Despite the numerous warnings about extreme weather, rising sea levels and mass extinctions, one message seems to have got lost in the debate about the impact of climate change. A warmer won't just be inconvenient. Huge swathes of it, including most of Europe, the US and Australia as well as all of Africa and China will actually be uninhabitable—too hot, dry or stormy to sustain a human population.

This is no mirage. It could materialize if the world warms by an average of just 4°C, which some models predict could happen as soon as 2050. This is the world our children and grandchildren are going to have to live in. So what are we going to do about it?

One option is to start planning to move the at-risk human population to parts of the world where it will still be cool and wet. It might seem like a drastic move, but this thought experiment is not about scaremongering. Every scenario is extrapolated from predictions of the latest climate models, and some say that 4°C may actually turn out to be a conservative estimate.

Clearly this glacier-free, decertified world—with its human population packed into high-rise cities closer to the poles—would be a last resort. Aside from anything else, it is far from being the most practical option: any attempt at mass migration is likely to fuel wars, political power struggles and infighting.

So what are the alternatives? The most obvious answer is to radically reduce carbon dioxide levels now, by fast-tracking green technologies and urgently implementing energy-efficient measures. But the changes aren't coming nearly quickly enough and global emissions are still rising. As a result, many scientists are now turning to "Earth's plan B".

Plan B involves making sure we have large scale geo-engineering technologies ready and waiting to either suck CO₂ out of the atmosphere or deflect the sun's heat. Most climate scientists were once firmly against fiddling with the Earth's thermostat, fearing that it may make a bad situation even worse, or provide politicians with an excuse to sit on their hands and do nothing.

Now they reluctantly acknowledge the sad truth that we haven't managed to reorder the world fast enough to reduce CO₂ emissions and that perhaps, given enough funding research and political muscle, we can indeed design, test and regulate geo-engineering projects in time to avert the more horrifying consequences of climate change.

Whatever we do now is the time to act. The alternative is to plan for a hothouse world that none of us would recognize as home.