A biolab creates a micro-organism that can consume the carbon out of the air. Of course some escape and they quickly grow to the size of skyscrapers. And with all that carbon abruptly pulled out of the air, temperatures drop substantially. We try to kill them with fire and that works but when it happen the carbon rich area that abruptly develops around them causes extreme heat. And they are drawn to the carbon-emitting factories that are used to manufacture ammunition. They don't eat "solids" but they are large enough that they destroy everything in their path by smashing it.
