Answer
$ 0.07^\circ\rm C$
Work Step by Step
We know that the time it takes to travel this 8000 km is given by
$$v=\dfrac{d}{t}$$
And hence,
$$t=\dfrac{d}{v}=\dfrac{8000\times 10^3}{1480}=\bf 5405.4\;\rm s$$
If the time decreased by 1 s, then the speed of the sound is then
$$v=\dfrac{8000\times 10^3}{5405.4-1}=\bf 1480.28\;\rm m/s$$
And since we know that 1$^\circ$C increase in temperature means an increase in speed of 4 m/s, then the increase in temperature is
$$\Delta T=\dfrac{1^\circ\rm C}{4\;\rm m/s}\times 0.28\;\rm m/s=\color{red}{\bf 0.07}^\circ\rm C$$
Thus, we can say that this is the smallest change in average temperature that can be measured by researchers.
