  Large-scale experiments are building blocks of the physics community: they
involve a large fraction of the scientific staff working in multiple countries,
and absorb a significant volume of the science budget. They are also a
collection of carbon-emitting sources and practices. As such, it is essential
to assess their environmental impact. We describe here a methodology to
estimate the main greenhouse gas (GHG) emissions of a large-scale astrophysics
collaboration project, using transparent open data. The goal is neither to
consider all possible emission sources of a project, nor to calculate accurate
values. It is rather to identify the biggest emission sources of the project,
obtain orders of magnitude for them and analyse their relative weights. We
discuss methods to quantify the GHG-generating activities and their related
emission factors for the three typical biggest emission sources that can be
controlled by the collaboration: travel, digital and hardware.
