Arctic permafrost and tropical forests are two of the most powerful natural drivers of our climate system, and both are approaching the point of tipping from carbon sinks to carbon sources–with potentially catastrophic consequences. At the same time, the ice sheets of Greenland and Antarctica are nearing points of no return, beyond which they may be committed to complete melting that would cause massive sea-level rise. Continuing to emit greenhouse gases without knowing where these tipping points lie is like driving toward a cliff in the fog. This gripping event will explore what we know–and need to know–to avoid going over the cliff.