The Liberators by Nathan Elliot

Hood and his army of freedom fighters are ready to start their counter attack against the K'Thraa invaders of Earth. By sabotaging a huge mirror which the aliens have placed in space to raise the temperature on the planet, hood is able to plunge Earth into a mini ice age overnight. Seriously weakened by the freezing conditions the K'Thraa are unable to keep Hoods Army back. But the aliens emperor, Ro'Sharok, is not finished yet. Unless the human forces surrender, he will blow up the entire planet...