Economist: IF PEOPLE can warm the Earth, they can probably cool it too. That is the idea behind geo-engineering, which holds that besides cutting the rate at which it is turning fossil fuels into climate-changing carbon dioxide, humanity should also consider planet-wide engineering projects intended to reduce the side-effects of this combustion. All sorts of ideas have been proposed, from filling the stratosphere with reflective particles to giant space-borne parasols designed to shade the Earth from …