  Cloud platforms' growing energy demand and carbon emissions are raising
concern about their environmental sustainability. The current approach to
enabling sustainable clouds focuses on improving energy-efficiency and
purchasing carbon offsets. These approaches have limits: many cloud data
centers already operate near peak efficiency, and carbon offsets cannot scale
to near zero carbon where there is little carbon left to offset. Instead,
enabling sustainable clouds will require applications to adapt to when and
where unreliable low-carbon energy is available. Applications cannot do this
today because their energy use and carbon emissions are not visible to them, as
the energy system provides the rigid abstraction of a continuous, reliable
energy supply. This vision paper instead advocates for a ``carbon first''
approach to cloud design that elevates carbon-efficiency to a first-class
metric. To do so, we argue that cloud platforms should virtualize the energy
system by exposing visibility into, and software-defined control of, it to
applications, enabling them to define their own abstractions for managing
energy and carbon emissions based on their own requirements.
