[Legbreaker]

Although the last scene appears to show some hope for the future, the the futility of the situation is detailed time and time again throughout the book.
Nothing grows anymore, even the rats have died off due to the extreme lack of food. Only a few crumbs scavanged from the most hidden and unlikely places remain to support the last few humans on the planet, but these can't last much longer.

The extreme lack of food is nearly the characters undoing on several occasions throughout the book, and is reinforced by the spectre of canablism. The baby for example is a very powerful image.

There may be some enclave of civilisation hidden away somewhere, but their entire ecosystem would have to be self contained. Sunlight is almost completely blocked out by clouds of ash so artifical lighting for greenhouses would be vital. Fuel to power those greenhouses is definately going to be an issue although hydroelectricity is a possible option if the ash could be prevented from fouling the system.

And then there's the roving bands of mauraders, etc. If word leaked out of such an enclave, it would be soon attacked and destroyed by those outside, desperate for just one more meal.

My reading of it is that the atmosphere may clear given a few hundred, or few thousand years, but there's not going to be any life left - even the seas have died by the time of the book (which I estimate to be only 5-6 years after the event - the boy doesn't seem much older than that).