With the growing concern over global warming and the signing and ratification by many countries of the Kyoto Protocol, alternative refrigerants to hydro-fluorocarbons are being evaluated. Carbon dioxide (CO₂ or R-744), one of the original refrigerants used more than 100 years ago, is nonflammable, inexpensive, widely available, and has low toxicity, zero Ozone Depletion Potential and low Global Warming Potential. It is thus an attractive alternative refrigerant and has been considered for automotive air-conditioning, centralized refrigeration systems used in supermarkets, hightemperature heat-pump applications, and military cooling systems.

The trends identified in this study may help in the design and formulation of new lubricants with greater thermal stability in CO₂ systems. However, compatibility and thermal stability are only two of the many lubricant properties (including lubricity, solubility, viscosity, miscibility) that require careful evaluation. Lubricant manufacturers should constantly be seeking to develop products that give performance enhancement with greater chemical stability.