We examine whether mandatory disclosure of greenhouse gas (GHG) emissions influences companies’ GHG emission levels. We identify the disclosure effect by exploiting a mandate requiring UK-incorporated listed companies to disclose information on GHG emissions in their annual reports. Using a difference-in-differences design, we show that disclosing GHG emissions in annual reports reduces emission levels by about 18% over three years. We find that emission reductions primarily occur for first-time mandatory reporters compared to firms who already voluntarily reported GHG information prior to the mandate. Further, we find that the emission reductions are permanent rather than transitory and we obtain stronger results for firms with larger savings potentials. Our effects are robust to various sample specifications, i.e., analysis at the installation- and firm-level, alternative control groups, and propensity score matching.