= Solution
Assume an <ideal gas> dominated by carbon dioxide, constant mean molar <heat capacity> $C_p$, and a dry convective <adiabat> connecting the stated emitting layer to the ground. Conservation of <entropy> gives $C_p\,d\ln T=R\,d\ln p$, where $R$ is the universal <molar gas constant>. Thus
$$
\boxed{T_s=260\left(\frac{90}{0.2}\right)^{8.31446/47}\,\mathrm K\simeq766\,\mathrm K.}
$$
The estimate extrapolates across a pressure ratio of $450$. It neglects radiatively stable layers, variable <heat capacity>, departures from an <ideal gas> and changes in composition; these approximations limit its accuracy. A brightness temperature observed from space cannot generally be equated to a unique material level, but this calculation uses the layer interpretation explicitly supplied by the question.
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