There Will Be Time
Poul Anderson
 Jack Havig, born in the American midwest in 1933, has a genetic mutation that allows him to travel effortlessly through time. He learns that an apocalypse will arrive around the turn of the 21st century, due to over-pollution and nuclear warfare. Farther still in the future, a New Zealand/Micronesian culture known as "the Maurai Federation" will come to dominate the world and impose their vision of a less industrialized, more ecologically balanced world. Jack reasons that there must be other people with the same ability to travel through time; to find them, he visits Jerusalem at the time of the crucifixion, walking through the street singing the Greek mass. He is identified by agents of a time-traveling organization, the "Eyrie," who take him to their headquarters in the far future. At first Jack joins their group, but eventually he rebels against the group's leader, a 19th-century racist whose goal is to stop the Maurai ascendancy. To defeat him, Jack must return to the 20th century, where he locates an independent group of time-travelers, eventually returning to the future to overthrow the Eyrie. Much of the story takes place in times past, including an interlude in the Byzantine empire, where Jack saves the life of a Byzantine girl during the carnage of the Fourth Crusade and falls in love with her.