  In this work, we argue that the $\alpha'\to 0$ limit of closed string theory
scattering amplitudes is a tropical limit. The motivation is to develop a
technology to systematize the extraction of Feynman graphs from string theory
amplitudes at higher genus. An important technical input from tropical geometry
is the use of tropical theta functions with characteristics to rigorously
derive the worldline limit of the worldsheet propagator. This enables us to
perform a non-trivial computation at two loops: we derive the tropical form of
the integrand of the genus-two four-graviton type II string amplitude, which
matches the direct field theory computations. At the mathematical level, this
limit is an implementation of the correspondence between the moduli space of
Riemann surfaces and the tropical moduli space.
