In this paper, the notion of greenhouse gas aversion (GHGA) is introduced into the mean–variance portfolio framework. GHGA is assumed to be a weighted sum of the portfolio holdings’ greenhouse gas emission intensities. A new portfolio performance measure, the GHGA-tilted Sharpe ratio, is offered for greenhouse-gas-averse investors. While the classical Sharpe ratio may monotonically decrease with growing GHGA, the GHGA-tilted Sharpe ratio has a maximum at intermediate values of GHGA, defining an optimal GHGA-based mean–variance portfolio. The main holdings of such a portfolio represent promising investment leads for socially responsible investors who do not want to abandon the “brown” industries altogether. An example of a GHGA-based mean–variance portfolio formed with the major constituents of the energy sector is discussed.