The classical picture of a two-phase diffuse interstellar medium (ISM), comprising a cold and a warm phase in thermal pressure equilibrium has to be augmented by additional components: a thermally unstable phase generated by supersonic turbulence in the warm neutral medium (WNM) and small-scale filamentary structures of warm gas within the cold neutral medium (CNM), generated by intermittent dissipation of turbulence. Unexpectedly, the 21cm line emission of the ensemble carries the statistical properties of Kolmogorov turbulence. Several signatures of the intermittent dissipation of turbulence at small scale have been observed in the diffuse ISM: rotationally excited H_2 not assigned to UV photons, large local velocity shears, chemical abundances far in excess to those produced by steady-state chemistry. Thus, turbulence in the warm diffuse ISM may drive rapid formation of cold structures and its dissipation in the CNM may trigger the first steps of interstellar chemistry. The high sensitivity of Her-schel/HIFI to hydrides rotational lines in absorption against star forming regions combined to the mosaic-ing capabilities of ALMA will be unique probes of these energetic processes whose exact nature is still debated.
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