I think this is "Portal: A Dataspace Retrieval" by Rob Swigart. A copy in LRF format is currently available at the MobileRead forums
The story, released in 1986 as an interactive fiction game, and in 1988 in book form, begins with the return of an astronaut from a failed mission to 61 Cygni, to find a park-like, depopulated Earth.
After wandering around for a while, he finds a barely functioning terminal with access to the global databases, which helps him to reconstruct the history of the last century or so.
Some of the points i remember are:
- All the cities were moved underground
- Some new technology allowed using environmentally clean cars, powered by liquid nitrogen
- Communication with dolphins
- At some point, there was an attack on research center based on Antarctica
- Humans dissapearing all around the world