This may be the one of the most important Horizon films of recent years. Climate scientists have just discovered a phenomenon that threatens to disrupt our world. It may already have contributed to the deaths of hundreds of thousands through drought and famine. Unchecked, it will strike again. The good news is that there is a cure. The bad news is that the cure may be worse than the disease. If they are right, then in tackling the one problem, we may unleash a climate catastrophe on our planet. This is a film about stark choices and about the dawning realisation that all our predictions about the world's climate may be completely wrong. At its heart is something that scientists are calling "global dimming".