  Turbulent flow physics regulates the aerodynamic properties of lifting
surfaces, the thermodynamic efficiency of vapor power systems, and exchanges of
natural and anthropogenic quantities between the atmosphere and ocean, to name
just a few applications. The dynamics of turbulent flows are described via
numerical integration of the non-linear Navier-Stokes equation -- a procedure
known as computational fluid dynamics (CFD). At the dawn of scientific
computing in the late 1950s, it would be many decades before terms such as
``carbon footprint'' or ``sustainability'' entered the lexicon, and longer
still before these themes attained national priority throughout advanced
economies. This paper introduces a framework designed to calculate the carbon
footprint of CFD and its contribution to carbon emission reduction strategies.
We will distinguish between "hero" and "routine" calculations, noting that the
carbon footprint of hero calculations is largely determined by the energy
source mix utilized. We will also review CFD of flows where turbulence effects
are modeled, thus reducing the degrees of freedom. Estimates of the carbon
footprint are presented for such fully- and partially-resolved simulations as
functions of turbulence activity and calculation year, demonstrating a
reduction in carbon emissions by two to five orders of magnitude at practical
conditions. Beyond analyzing CO2 emissions, we quantify the benefits of
applying CFD towards overall carbon emission reduction. The community's effort
to avoid redundant calculations via turbulence databases merits particular
attention, with estimates indicating that a single database could potentially
reduce CO2 emissions by approximately O(1) million metric tons. Additionally,
implementing CFD in the fluids industry has markedly decreased dependence on
wind tunnel testing, which is anticipated to lead to CO2 emission reduction.
