The adventurers in this novel travel through time and into dimensions of myths and legends including that of Yggdrasil the World Tree. What they encounter and bring back causes problems including the environmental catastrophe you described.
Time, space and a reader's patience become vertiginously distorted in this dizzying compilation of six linked tales written over a 30-year time span by Niven (Destiny's Road, etc.), winner of five Hugo Awards. Five short stories written between 1969 and 1973 follow the title novella (copyrighted 1999) but relate events predating its Martian adventures of Niven's klutzy time-traveler hero, Hanville Svetz, as he scours previous centuries for animals extinct in his environmentally devastated 2300 A.D. Earth. When Svetz's Institute for Temporal Research is transferred to the Bureau for Sky Domains, the resulting power struggle launches Svetz into Mars's inhabited past, accompanied by two lissome, stretch-suited astronauts, Zeera and Miya, on a mission to save Earth from Mars's dried-up fate and to colonize the solar system. After surviving Miya's lusty libido and multitudinous hokey alien monsters, Svetz solves one of the chief mysteries of Niven's universe, that of the Beanstalk stretching from earth to the heavens. Occasional satiric sparks light up Svetz's perils, but internal consistency is weak, while a generally gluey pace retards Niven's intended flights of imagination.
— Publishers Weekly