TimeRiders: City of Shadows (Book 6) by Alex Scarrow

being fought for the
     remaining oil fields, while the world continued to warm up as we ferociously burned our
     dwindling supply of fossil fuels and the oceans continued to rise.
    ‘I have a question for you.’
     Rashim lifted his head and looked at them all. ‘Any of you heard of the Fermi
     Paradox?’
    Maddy did, or thought she did.
     ‘Isn’t that the puzzle to do with why we haven’t yet found any alien
     civilizations out there in the universe?’
    He nodded. ‘A mathematician called
     Fermi calculated the odds of there being other alien life forms out there in the big
     wide galaxy. He took into account all the usual variables: the number of stars at the
     right point in their life cycles, the average number of likely planets per star, the
     probability of any of those planets existing within the “Goldilocks Zone”
     around the star, the likelihood of a planet having liquid water … all those
     important variables.
    ‘Anyway, while the odds were stacked
     against any one solar system containing intelligent life, given that there are literally
     trillions of stars, his maths delivered an answer that there must be hundreds of
     thousands of alien civilizations out there, and tens of thousands of civilizations
     advanced enough in technology to be putting out radio waves, intentionally or not.
    ‘So the point is,’ continued
     Rashim, ‘when we started looking into space for radio signals, we
should
have stumbled across them almost immediately. According to Fermi’s maths, we
     should have been swimming in alien radio signals.’
    ‘But instead we never found
     anything,’ said Maddy.
    ‘Right. And
that’s
the
     Fermi Paradox. Why isn’t every frequency full of alien signals?’ He sighed.
     ‘Because we’re alone. And why are we alone?’ He smiled. He
     wasn’t expecting them to answer. ‘Well … in my time we figured
     that out for ourselves. Within a century of discovering radio waves, mankind managed to
     exhaust the raw materials of the planet. The raw materials, the free energy source that
     every emerging technological civilization gets as a gift from its historical past –
     fossil fuels. It’s that package of free energy that we should have used carefully
     while we took our time to discover and harness quantum energy. Humankind never got a
     chance to take anything more than a few baby steps into space. We never got the time to
     mature, to reach out into space, for other worlds. Hydrocarbons. Fossil fuels. Oil. We
     used it all up far too quickly. Too many people wanting too many things. We used it
     up,’ he said, sighing, ‘and then, as it began to run out, we turned on each
     other.’
    ‘The Oil Wars?’ said Liam. He
     had heard another traveller from Rashim’s time mention them. A man called
     Locke.
    ‘Yes. Wars between India and China.
     Japan and Korea. The first of those was in the 2040s. Russia and the European Bloc,
     there was a short war between those. And, of course, what we should have been doing is
     trying to fix another bigger problem. The world itself dying: warming up, rising tides,
     poisoned blooms of algae killing the seas.’
    Rashim fell silent for a moment.
     ‘Anyway, that’s the answer to the Fermi Paradox; most – if not
all
– civilizations either destroy themselves or mine themselves dry long before they ever
     spread out to other planets and are able to mine, harvest them for resources. Once
     you’ve exhausted your home planet … it’s all over for you. Either
     you become extinct, or you eventually end up being cavemen once more.’
    ‘It’s a one-shot deal?’ said
     Maddy.
    He nodded. ‘And perhaps every
     civilization makes the same mistake. Spends what it has, thinking it will never run out.
     Then, all of a sudden, it does.’
    ‘Wonderful,’ sighed Maddy.
    ‘But on Earth we didn’t just run
     out. We decided to destroy ourselves in style.’ Rashim snorted. ‘It was some
     kind of a genetically engineered