Carbon dioxide is the primary greenhouse culprit in human-generated global warming, most scientists agree, but CO2 itself, and a handful of other substances, are now being promoted as good alternatives to commonly used refrigerants that threaten Earth's atmosphere and climate.

To understand this paradoxical turn of events, it helps to recall the 1980s, when the world's governments banded together to fix the Antarctic ozone hole, a continent-size gap in the atmospheric layer that protects human beings, among other living things, from the sun's damaging ultraviolet radiation. Via the international treaty that entered into force in 1989 known as the Montreal Protocol, participants agreed to phase out the chemicals that harmed ozone. Closing the hole became one of the globe's greatest and most successful environmental restoration projects. But today, there is a glitch: The touted solution for the ozone predicament could in fact exacerbate our greatest environmental challenge—climate change.

The problem is that under the Montreal Protocol, which the U.S. has signed, hydrofluorocarbons (HFCs) were promoted as the environmental alternative to ozone-depleting hydrochlorofluorocarbons (HCFCs), which had become the standard working coolant in refrigerators, air conditioners and aerosol cans. HCFCs, for their part, originally replaced the even more potent ozone-depleting chlorofluorocarbons (CFCs) that were used liberally until the early 1990s. Whereas HFCs do not destroy the ozone layer, they can be thousands of times more harmful to Earth's climate than carbon dioxide, posing a significant threat should they become HCFCs' main replacement.

"HCFCs and HFCs are two chemicals designed by chemists to trap heat; the fluorinated part of the compound turns what would be a normal hydrocarbon into something that is much more durable," explains Kert Davies, director of research for Greenpeace USA. "When you combine those two properties—heat trapping and durability—in the atmosphere, it creates a greenhouse gas. We created another problem by replacing the ozone depleters with chemicals that cause global warming, and now we need to replace these because they are going to be banned."

Starting January 1, under the Montreal Protocol, the world's developed nations must cut HCFC consumption and production by 75 percent. It will then become illegal to import, produce or sell Freon (HCFC-22) and HCFC-142b, the ubiquitous refrigerants, for use in new equipment. At the same time, Europe is implementing a ban on HFC-134a (a common car air-conditioning refrigerant that can trap 3,400 times more heat in the atmosphere than CO2), beginning in 2011.

The alternatives? Natural refrigerants and a new group of fluorochemicals called hydrofluoro-olefins (HFOs).

Naturally occurring refrigerants, such as hydrocarbons (propane, isobutane and cyclopentane), ammonia and even that climate culprit carbon dioxide, can be used as cooling agents in refrigerators and air conditioners. They all have relatively low or lower global warming potentials, exist in large quantities, and do not have unknown side effects.

Ironically, CO2 seems to rise above the rest. "CO2 is an excellent refrigerant with superior thermodynamic and transport properties, compared to the HFCs in use today," saysDavid Hinde, manager of research and development for Conyers, Ga.–based Hill Phoenix, which recently became the first company to receive EPA approval to replace HCFCs with CO2 in supermarkets. "By using CO2, refrigeration systems will be able to reduce HFC leaks as well as dramatically reduce the HFC charge (the amount used in a system)."

Like most refrigerants, CO2 can remove heat from the air. The process starts when a refrigerator's compressor condenses CO2, raising its pressure and temperature. The gas is then transferred to a gas cooler where the heat is released cooling the refrigerant, which casts off the heat from a radiator on the back or bottom of the fridge. The CO2 (now between the liquid and vapor phases) then travels through an expansion valve, instantly reducing the liquid's pressure and causing it to rapidly expand into vapor. As the CO2 evaporates it absorbs heat, thereby cooling the air inside the refrigerator compartment. CO2 must be used at a much higher pressure than HFC refrigerants, and therefore requires stronger piping.
Already, companies such as Coca-Cola have begun using CO2 as a refrigerant in vending machines and other retail refrigerators outside of the U.S., as in China during the Olympics. And PepsiCo is now testing these vending machines in Washington, D.C., including in the Capitol.

Another potential alternative is hydrocarbon-based refrigerants, such as isobutane, which has been used in household refrigerators throughout Europe and in parts of Asia for a number of years. General Electric is currently seeking U.S. Environmental Protection Agency approval for its use in the U.S. Ben & Jerry's, a division of Unilever, has applied to the EPA to use propane as a refrigerant in ice-cream freezers. Both requests are pending approval due to fears over combustibility.

Then there are the synthetic refrigerants. In response to the ban on HFC-134a in Europe, chemical industry giants DuPont and Honeywell combined their years of fluorochemical expertise to find an alternative. HFO-1234yf was their answer. It doesn't cause ozone depletion, has an extremely low global warming potential, and is predicted to be ready for commercial use midway through 2011.

Concerns have been raised, however, over possible toxic side effects to the employees working around these chemicals as well as for automobile owners should it ignite. DuPont acknowledges that though HFO-1234yf is flammable, it is no more hazardous than the chemical it is replacing.

As most of these alternatives await approval, the possibility of HFC proliferation hangs in the air. But, not if Deputy Assistant Secretary of State for Environment Dan Reifsnyder has a say in the matter. Reifsnyder, as head of the U.S. delegation to the Montreal Protocol, along with Canada and Mexico, has proposed an amendment to the treaty, calling for an HFC phasedown.

"HFCs today are not yet in wide use and embedded in the world, but if you look out to 2050 without any action being taken, they will be where people go when they leave HCFCs," Reifsnyder says. "The value of this proposal is it will send a signal to the private sector for the need for alternatives that are benign in the ozone and climate sense, and give the private sector time to work on new compounds."

The amendment to the protocol will be negotiated this week in Port Ghalib, Egypt, by the United Nations. If passed, it would mark the first time that language on a greenhouse gas is incorporated into the ozone treaty. It would also mean the EPA could expand its mandate to regulate ozone-depleting substances to include a greenhouse gas, because it is the agency that implements the Montreal Protocol in the U.S.

"Before we end up with a big problem, let's avoid a big problem," Reifsnyder says.