As the world’s glaciers melt, Perito Moreno glacier in Argentina is a rare exception. But for how long? Manuel Sulzer/Getty

WELCOME to Icehouse Earth. It may not feel like it but, right now, our planet is in an ice age. It started about 2.6 million years ago and, until recently, showed little sign of letting up. In the 1970s, scientists were even worried that we were about to plunge into another full-blown icy spell.

Today, those fears have evaporated into a fog of greenhouse gases. Unless we do something, fast, the exact opposite is going to happen. If emissions continue to accumulate in the atmosphere, Earth will blow its cool, with potentially disastrous consequences for humanity and the living world.

As the climate hots up, so does the race to understand what really happens when we crank up the thermostat. The standard approach is computer modelling, but we need every insight we can get, which is why some climatologists are turning their attention to the deep past, searching for global warming events to help predict the future. The good news is that the biosphere has endured some very hot periods and lived to tell the tale. The bad news is that the next hothouse may be more extreme than anything Earth has experienced before. In which case, it really is goodbye, cool world.

The earliest inklings that Earth’s climate was radically different in the past came in the 1800s, when geologists were stumped by phenomena such as glacial deposits and desert sandstones at temperate latitudes. In the early 20th century, the theory of continental drift appeared to offer an explanation – maybe the deposits had been …