Your car's air conditioner might be your best friend on a hot summer day, but it is certainly no friend of the environment thanks to the ozone-depleting refrigerant used to keep the cockpit cool. This could change in a few years as carmakers in some parts of the world are being forced to charge their AC units with chemicals that have lower global warming potential.

General Motors Co. has put its considerable weight behind a new, and supposedly greener, AC refrigerant called HFO-1234yf, which is being developed by Honeywell and DuPont. The carmaker on July 23 announced it will use HFO-1234yf beginning in 2013 Chevrolet, Buick, GMC and Cadillac models in the U.S.

HFO-1234yf is designed to break down faster in the atmosphere than the R-134a refrigerant currently used. On average, R-134a has an atmospheric life of more than 13 years compared with 11 days for HFO-1234yf, according to GM. Put another way, R-134a has a global warming potential (GWP) of over 1,400 compared to HFO-1234yf's GWP of four. GWP is a value used to compare different greenhouse gases that trap heat in the atmosphere—carbon dioxide (CO2) serves as the base measurement with a GWP of one.

Automotive AC systems are prone to leakage due to the use of an open drive compressor and the need for flexible tubing to accommodate engine vibration. "There have been reports that indicate that older automotive AC systems leak up to 50 [percent] of their charge over two years," Eckhard Groll, a Purdue University mechanical engineering professor and director of the Office of Professional Practice, wrote in an e-mail to Scientific American. "Even if modern automotive AC systems are tighter most of them still leak at some rate." The U.S. alone has about 200 million cars with AC, and the typical amount of refrigerant used in each of those vehicles is about one kilogram.

HFO-1234yf is expected to help GM vehicles meet the overall requirements of the U.S. Environmental Protection Agency's new motor vehicle greenhouse gas regulations, which require a 40 percent improvement in the average overall vehicle fuel economy of the U.S. fleet by 2016, the company says. The European Union's mobile air-conditioning (MAC) directive requires that, starting in 2011, all new vehicle models use a refrigerant with a GWP below 150; by 2017, all new automobiles sold in Europe will be required to use a low-GWP refrigerant.

Automakers have another option—air-conditioning systems that use CO2. Members of the German Association of the Automotive Industry (VDA), spurred by a European Union mandate, jointly decided in the summer of 2007 to employ air-conditioning systems using CO2, rather than a fluorocarbon, as the refrigerant. If CO2 technology is adopted throughout Europe, manufacturers of fluorocarbon refrigerants will see the large MAC market significantly diminished, ICIS Chemical Business reported on its Web site in February 2008.

CO2 systems would require carmakers to modify their existing fluorocarbon-based AC systems. For one, a CO2-based system would require up to 10 times as much pressure as a fluorocarbon-based system to function properly. "Car makers would have to make major modifications to accommodate CO2 as a refrigerant, whereas HFO-1234yf can be used as a drop-in replacement refrigerant in existing AC systems," according to Groll. "I have seen presentations that show that the price of CO2 automotive AC systems will be 30 [percent] higher than that of R-134a systems."

Another challenge will be ensuring that any alternatives to R-134a do not make the situation worse. "My personal opinion is that with any synthetically manufactured substance there is some uncertainty about the long-term environmental impact," Groll says. "When the CFC refrigerants were developed in the 1930s they were celebrated as the best possible substances on Earth for this application. [Fifty] years later we found out that they deplete the ozone layer and lead to global warming."