Sustainability is a topic that demands the attention of enterprises. In the software development space its importance has increased, and we're now seeing different ways to approach this topic. Looking at the carbon footprint of building software, for example, we recommend assessing carbon efficiency as an architectural characteristic. An architecture that takes into consideration carbon efficiency is one where design and infrastructure choices have been made in order to to minimize energy consumption and therefore carbon emissions. The measurement tooling and advice in this space is maturing, making it feasible for teams to consider carbon efficiency alongside other factors such as performance, scalability, financial cost and security. Like almost everything in software architecture, this should be considered a trade-off; our advice is to think about this as one additional characteristic in a whole set of relevant quality attributes that are driven and prioritized by organizational goals and not left to a small cadre of experts to ponder in a siloed manner.