Since 2005, European companies that emit greenhouse gases (GHG) have been allocated emission credits that cover only part of their actual emissions. At the end of each year, they must present a number of credits that corresponds to their actual emissions and, if they do not, they must pay a somewhat dissuasive fine. In order to satisfy this obligation, they can either obtain the credits they need through the emission allowance trading scheme or make investments that are designed to reduce the quantity of GHG emitted. In this article, first a model will make it possible to determine the optimum investment to make, integrating the price of credits via the trading scheme and the amount of the fine, taking into account random emissions. In the second part, and on the basis of simulations, the optimum investment figure will be determined in relation to its impact on average and/or the perceived volatility of emissions. We will then be able to observe that an optimum investment strategy can result in a company reducing its future emissions to a level that is lower than that of the credits it can obtain through allocation and purchase through the trading scheme, even if the marginal investment cost is higher than the anticipated price of credits.