While the direct environmental impacts of digital technologies are now well documented, it is often said that they could also help reduce greenhouse gas (GHG) emissions significantly in many domains such as transportation, building, manufacturing, agriculture, and energy. Assessing such claims is essential to avoid delaying alternative action or research. This also applies to related claims about how much GHG emissions existing digital technologies are already avoiding. In this paper, we point out critical issues related to these topics in the state of the art. First, most papers do not provide enough details on the scenarios underlying their evaluations: which hypotheses they are based on and why, and why specific scenarios are chosen as the baseline. This is a key point because it may lead to overestimating the current or potential benefits of digital solutions. Second, results are rarely discussed in the context of global strategies for GHG emissions reduction. These leaves open how the proposed technologies would fit into a realistic plan for meeting current GHG reduction goals. To overcome the underlined limitations, we propose a set of guidelines that all studies on digital solutions for mitigating GHG emissions should satisfy, point out overlooked research directions, and provide concrete examples and initial results for the specific case of carpooling.