  Warped accretion discs of low viscosity are prone to hydrodynamic instability
due to parametric resonance of inertial waves as confirmed by local
simulations. Global simulations of warped discs, using either smoothed particle
hydrodynamics (SPH) or grid-based codes, are ubiquitous but no such instability
has been seen. Here we utilize a hybrid Godunov-type Lagrangian method to study
parametric instability in global simulations of warped Keplerian discs at
unprecedentedly high resolution (up to 120 million particles). In the global
simulations, the propagation of the warp is well described by the linear
bending-wave equations before the instability sets in. The ensuing turbulence,
captured for the first time in a global simulation, damps relative orbital
inclinations and leads to a decrease in the angular momentum deficit. As a
result, the warp undergoes significant damping within one bending-wave crossing
time. Observed protoplanetary disc warps are likely maintained by companions or
aftermath of disc breaking.
