  We present a pioneering estimate of the global yearly greenhouse gas
emissions of a large-scale Astrophysics experiment over several decades: the
Giant Array for Neutrino Detection (GRAND). The project aims at detecting
ultra-high energy neutrinos with a 200,000 radio antenna array over
200,000\,km$^2$ as of the 2030s. With a fully transparent methodology based on
open source data, we calculate the emissions related to three unavoidable
sources: travel, digital technologies and hardware equipment. We find that
these emission sources have a different impact depending on the stages of the
experiment. Digital technologies and travel prevail for the small-scale
prototyping phase (GRANDProto300), whereas hardware equipment (material
production and transportation) and data transfer/storage largely outweigh the
other emission sources in the large-scale phase (GRAND200k). In the mid-scale
phase (GRAND10k), the three sources contribute equally. This study highlights
the considerable carbon footprint of a large-scale astrophysics experiment, but
also shows that there is room for improvement. We discuss various lines of
actions that could be implemented. The GRAND project being still in its
prototyping stage, our results provide guidance to the future collaborative
practices and instrumental design in order to reduce its carbon footprint.
