Challenges to the Greenhouse Effect
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I'm frequently asked to supply lists of studies supporting the scientific conclusions that we've understood for decades. Sometimes they challenge scientific conclusions that have stood the test of time for centuries. I thought it would be good collect my standard responses in one place. Links in the text descriptions following titles point to my posts that discuss the studies further.
Links to Sections:
I. CO2 Causes Warming
Scientists have understood that increasing CO2 causes global warming since the 19th century, but over the last decade or so, scientists have documented empirical evidence for causation from CO2 and GMST data,[1] observations at the surface, [2] observations at the top of the atmosphere,[3] and quantum mechanics.[4]
[2] Feldman DR, Collins WD, Gero PJ, Torn MS, Mlawer EJ, Shippert TR. Observational determination of surface radiative forcing by CO2 from 2000 to 2010. Nature. 2015;519 (7543):339‐343. doi:10.1038/nature14240.
https://escholarship.org/content/qt3428v1r6/qt3428v1r6_noSplash_b5903aebfe105b4071103e11197138f8.pdf
[3] Kramer, R. J., He, H., Soden, B. J., Oreopoulos, L., Myhre, G., Forster, P. M., & Smith, C. J. (2021). Observational evidence of increasing global radiative forcing. Geophysical Research Letters, 48, e2020GL091585. https://doi.org/10.1029/2020GL091585
[4] Wordsworth, R., Seeley, J. T., & Shine, K. P. (2024). Fermi Resonance and the Quantum Mechanical Basis of Global Warming. The Planetary Science Journal, 5(3), 67. DOI: 10.3847/PSJ/ad226d. https://iopscience.iop.org/article/10.3847/PSJ/ad226d/pdf
II. CO2 Sensitivity
2.1. ECS: 2xCO2 Causes ~3 C Warming
The most thorough evaluation of equilibrium climate sensitivity (ECS) arrived at a likely range of 2.6°C to 4.1°C.[1] A meta-analysis from 2017 shows that the value of 3°C is also a central estimate in the peer-reviewed literature, agreeing with the values estimated by the IPCC. This is reinforced by estimates of cloud feedbacks that make low estimates for ECS less likely.[3] A recent analysis, however, found that the models that do the best job of predicting global warming and changes in EEI trends are on the higher end of this estimate, especially when looking at models with strong trends of increasing absorbed solar radiation and a strong temperature response to forcings.
[2] Knutti, R., Rugenstein, M. & Hegerl, G. Beyond equilibrium climate sensitivity. Nature Geosci 10, 727–736 (2017). https://doi.org/10.1038/ngeo3017
[3] Paulo Ceppi, Peer Nowack. Observational evidence that cloud feedback amplifies global warming. Proceedings of the National Academy of Sciences Jul 2021, 118 (30) e2026290118; DOI: 10.1073/pnas.2026290118 https://www.pnas.org/content/118/30/e2026290118
[3] Paulo Ceppi, Peer Nowack. Observational evidence that cloud feedback amplifies global warming. Proceedings of the National Academy of Sciences Jul 2021, 118 (30) e2026290118; DOI: 10.1073/pnas.2026290118 https://www.pnas.org/content/118/30/e2026290118
[4] Gunnar Myhre et al. Observed trend in Earth energy imbalance may provide a constraint for low climate sensitivity models.Science388,1210-1213 (2025).DOI:10.1126/science.adt0647
2.2. ESS: 2xCO2 Causes ~6 C Warming
ECS only accounts for rapid feedbacks. On longer time scales, slower feedbacks like reductions in glacial ice and the poleward movement of boreal forests continue to add warming on millennial time scales. Generally speaking, Earth System Sensitivity (ESS) is estimated to be about 2x ECS.
[2] Snyder, C. W. (2016). Evolution of global temperature over the past two million years. Nature, 538(7624), 226–228. doi:10.1038/nature19798
[3] The Cenozoic CO2 Proxy Integration Project (CenCO2PIP) Consortium, Toward a Cenozoic history of atmospheric CO2. Science 382,eadi5177(2023). DOI:10.1126/science.adi5177. Accepted version online at: https://oro.open.ac.uk/94676/1/Accepted_manuscript_combinepdf.pdf
[4] Emily J. Judd et al., A 485-million-year history of Earth’s surface temperature. Science 385,eadk3705 (2024).DOI:10.1126/science.adk3705
III. Answering Contrarian Misinformation
about the Greenhouse Effect
3.1. Studies Resolving Lead-Lag Issues between CO2 and GMST
Scientific evidence has been clear that, while warming in Antarctica is triggered by orbital forcings, CO2 increases at about the same time[2] as a feedback, and global warming follows this increase in CO2.[1] The lead-lag issue is also resolved with causative analysis.[3] There is evidence that natural cycles like ENSO affect variability in the rates at which CO2 increases, but the the overall increase in CO2 since 1850 comes from human activity. We know that CO2 leads GMST.[4]
[1] Shakun, J.D., Clark, P.U., He, F., Marcott, S.A., Mix, A.C., Liu, Z., Otto-Bliesner, B., Schmittner, A., and Bard, E.: Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation, Nature, 484, 49–54, https://doi.org/10.1038/nature10915, 2012.
https://www.researchgate.net/publication/223987444_Global_Warming_Preceded_by_Increasing_Carbon_Dioxide_Concentrations_during_the_Last_Deglaciation
[2] Parrenin, F. et al. “Synchronous Change of Atmospheric CO2 and Antarctic Temperature During the Last Deglacial Warming.” Science 339, 1060 (2013). DOI: 10.1126/science.1226368
https://pdfs.semanticscholar.org/d61d/0fbcb5828af1d434d1bd0282ed36e0f00d2a.pdf
[3] Stips, A., Macias, D., Coughlan, C. et al. On the causal structure between CO2 and global temperature. Sci Rep 6, 21691 (2016). https://www.nature.com/articles/srep21691
[4] W. Wang, P. Ciais, R.R. Nemani, J.G. Canadell, S. Piao, S. Sitch, M.A. White, H. Hashimoto, C. Milesi, & R.B. Myneni, Variations in atmospheric CO2 growth rates coupled with tropical temperature, Proc. Natl. Acad. Sci. U.S.A. 110 (32) 13061-13066, https://doi.org/10.1073/pnas.1219683110 (2013).
[1] Shakun, J.D., Clark, P.U., He, F., Marcott, S.A., Mix, A.C., Liu, Z., Otto-Bliesner, B., Schmittner, A., and Bard, E.: Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation, Nature, 484, 49–54, https://doi.org/10.1038/nature10915, 2012.
https://www.researchgate.net/publication/223987444_Global_Warming_Preceded_by_Increasing_Carbon_Dioxide_Concentrations_during_the_Last_Deglaciation
[2] Parrenin, F. et al. “Synchronous Change of Atmospheric CO2 and Antarctic Temperature During the Last Deglacial Warming.” Science 339, 1060 (2013). DOI: 10.1126/science.1226368
https://pdfs.semanticscholar.org/d61d/0fbcb5828af1d434d1bd0282ed36e0f00d2a.pdf
[3] Stips, A., Macias, D., Coughlan, C. et al. On the causal structure between CO2 and global temperature. Sci Rep 6, 21691 (2016). https://www.nature.com/articles/srep21691
[4] W. Wang, P. Ciais, R.R. Nemani, J.G. Canadell, S. Piao, S. Sitch, M.A. White, H. Hashimoto, C. Milesi, & R.B. Myneni, Variations in atmospheric CO2 growth rates coupled with tropical temperature, Proc. Natl. Acad. Sci. U.S.A. 110 (32) 13061-13066, https://doi.org/10.1073/pnas.1219683110 (2013).
3.2. Studies Showing all the Increase in CO2 above Preindustrial Levels come from Human Activity
There are at least five lines of independent evidence demonstrating that humans are responsible for virtually all the increase in atmospheric CO2 above preindustrial levels. In many ways, the issue is fully decided by evidence we about human carbon emissions since 1750. Estimates of our carbon emissions from fossil fuels and industry (FFI) and land use change (LUC) show that humans have added 720 GtC to the atmosphere, which is more carbon than was in the atmosphere in 1750 (~600 GtC or ~280 ppm). But atmospheric CO2 has only increased by 50% (~300 GtC or ~140 ppm). So human emissions have actually been large enough not only to be responsible for 100% of the increase in atmospheric CO2 but also to add ~420 GtC to the land and ocean sinks. I made a chart that summarizes this below (data from the 2024 Global Carbon Budget and CO2 concentrations from Mauna Loa and ice core data).
https://essd.copernicus.org/articles/17/965/2025/essd-17-965-2025.html
[2] Rebecca Lindsay. Climate Change: Atmospheric Carbon Dioxide. https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide
[3] Akira Tomizuka. Why Is Atmospheric Carbon Dioxide Concentration Higher in the Northern Hemisphere. Environmental Science 26.4 (2013): 374-387. www.jstage.jst.go.jp/article/sesj/26/4/26_374/_pdf
[4] Iris Crawford and Andrew Babbin. How will future warming and CO2 emissions affect oxygen concentrations? Ask MIT Climate. https://climate.mit.edu/ask-mit/how-will-future-warming-and-co2-emissions-affect-oxygen-concentrations
[5] Graven, H., Keeling, R. F., & Rogelj, J. (2020). Changes to carbon isotopes in atmospheric CO2 over the industrial era and into the future. Global Biogeochemical Cycles, 34, e2019GB006170. https://doi.org/10.1029/2019GB006170
https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019GB006170
[6] Watson, A.J., Schuster, U., Shutler, J.D. et al. Revised estimates of ocean-atmosphere CO2 flux are consistent with ocean carbon inventory. Nat Commun 11, 4422 (2020). https://doi.org/10.1038/s41467-020-18203-3
[6] Watson, A.J., Schuster, U., Shutler, J.D. et al. Revised estimates of ocean-atmosphere CO2 flux are consistent with ocean carbon inventory. Nat Commun 11, 4422 (2020). https://doi.org/10.1038/s41467-020-18203-3
3.3. Studies Demonstrating that Solar Variability Does Not Explain Global Warming
[1] Benestad R. E. 2006 Solar Activity and Earth's Climate (Chichester: Springer/Praxis)
[2] Benestad, R. E. (2015). The debate about solar activity and climate change. Eathʼs Climate Response to a Changing Sun (Les Elis: EDP Sci.).
https://www.degruyter.com/document/doi/10.1051/978-2-7598-1849-5.c008/pdf?licenseType=open-access
[3] Benestad, R. E., and G. A. Schmidt (2009), Solar trends and global warming, J. Geophys. Res., 114, D14101, doi:10.1029/2008JD011639.
[4] Rasmus Benestad, "How large were the past changes in the sun?"
https://www.realclimate.org/index.php/archives/author/rasmus/
[5] Mark T. Richardson and Rasmus E. Benestad 2022 Res. Astron. Astrophys. 22 125008
https://iopscience.iop.org/article/10.1088/1674-4527/ac981c
[6] Kopp. "Historical Total Solar Irradiance Reconstruction, Time Series." https://lasp.colorado.edu/lisird/data/historical_tsi/
[7] Lean, J. L. (2018). Estimating solar irradiance since 850 CE. Earth and Space Science, 5, 133– 149. https://doi.org/10.1002/2017EA000357
[2] Benestad, R. E. (2015). The debate about solar activity and climate change. Eathʼs Climate Response to a Changing Sun (Les Elis: EDP Sci.).
https://www.degruyter.com/document/doi/10.1051/978-2-7598-1849-5.c008/pdf?licenseType=open-access
[3] Benestad, R. E., and G. A. Schmidt (2009), Solar trends and global warming, J. Geophys. Res., 114, D14101, doi:10.1029/2008JD011639.
[4] Rasmus Benestad, "How large were the past changes in the sun?"
https://www.realclimate.org/index.php/archives/author/rasmus/
[5] Mark T. Richardson and Rasmus E. Benestad 2022 Res. Astron. Astrophys. 22 125008
https://iopscience.iop.org/article/10.1088/1674-4527/ac981c
[6] Kopp. "Historical Total Solar Irradiance Reconstruction, Time Series." https://lasp.colorado.edu/lisird/data/historical_tsi/
[7] Lean, J. L. (2018). Estimating solar irradiance since 850 CE. Earth and Space Science, 5, 133– 149. https://doi.org/10.1002/2017EA000357
3.4. Studies Demonstrating that Cosmic Rays Do Not Explain Global Warming
[1] Agee, E. M., K. Kiefer, and E. Cornett, 2012: Relationship of Lower-Troposphere Cloud Cover and Cosmic Rays: An Updated Perspective. J. Climate, 25, 1057–1060, https://doi.org/10.1175/JCLI-D-11-00169.1.
https://www.pik-potsdam.de/en/news/latest-news/archive-news/2004-2005/pm_Shaviv_Veizer_e.html
[3] Rasmus E Benestad 2013 Environ. Res. Lett. 8 035049. DOI 10.1088/1748-9326/8/3/035049. https://iopscience.iop.org/article/10.1088/1748-9326/8/3/035049
[5] Erlykin, A.D., Sloan, T. & Wolfendale, A.W. A review of the relevance of the ‘CLOUD’ results and other recent observations to the possible effect of cosmic rays on the terrestrial climate. Meteorol Atmos Phys 121, 137–142 (2013). https://doi.org/10.1007/s00703-013-0260-x
[4] Eimear M. Dunne et al. ,Global atmospheric particle formation from CERN CLOUD measurements. Science354,1119-1124(2016). DOI:10.1126/science.aaf2649
[7] Laken, Benjamin A., Enric PallĂ©, JaÅ¡a ÄŒalogović and Eimear M. Dunne. A cosmic ray-climate link and cloud observations. J. Space Weather Space Clim., 2 (2012) A18. DOI: https://doi.org/10.1051/swsc/2012018
[8] Pierce, J. R., and P. J. Adams (2009), Can cosmic rays affect cloud condensation nuclei by altering new particle formation rates? Geophys. Res. Lett., 36, L09820, doi:10.1029/2009GL037946.
doi: 10.1130/1052-5173(2004)014<4:CAAPDO>2.0.CO;2.
https://www.geosociety.org/gsatoday/archive/14/3/pdf/i1052-5173-14-3-4.pdf
[11] Sloan, T and Wolfendale, A W. "Cosmic Rays and Global Warming." AIP Conference Proceedings, vol. 972, no. 1, Jan. 2008. https://doi.org/10.1063/1.2870330
[12] T. Sloan, A.W. Wolfendale, Cosmic rays and climate change over the past 1000 million years, New Astronomy, Volume 25, 2013, Pages 45-49, ISSN 1384-1076, https://doi.org/10.1016/j.newast.2013.03.008.
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