The way that environmentalists bad-mouth carbon dioxide, it should be a four-letter word. Of course it is abbreviated to three characters - CO2.

High concentrations of it in the atmosphere since the advent of the Industrial Revolution are accused of contributing to global warming. So clean-air authorities have begun regulating it, with limits on truck engines beginning in 2014.

But there's a very good side to CO2, says Carrier Transicold. Aside from putting the bubbles in champagne and soft drinks, the gas occurs naturally: Among other things, we mammals exhale it.

And CO2 is a decent refrigerant, called R-744. If leaked, its environmental effects are neutral, and it is said to have a global warming potential, or GWP, of one.

By comparison, synthetic compounds like R-134a and R-404A have GWPs of 1,300 and 3,260, respectively. R-12, also called CFC-12 and best known as Freon, has a GWP of 8,050; it was banned after scientists found that when vented, it was depleting the earth's ozone layer, which protects us from the sun's ultraviolet rays.

Carrier uses CO2 recycled from the atmosphere for refrigerant cooling, so it adds no new environmental risk, the company said in a statement. In other words, the CO2 is there, no harm is done in harnessing it for refrigeration, and it makes immense sense to do so.

"We have an ongoing commitment to apply refrigerant alternatives that minimize environmental impact while serving customer needs," said David Appel, president of Carrier Transicold. "That's why the goal of the NaturaLine container unit and the CO2 concept trailer programs is to significantly reduce the global environmental impact, and improve energy efficiency at the same time."

Following success with carbon dioxide stationary cooling in Europe, Carrier began testing CO2 in a new series of transport refrigeration units called NaturaLine. The reefers are for ocean containers. That's the application now because steamship lines are under pressure from authorities in Europe to clean up their operations.

In three voyages by Hapag-Lloyd, a major steamship line, the concept proved sound. Now the NaturaLine reefers are entering additional sea trials, the company announced at an intermodal trade show in Hamburg, Germany, a few weeks ago. If successful, they'll be marketed worldwide, and trailer reefer units using CO2 may follow.

Mechanically the new units have "transcritical" distinctions from existing products to deal with CO2's special thermodynamic properties. This includes a new purpose-built multi-stage compressor, a "gas cooler" with a wrap-around coil design, multi-speed fans, and a flash tank. At light loads, the compressor can run on just one cylinder while the other cylinder is idled.

Otherwise the NaturaLine design is similar to Carrier's PrimeLine units, which have low on-board power generation requirements. That saves fuel for shipping companies. The basic frame, the evaporator and evaporator fans, and the controller and control box are virtually identical to existing models, the company said. The control interface is the same, but with new alarm and function codes, and servicing will be similar.

So, next time you hear CO2 being described in sky-is-falling terms, remember the refrigerant alternatives and breathe deeply.