As the trend towards clouds and the delivery of software applications “as a service” continues, the importance of controlling, and if possible reducing, the environmental impact of the supporting data centers assumes ever more importance. However, increasing efficiency alone will not necessarily have the desired impact if computing service consumption levels grow out of control. Increased efficiency needs to be accompanied by responsible usage, which in turns requires users to be aware of the impact of their computing consumption choices. This paper makes a first step in this direction by proposing some techniques for making statements about the environmental impact of specific applications based on formal, but human readable, specifications of their functionality. It then shows how these techniques can be extended to support the definition of application specific, GreenSLAs (Service Level Agreements) which allow the contract between computing service suppliers and consumers to include constraints on the acceptable environmental impact (e.g. energy usage or CO₂ emissions).