Effort to scientifically ascertain mechanisms responsible for recent global warming
This article is about the physical causes of current climate change. For the study of how climate change affects specific extreme events, see Extreme event attribution.
The scientific community has been investigating the causes of climate change for decades. After thousands of studies, the scientific consensus is that it is "unequivocal that human influence has warmed the atmosphere, ocean and land since pre-industrial times."[1]: 3 This consensus is supported by around 200 scientific organizations worldwide.[2] The scientific principle underlying current climate change is the greenhouse effect, which provides that greenhouse gases pass sunlight that heats the earth, but trap some of the resulting heat that radiates from the planet's surface. Large amounts of greenhouse gases such as carbon dioxide and methane have been released into the atmosphere through burning of fossil fuels since the industrial revolution. Indirect emissions from land use change, emissions of other greenhouse gases such as nitrous oxide, and increased concentrations of water vapor in the atmosphere, also contribute to climate change.[1]
The warming from the greenhouse effect has a logarithmic relationship with the concentration of greenhouse gases. This means that every additional fraction of CO2 and the other greenhouse gases in the atmosphere has a slightly smaller warming effect than the fractions before it as the total concentration increases. However, only around half of CO2 emissions continually reside in the atmosphere in the first place, as the other half is quickly absorbed by carbon sinks in the land and oceans.[6]: 450 Further, the warming per unit of greenhouse gases is also affected by feedbacks, such as the changes in water vapor concentrations or Earth's albedo (reflectivity).[7]: 2233
As the warming from CO2 increases, carbon sinks absorb a smaller fraction of total emissions, while the "fast" climate change feedbacks amplify greenhouse gas warming. Thus, both effects are considered to each other out, and the warming from each unit of CO2 emitted by humans increases temperature in linear proportion to the total amount of emissions.[8]: 746 [citation needed] Further, some fraction of the greenhouse warming has been "masked" by the human-caused emissions of sulfur dioxide, which forms aerosols that have a cooling effect. However, this masking has been receding in the recent years, due to measures to combat acid rain and air pollution caused by sulfates.[9][10]
^Bindoff, N.L., W.W.L. Cheung, J.G. Kairo, J. Arístegui, V.A. Guinder, R. Hallberg, N. Hilmi, N. Jiao, M.S. Karim, L. Levin, S. O’Donoghue, S.R. Purca Cuicapusa, B. Rinkevich, T. Suga, A. Tagliabue, and P. Williamson, 2019: Chapter 5: Changing Ocean, Marine Ecosystems, and Dependent Communities. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 447–587. https://doi.org/10.1017/9781009157964.007.
^Canadell, J. G.; Monteiro, P. M. S.; Costa, M. H.; Cotrim da Cunha, L.; Ishii, M.; Jaccard, S.; Cox, P. M.; Eliseev, A. V.; Henson, S.; Koven, C.; Lohila, A.; Patra, P. K.; Piao, S.; Rogelj, J.; Syampungani, S.; Zaehle, S.; Zickfeld, K. (2021). "Global Carbon and Other Biogeochemical Cycles and Feedbacks"(PDF). IPCC AR6 WG1 2021.