The Long Earth by Terry Pratchett and Stephen Baxter

What if almost everyone suddenly discovered they could step sideways to a parallel world. What if there wasn't just one world but any number of them. Everyone can have their own private world with all the resources in it. This collaborative novel explores that premise in a simple approachable way and it makes for a very entertaining read. In order to keep things under control (and easy to comprehend) these worlds are linked serially each to two neighbours. Most people need a simple device (a stepper) to swap between them and it takes them some time to traverse more than a few worlds. Settlers set out on epic convoys to travel to far distant worlds like pioneers of the American West. Joshua Valente however is a rare individual who can step without the aid of any device and he can travel much farther and much faster than others. He sets out with an inquisitive AI to discover just what surprises lay in store millions of worlds away from datum Earth.

Flood by Stephen Baxter

Forget the 10m rise in sea levels predicted by proponents of global warming. What if the waters kept rising until every piece of land on the planet was submerged. Baxter's terrifying novel postulates vast sub surface aquifers busting through and flooding the planet's surface over a few short decades.

The back story to this novel relates the experience of a group of former hostages who somehow live long enough to experience all the stages of this extinction level event. The real story though is Baxter's description of the collapse of humanity in the face of overwhelming natural forces. I found it genuinely scary and it brought home to me how precariously balanced out existence really is.  At first I was disappointed in the portrayal of the surviving members of humanity fighting over ever diminishing scraps of land rather than investing serious efforts into a transition to a water bound world but to be honest this is probably a correct prediction of our response.