Michael Crichton's "Jurassic Park" (to the uninitiated this is a Science Fiction novel) takes place around the close of the 20th century. The novel draws upon molecular biology/genetic engineering to raise (pardon the pun!) dinosaurs from the grave - and place them squarely in our time.
Wikipedia (if one may cite it) writes to say about the Jurassic
O2 130 % of modern levels CO2 7 times pre-industrial level
Granted the novel falls in the genre of Science Fiction - is it at all possible the author's scenario was improbable beyond the realm of fiction? Could dinosaur metabolism possibly survive in contemporary climate which is so different from that of dinosaur era?