The Time Travel Journals: Bridgebuilders - The Time Travel Journals, #2

The second novel in the Time Travel Journal seriesWhen Sam Altair went back in time to 1906, he created a second universe, one with its own alternate future. In this universe, humans surpass the technology of the first, so that by 1980, their world is cleaner, more free, and less populated than the original. In the alternate universe, a second Sam Altair is born in 1946. As he carries on the original Sam's work, young Sam discovers how to build a bridge back to the first universe.
In 1980, he prepares to make the trip, along with his partner, Sarah Andrews. In the first timeline, the year is 2080, and Sam and Sarah find a planet that is reeling under the effects of advanced climate change, its human population decimated by famines, wars, and pandemics. The few billion who remain are hanging on by the threads of religious law and totalitarian regimes. Taken prisoner by a powerful cartel, Sam and Sarah escape with two young scientists who are on the run from a death sentence.
They hope to find an elusive rebel army and offer their assistance in return for protection. The price is steep, for the rebels are facing a bloody attack that might destroy their organization for good, and trap Sam and Sarah in this universe forever. And Sam's bridges have just made them the most wanted people on the planet.