This paper addresses the problem of R314a substitution with a natural refrigerant fluid. Attention is devoted to the evaluation of the environmental impact, in terms of greenhouse effect. R134a and R744 (CO2) are compared to one another. The hydrofluorocarbon R134a has a large direct warming impact (GWP), whereas the R744 contribution is negligible. The greenhouse effect is determined by the experimental evaluation of the TEWI index (Total Equivalent Warming Impact) that takes into account both direct and indirect contributions to global warming. This paper compares a commercial R134a refrigeration plant and a prototype R744 system working in a trans-critical cycle. The experimental results clearly show that the latter has a larger TEWI than the system operating with R134a. The indirect contribution to global warming provided by R744 is always greater than that of R134a. This contribution prevails in most cases. Only few operating conditions corresponding to a refrigerating plant working as a classical split system benefits, in terms of greenhouse effect, of the substitution of R134a with R744.