  Using as a thermometer the temperature dependent magneto-transport of a
two-dimensional electron gas, we find that effective temperature scales with
current as $T_{\rm e} \sim I^a$, where $a=0.4 \pm 2\%$ in the {\it Shubnikov
de-Haas} regime, and $0.53 \pm 2\%$ in both the {\it integer and fractional}
quantum Hall effect. This implies the phonon energy emission rate changes from
the expected $P\sim T^5$ to $P\sim T^4$. We explain this, as well as the
dramatic enhancement in phonon emission efficiency using a hydrodynamic model.
