We report on a $2.05\mu \mathrm{m}$ nanosecond master oscillator power amplifier optical parametric source for $\mathrm{C}{\mathrm{O}}_2$ differential-absorption lidar. The master oscillator consists of an entangled-cavity nanosecond optical parametric oscillator based on a type II periodically poled lithium niobate crystal that provides highly stable single-longitudinal-mode radiation. The signal emission is amplified by a multistage parametric amplifier to generate up to $11\mathrm{mJ}$ in a nearly diffraction-limited beam with an $M^2$ quality factor of $\approx 1.5$ while maintaining single-longitudinal-mode emission with a frequency stability better than $3\mathrm{MHz}$ rms. This approach can be readily applied to the detection of various greenhouse gases.