ChemCases: Refrigerants - 17. Global Warming

The Kyoto Protocol agrees that nations will strive for:

(i)Enhancement of energy efficiency in relevant sectors of the national economy;

(ii)Protection and enhancement of sinks and reservoirs of greenhouse gases not controlled by the Montreal Protocol, taking into account its commitments under relevant international environmental agreements; promotion of sustainable forest management practices, afforestation and reforestation;

(iii) Promotion of sustainable forms of agriculture in light of climate change considerations;

(iv) Promotion, research, development and increased use of new and renewable forms of energy, of carbon dioxide sequestration technologies andof advanced and innovative environmentally sound technologies;

(v) Progressive reduction or phasing out of market imperfections, fiscal incentives, tax and duty exemptions and subsidies in all greenhouse gas emitting sectors that run counter to the objective of the Convention and apply market instruments;

(vi) Encouragement of appropriate reforms in relevant sectors aimed at promoting policies and measures which limit or reduce emissions of greenhouse gases not controlled by the Montreal Protocol;

(vii) Measures to limit and/or reduce emissions of greenhouse gases not controlled by the Montreal Protocol in the transport sector;

(viii) Limitation and/or reduction of methane through recovery and use in waste management, as well as in the production, transport and distribution of energy.

Unfortunately, the need for corrective action by the nations of the world has not been as widely accepted as the need to reduce long-lived chlorinated species emissions, such as CFC's, to the atmosphere. One of the reasons, of course, is that the biggest single cause of global warming is CO2, mostly (80-85%) resulting from combustion of fossil fuels - coal, gasoline, diesel fuel and natural gas. Another is that evidence that global warming is indeed occurring is far less convincing than damage to stratospheric O3. The average temperature rise over the surface of the earth is "only" 0.5^oC. over the past 100 years. Yet, in the history of the earth, there have been much more dramatic temperature changes, e.g., the Ice Age. But sophisticated computer models, based on CO2 emissions alone, predict as much as a 5^oC. temperature rise over the next 200 years. This would be catastrophic, causing severe ice melting and an almost unbearably hot climate over much of the earth.

Why should global warming take place? Over the aeons a balance had developed in the amount of solar radiation reaching the earth's surface and that portion of it, when converted to heat, that mostly escapes as infrared radiation. However, most molecules (even water vapor) are capable of infrared absorption, trapping a portion of the heat close to the earth's surface. If new molecules, generated by mankind, escape to the atmosphere, more heat will be trapped.

As it turns out, the F-C bond in fluorocarbons very strongly absorbs infrared radiation - even more so than CO2. Here, the problem is not the C-Cl bond that impacts stratospheric O3.

Scientists can measure infrared absorption by molecules very accurately. In order to compare the potential impact of various "greenhouse gases", a scale similar to the ozone depletion potential (ODP) scale has been set up. This scale is called global warming potential (GWP). In the case of fluorocarbons, it is not just sufficient to look at the GWP values. Atmospheric lifetimes must be taken into account. The following table shows these values for some key compounds. The GWP of CO2 is arbitrarily set at 1.0 for comparison.

Chemical Composition	Designation	Atmospheric Lifetime (years)	GWP
CO2		(removed by rain, photosynthesis)	1.0
CF2Cl2	CFC-12	102	8500
CFCl3	CFC-11	50	4000
C2F6	FC-116	10000	12500
CF3CH2F	HFC-134a	14	1200
CF3CHF2	HFC-125	36	3200

FC-116 is included in the table to show the extreme impact of atmospheric lifetime. Tetrafluoromethane (CF4; FC-14) is even more stable; there are no known ways for it to be destroyed in the atmosphere. Therefore, its GWP is even greater than FC-116. The reason FC-14 and FC-116 are of concern is that both are emitted to the atmosphere in significant quantities from the Hall process for aluminum as a result of fluorination of the graphite electrodes used in it.

So, even the zero ODP compounds (they contain no C-Cl bonds) have the potential for causing damage to our atmosphere. However, their contribution to global warming is a good deal less than the other "greenhouse gases" (CO2, methane, nitrous oxide) we are emitting.