. 15 (100221): 1–9. doi:10.1016/j.aosl.2022.100221.
  7. Wigley, T. M. L. (17 March 2005). "The Climate Change Commitment" (PDF). Science. 307 (5716): 1766–9. Bibcode:2005Sci...307.1766W. doi:10.1126/science.1103934. PMID 15774756.
  8. Lockwood, Deirdre (2005-05-17). "Oceans extend effects of climate change". Nature News. doi:10.1038/news050314-13.
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