  No present observations suggest a technologically advanced extraterrestrial
intelligence (ETI) has spread through the galaxy. However, under commonplace
assumptions about galactic civilization formation and expansion, this absence
of observation is highly unlikely. This improbability is the heart of the Fermi
Paradox. The Fermi Paradox leads some to conclude that humans have the only
advanced civilization in this galaxy, either because civilization formation is
very rare or because intelligent civilizations inevitably destroy themselves.
In this paper, we argue that this conclusion is premature by introducing the
"Sustainability Solution" to the Fermi Paradox, which questions the Paradox's
assumption of faster (e.g. exponential) civilization growth. Drawing on
insights from the sustainability of human civilization on Earth, we propose
that faster-growth may not be sustainable on the galactic scale. If this is the
case, then there may exist ETI that have not expanded throughout the galaxy or
have done so but collapsed. These possibilities have implications for both
searches for ETI and for human civilization management.
