Greenhouse effect due to chlorofluorocarbons: Climatic implications.

Ramanathan, V., 1975: Greenhouse effect due to chlorofluorocarbons: Climatic implications. Science, 190, 50-52.

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The potentially damaging effects of human activities on the upper atmosphere was first pointed out in 1962 by Harry Wexler, Chief of Scientific Services at the U. S. Weather Bureau, who identified ozone destroying catalytic reactions involving chlorine and bromine. It became an internationally prominent issue in the early 1970s when a fleet of supersonic transports (SSTs) was proposed that would travel in the stratosphere. In 1974, atmospheric chemists Mario Molina and F. Sherwood Rowland implicated chlorofluorocarbons (CFCs) in ozone depletion. At the time, CFCs and related chemicals were being produced and released to the environment in record amounts and, since there were no known mechanisms for removing them from the atmosphere, their concentration was projected to continue to increase unless their use could be somehow banned. Remember this was more than a decade before the actual measurements of ozone depletion in 1985 over Antarctica and the enactment of the 1987 Montreal Protocol.

In 1975, V. Ramanathan published the results of his study of the climatic implications of CFCs and related compounds. With primary infrared absorption bands between 8-12 microns, CFCs act as powerful greenhouse gases, and, if concentrations continue to increase, could cause significant planetary warming over and above their ongoing damage to the ozone layer. Using a radiation model that includes infrared absorption and emission by CFCs, he finds that if their concentration reaches two parts per billion—an amount projected for the year 2000 unless something drastic is done to limit their release—they could cause an increase of the Earth’s average temperature of 0.9 oC, over and above the expected warming caused by the anthropogenic CO2 greenhouse effect. Ramanathan warned that such rapid warming could have significant climatic impacts on rainfall and on ice-covered regions.

In addition to the ongoing concern about the effect of CFCs on the chemical balance of the atmosphere, Ramanathan concluded that their effect on the Earth’s thermal energy balance must also be given serious consideration.

Discussion Questions

a. What is stratospheric ozone depletion and how is it different from, yet related to climate warming?

b. Name as many air pollutants as you can and discuss their atmospheric effects on various spatial scales, from local to global.

c. What is greenhouse warming potential, over and above the radiative effects of carbon dioxide? Identify and discuss as many gases and aerosols as you can that affect climate.

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Hansen, J., D. Johnson, et al. (1981). "CLIMATE IMPACT OF INCREASING ATMOSPHERIC CARBON-DIOXIDE." Science 213(4511): 957-966.

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Climate impact of increasing atmospheric carbon dioxide.

Hansen, J., D. Johnson, A. Lacis, S. Lebedeff, P. Lee, D. Rind, and G. Russell 1981. Climate impact of increasing atmospheric carbon dioxide. Science 213, 957-966.

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In this article, Hansen, et al. take a comprehensive look at climate models and Earth’s carbon induced climatic future. They begin by examining the fundamental components of current climate models including cloud, ice and land cover albedo; atmospheric, cloud, and ocean dynamics; solar luminosity; and the radiative behavior of trace gases. Although many processes are thought to have been accurately represented such as fixing the relative humidity and implementing a moist adiabatic lapse rate within convective processes (Manabe and Wetherald, 1967), there are several aspects that still elude climatologists.

One of the most pressing problems is model implementation of ocean dynamics and how it might affect climate change in the future. The oceans are complex reservoirs with changing currents, salinities, depths, and temperatures. They interact dynamically with the atmosphere and have a large capacity for heat storage and carbon sequestration . Because of this they may delay the signs of global warming, but all models agree that eventually a new equilibrium will be reached with increased atmospheric temperatures.

Clouds too complicate the picture, since high-level clouds induce a greenhouse warming effect by trapping long wave radiation while low-level clouds decrease surface temperatures by scattering solar radiation. Scientists lack a full understanding of cloud feedbacks, a limitation that may compromise the accuracy of current climate model predictions.

Hansen, et al. identify three future scenarios in which humans either (a) continue to exploit fossil fuels at the current increasing rate, (b) slow the rate of fossil fuel consumption, or (c) essentially stop the use of fossil fuels. Using these scenarios the models predict warmings of, respectively, (a) 3 to 4.5 oC, (b) 2.5 oC, and (c) 1 oC.

The authors present a reconstruction of global temperatures indicating an increase of 0.2 oC between the middle 1960s and 1980, and a total warming of 0.4 oC in the past century. This temperature increase is consistent with the calculated greenhouse effect due to measured increases of atmospheric carbon dioxide. Minor cooling episodes are attributed to volcanic aerosols and possibly changes in solar luminosity, although this is not at all certain. The authors anticipate that the anthropogenic carbon dioxide warming signal should emerge from the noise level of natural climate variability by the end of the century. They envision a 21st century world with increased drought in the centers of continents, possible collapse of the West Antarctic ice sheet with a consequent worldwide rise in sea level, and erosion of Arctic sea ice sufficient for navigation through the fabled Northwest Passage.

As reported on the front page of the New York Times on August 22, 1981, the study predicted a global warming of “almost unprecedented magnitude” with potential collapse of the West Antarctic ice sheet, sea level rise, coastal flooding, and widespread disruption of agriculture. As “an appropriate strategy,” the report emphasized energy conservation and the development of alternative energy sources, while using fossil fuels only “as necessary'” in the coming decades.

Discussion Questions

a. What were the outstanding uncertainties in 1981 concerning climate feedback systems?

b. Describe three scenarios concerning fossil fuel use and their implications for climate change and society.

c. Given the public warning that emerged from this study, what happened to public attitudes?

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Ray, S., N. Chowdhury, et al. (2008). "Impact of initial pH and linoleic acid (C18 : 2) on hydrogen production by a mesophilic anaerobic mixed culture." Journal of Environmental Engineering-Asce 134(2): 110-117.

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Bond, T. C. and R. W. Bergstrom (2006). "Light absorption by carbonaceous particles: An investigative review." Aerosol Science and Technology 40(1): 27-67.

Liousse, C., J. E. Penner, et al. (1996). "A global three-dimensional model study of carbonaceous aerosols." Journal of Geophysical Research-Atmospheres 101(D14): 19411-19432.


Hansen, J., I. Fung, et al. (1988). "GLOBAL CLIMATE CHANGES AS FORECAST BY GODDARD INSTITUTE FOR SPACE STUDIES 3-DIMENSIONAL MODEL." Journal of Geophysical Research-Atmospheres 93(D8): 9341-9364.

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Hansen, J. and S. Lebedeff (1987). "GLOBAL TRENDS OF MEASURED SURFACE AIR-TEMPERATURE." Journal of Geophysical Research-Atmospheres 92(D11): 13345-13372.

Jones, P. D., T. M. L. Wigley, et al. (1986). "GLOBAL TEMPERATURE-VARIATIONS BETWEEN 1861 AND 1984." Nature 322(6078): 430-434.

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Interpretation of cloud-climate feedback as produced by 14 atmospheric general circulation models.

Cess, R. D., G. L. Potter, J. P. Blanchet, G. J. Boer, S. J. Ghan, J. T. Kiehl, H. Le Treut, Z.-X. Liang, J. F. B. Mitchell, J.-J. Morcrette, D. A. Randall, M. R. Riches, E. Reockner, U. Schlese, A. Slingo, K. E. Taylor. W. M. Washington, R. T. Wetherald and I. Yagai, 1989: Interpretation of cloud-climate feedback as produced by 14 atmospheric general circulation models. Science, 245, 513-516.

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Essay about this article

Cess, et al. compared different projections of carbon dioxide-induced climate change in fourteen general circulation models, seeking to identify, if possible, the causes of these differences. Although the different models have similar architectures and inputs from a community of interactive modelers, their projections of climatic warming as induced by increasing levels of carbon dioxide are quite different for reasons that are not fully understood. Their goal was to improve the models. Cloud feedbacks appear to be the “x-factor” in the models, not just in their radiative properties, but also in many other ways, including the ways models parameterize their formation, transformation, and dissipation.

Cess, et al. employed forced perturbations in model sea surface temperature as a surrogate for climate change, and examined the responses. First of all they removed the cloud dynamics and related parameterizations from the models to compare the “clear sky” results. These were in good agreement. But when cloud feedbacks were put back in, model compatibility vanished and a three-fold variation in climate sensitivity emerged. They attributed this to differences in how the models handled clouds, especially their formation and radiative feedbacks.

Earlier modelers basically guessed that “cloud-climate feedback” (as if clouds were not integral part of the climate system) amplified global warming by a factor ranging from 1.3 to 1.8 the clear sky value. But depending on their height, vertical structure, and thickness, clouds can act as positive or negative feedback mechanisms to the surface temperature. Cess, et al. emphasized that improvements are needed in the ways the effects of clouds are treated in the models if the models are to be useful as climatic predictors. This result, although valid scientifically, led policymakers, to the conclusion that the models were not trustworthy and that the uncertainties in climate models were currently too many and too large. By focusing on prediction, this conclusion downplayed the heuristic and diagnostic strengths of the models. This supported a “wait and see” approach for some, while feeding a scramble to “reduce uncertainties” in climate change prior to taking effective action (Cess, et al., 1993).

At the time climate modelers presented a unified front to the public, but were voicing explicit skepticism, at least in their unpublished remarks. For example in 1992, in a symposium at MIT titled "Perspectives on Climate Modeling," NASA-GISS climate modeler Anthony Del Genio presented a paper titled, "Cloud Feedbacks: Have It Your Way." He spoke on the tremendous uncertainties involved in putting clouds and aerosols into the climate models. Since clouds are very complex phenomena, the main problems involve the choice of parameterizations and the observational problem of measuring the radiative properties and liquid water content of clouds, which vary widely. According to Del Genio, the published measurements are "all over the place," all disagree, yet none of the authors attack the other authors as being wrong. Modelers can pick and choose among any number of computational subroutines and from a variety of published observations. "Have it your way.”

At the same symposium Edwin Schneider, a climate modeler then at the University of Maryland, began his talk with the question, "Do Climate Modelers Think?" He went on to explain that the physics inside the models was insufficient to replicate reality and that unless the models were constantly adjusted and "tuned" there would be no results at all. Asking, "Why do we need these corrections?" he responded, "We need to convince society that the model's predictions are worth heeding." He concluded with the statement, "I don't understand what's happening in the models, I just report on them." Assuredly, these are extreme comments, and were meant for an in-house group of climate modelers. Yet these statements accurately represent the discourse going on amongst the initiated.

Additional uncertainties regarding global warming include the response of the cryosphere, hydrosphere, and biosphere; local and regional effects; and the “real wild card,” human behavior.

Discussion Questions

a. What are cloud-climate feedbacks, why are they important, and what are the uncertainties?

b. How can evaluating multiple atmospheric general circulation models help reduce uncertainty? What assumptions are needed to make such comparisons?

c. What are the pitfalls of “parameterizing” clouds and other fundamental physical processes?

d. If basic physical interactions within clouds occur at very small scales involving molecules and cloud droplets, how can such microphysics be incorporated into global models?


Cess, R.D., M.-H. Zhang, G.L. Potter, H.W. Barker, R.A. Colman, D.A. Dazlich, A.D. Del Genio, M. Esch, J.R. Fraser, V. Galin, W.L. Gates, J.J. Hack, W. Ingram, J.T. Kiehl, A.A. Lacis, H. Le Treut, Z.-X. Li, X.-Z. Liang, J.-F. Mahfouf, B.J. McAvaney, V.P. Meleshko, J.-J. Morcrette, D.A. Randall, E. Roeckner, J.-F. Royer, A.P. Sokolov, P.V. Sporyshev, K.E. Taylor, W.-C. Wang and R.T. Wetherald, 1993. Uncertainties in carbon dioxide radiative forcing in atmospheric general circulation models. Science 262, 1252-1255.

Mahlman, J. D., 1997. Uncertainties in projections of human-caused climate warming. Science 278, 1416-1417

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Chylek, P., U. Lohmann, et al. (2007). "Limits on climate sensitivity derived from recent satellite and surface observations." Journal of Geophysical Research-Atmospheres 112(D24).

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