  The metaverse has recently gained increasing attention from the public. It
builds up a virtual world where we can live as a new role regardless of the
role we play in the physical world. However, building and operating this
virtual world will generate an extraordinary amount of carbon emissions for
computing, communicating, displaying, and so on. This inevitably hinders the
realization of carbon neutrality as a priority of our society, adding heavy
burden to our earth. In this survey, we first present a green viewpoint of the
metaverse by investigating the carbon issues in its three core layers, namely
the infrastructure layer, the interaction layer, and the economy layer, and
estimate their carbon footprints in the near future. Next, we analyze a range
of current and emerging applicable green techniques for the purpose of reducing
energy usage and carbon emissions of the metaverse, and discuss their
limitations in supporting metaverse workloads. Then, in view of these
limitations, we discuss important implications and bring forth several insights
and future directions to make each metaverse layer greener. After that, we
investigate green solutions from the governance perspective, including both
public policies in the physical world and regulation of users in the virtual
world, and propose an indicator Carbon Utility (CU) to quantify the service
quality brought by an user activity per unit of carbon emissions. Finally, we
identify an issue for the metaverse as a whole and summarize three directions:
(1) a comprehensive consideration of necessary performance metrics, (2) a
comprehensive consideration of involved layers and multiple internal
components, and (3) a new assessing, recording, and regulating mechanism on
carbon footprints of user activities. Our proposed quantitative indicator CU
would be helpful in regulating user activities in the metaverse world.
