Beyond CO2

While carbon dioxide is by far the most important greenhouse gas associated with the use of motor vehicles, small amounts of other greenhouse gases are also emitted, notably methane (CH4), nitrous oxide (N2O) and hydrofluorocarbon-134a (HFC-134a). We take a holistic view of climate change and are addressing non-CO2 emissions in our research, product development and operations.

Methane and nitrous oxide are combustion products formed in the engine and emitted into the atmosphere. Although the overall contribution of these pollutants is small – the U.S. Environmental Protection Agency (EPA) estimates that they contribute less than 1 percent of vehicle greenhouse gas emissions – manufacturers must meet new standards for these emission constituents starting in 2012. We have assessed the contribution to climate change made by methane emissions from vehicles as about 0.3 to 0.4 percent of that of the CO2 emissions from vehicles. We have also assessed the contribution to climate change from N2O emissions from vehicle tailpipe emissions (i.e., not including potential emissions associated with fuel production) as about 1 to 3 percent of that of tailpipe CO2 emissions. The contribution from HFC-134a is slightly higher. We have estimated that the radiative forcing contribution of HFC-134a leakage from an air-conditioner-equipped vehicle is approximately 3 to 5 percent of that of the CO2 emitted by the vehicle.1 When expressed in terms of “CO2 equivalents,” the contribution of vehicle emissions to radiative forcing of climate change is dominated by emissions of CO2.

We are also addressing other non-CO2 greenhouse gases such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6). Through our Restricted Substance Management Standard we have prohibited SF6 in magnesium casting. We are continuing our scientific research to determine the relative contribution of a wide range of long-lived greenhouse gases on the radiative forcing of climate change. In 2012, for example, we worked with an international team of climate and atmospheric scientists to assess the global warming potentials of long-lived greenhouse gases.

And, we have assessed the contribution to climate change made by “criteria emissions” from light-duty vehicles, including hydrocarbons, nitrogen oxides (NOx), particulate matter and carbon monoxide. Given the impressive reductions in criteria emissions enabled by improvements in engine and exhaust after-treatment technology, we believe that these short-lived emission constituents from light-duty vehicles will continue to have a relatively minor influence on climate change in the future.2 We have presented a technical assessment arguing that time horizons of 20 years, or longer, are needed in assessments of the contribution of road transport to the radiative forcing of climate change.3

Reducing the Climate and Ozone Impacts of Vehicle Air Conditioning Refrigerants

Chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs), which are used as refrigerants in vehicle air conditioning (AC) units, also have warming effects on the atmosphere and contribute to climate change. CFCs, which are commonly known for their negative impact on the Earth’s ozone layer, also have the highest global warming potential of these three refrigerants. Ford has been a leader in conducting research on CFC replacements to eliminate their ozone impacts and reduce the overall global warming potential of air conditioning refrigerants.

In the 1980s and early 1990s, all vehicle manufacturers used CFC-12 (CF2Cl2) as the refrigerant in AC units. By the mid-1990s, in response to the Montreal Protocol on Substances that Deplete the Ozone Layer (1987), vehicle manufacturers switched to HFC-134a (CF3CFH2). Hydrofluorocarbons such as HFC-134a contain only hydrogen, fluorine and carbon; they do not contain chlorine and hence do not contribute to stratospheric ozone depletion. HFC-134a also has a shorter atmospheric lifetime and a substantially smaller global warming potential than CFC-12. As shown in Table 1 below, the global warming potential of HFC-134a is 1,370,4 compared to CFC-12’s global warming potential of 10,900.

As seen in Figure 1 below, the lifecycle emissions of CFC-12 from an AC-equipped car in 1990 were approximately 400g per vehicle5 – i.e., a CO2 equivalent radiative forcing impact comparable to that of the CO2 emitted from the tailpipe of the car. Replacement of CFC-12 with HFC-134a, together with improvements in AC systems, has led to a dramatic (approximately 30-fold) decrease in the climate impact of refrigerant emissions per vehicle for an AC-equipped vehicle. (This can be seen by comparing the two left-hand bars in Figure 1.) We estimate that lifecycle emissions of HFC-134a from vehicles manufactured in 2010 are approximately 100 g per vehicle per year.6 Looking to the future, based on published assessments,7 we believe that HFC-134a emissions from a typical light-duty vehicle manufactured in 2017 will be approximately 50g per vehicle per year, further decreasing in the impact of HFC-134a emissions on a per-vehicle basis by a factor of two (see the third bar in Figure 1).

In the EU, we are required to use compounds with global warming potentials of 150 or less in the AC units for all approvals of new vehicle types beginning on January 1, 2011 (though this deadline was extended by moratorium until the end of 2012) and all registered vehicles beginning on January 1, 2017. Because HFC-134a has a global warming potential of 1,370, it does not meet the new regulation. Hydrofluoroolefins (HFOs) are a class of compounds that are safe for the ozone layer and have very low global warming potential (typically less than 10). Based upon engineering, environmental and safety assessments, many automakers, including Ford, have chosen the compound known as HFO-1234yf (also known as HFC-1234yf or CF3CF=CH2) for use in European vehicles subject to the above-mentioned legislation timing. Research at Ford8 has established that HFO-1234yf has a global warming potential of 4. As seen in the right-hand bar of Figure 1 below, by using HFO-1234yf, the AC refrigerant impact on global warming ceases to be discernible. In addition to using new refrigerants, Ford has also implemented new lower-leakage fitting designs in our AC systems, to reduce refrigerant leakage.

In the U.S., the EPA has proposed that HFCs such as HFC-134a should be added to, and regulated as part of, the Montreal Protocol. We do not support the inclusion of HFCs within the Montreal Protocol for the three reasons stated below:

  • HFCs do not contribute to the depletion of stratospheric ozone. HFCs should therefore not be included in the Montreal Protocol on Substances that Deplete the Ozone Layer.
  • As seen in Figure 1, replacing CFC-12 with HFC-134a has been a major step forward in environmental protection. Retaining the option to use HFC-134a in the future increases our ability to deliver cost-effective solutions for our customers.
  • Emissions of CO2, CH4 and N2O, not HFCs, are the main driver of climate change. (HFCs are currently responsible for less than 1 percent of the radiative forcing by long-lived GHGs.) Regulations focused on less than 1 percent of the problem are not very useful. We need to adopt a lifecycle perspective and focus on the most cost-effective options. Assessment of cost effectiveness is required before enacting blanket restrictions on HFCs.

Figure 1: Annual In-Use Greenhouse Gas (GHG) Emissions From Typical AC-Equipped Cars in the U.S in 1900, 2010 and 2016 Using Either CFC-12 (in 1990, Left-Hand Bar), HFC-134a (2010 and 2016, Middle Bars), or HFO-1234yf (Right-Hand Bar) Refrigerants.

Annual in-use greenhouse gas (GHG) emissions

Table 1: Comparison of CFC-12, HFC-134a and HFO-1234yf

Compound Chemical Formula Safe for Ozone? Atmospheric Lifetime9 Global Warming Potential9
CFC-12 CF2CI2 No 100 years 10,900
HFC-134a CF3CFH2 Yes 13.4 years 1,370
HFO-1234yf CF3CF=CH2 Yes 11 days 4
  1. T.J. Wallington, J.L. Sullivan and M.D. Hurley, “Emissions of CO2, CO, NOx, HC, PM, HFC-134a, N2O and CH4 from the Global Light Duty Vehicle Fleet,” Meteorol. Z. 17, 109 (2008).
  2. T.J. Wallington, J.E. Anderson, S.A. Mueller, S. Winkler and J.M. Ginder, “Emissions Omissions,” Science 327, 268, (2010).
  3. T.J. Wallington, J.E. Anderson, S.A. Mueller, S. Winkler, J.M. Ginder and O.J. Nielsen, “Time Horizons for Transport Climate Impact Assessments,” Environ. Sci. Technol. 45, 3169 (2011).
  4. World Meteorological Organization, Scientific Assessment of Ozone Depletion: 2010, Geneva (2010).
  5. IPCC/TEAP, Special Report: Safeguarding the Ozone Layer and the Climate System, Cambridge University Press, 2005.
  6. T.J. Wallington, J.L. Sullivan and M.D. Hurley, “Emissions of CO2, CO, NOx, HC, PM, HFC-134a, N2O and CH4 from the Global Light Duty Vehicle Fleet,” Meteorol. Z. 17, 109 (2008).
  7. S. Papasavva, D.J. Luecken, R.L. Waterland, K.N. Taddonio and S.O. Andersen, “Estimated 2017 Refrigerant Emissions of 2,3,3,3-tetrafluoropropene (HFC-1234yf) in the United States Resulting from Automobile Air Conditioning,” Environ. Sci. Technol. 43, 9252 (2009).
  8. O.J. Nielsen, M.S. Javadi, M.P. Sulbaek Andersen, M.D. Hurley, T.J. Wallington and R. Singh, “Atmospheric Chemistry of CF3CF=CH2: Kinetics and Mechanisms of Gas-Phase Reactions with Cl Atoms, OH radicals, and O3,” Chem. Phys. Lett. 439, 18 (2007).
  9. WMO/UNEP, Scientific Assessment of Ozone Depletion: 2010, Geneva (2010). Global Warming Potential (GWP) is a relative measure of how much heat a greenhouse gas traps in the atmosphere. It compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide. A GWP is calculated over a specific time interval, commonly 20, 100 or 500 years. GWP is expressed as a factor of carbon dioxide (whose GWP is standardized to 1).


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