Earth is the core world of humanity and the cradle it finally left. Though it has the highest population of any human settled world, there are now more people living off-Earth than not.
Most of the population lives in sprawling and slightly cramped megalopolises of threedimensional expansion.
In 2000 the global population numbered 6.1 billion people, compared to 1.6 billion in 1900. This exponential growth mellowed somewhat, but only stabilized and declined in the 22nd century, mayhaps in part due to the pogroms by the hyper-rich in the states later forming the Freedom Alliance.
Is is unknown how many people survived the Unification, but
With the success of Governance-initiated Project Janus and subsequent colonization efforts both by Governance directly and independently organized groups alleviated some of the pressure created by the clinical immortality following Project Eden.
Currently the population is stable at 12 billion people (12 x 109). Contraceptive measures are mandatory and childhood licenses are granted by Governance to keep the societal demands and environmental limitations in an equilibrium. There is still a constant outflow of people to old and new colonies as well as to the military.
With large scale industrialization on a global scale an accidental climate change was initiated mostly acknowledge arount the turn of the millennium. Nevertheless the ongoing processes lead to extinction of many species, a measurable rise in sea level and desertification in lieu of first measures trying to reverse the process.
The environmental changes only grew cascadically worse during the 21st century, with a slight mellowing in the early 22nd due to geoengineering efforts. Sadly, the Unification wars and connected deployment of nuclear weaponry, abandonment/destruction of geoengineering projects removed the stop-gap measures in progress.
Post-Unification most of the population was concentrated in megalopolises, leaving devasted and desertified stretches of land for nature to reclaim, a still ongoing progress.
It is assumed and probably a given that further advances in terraforming technologies will not only be used in colonization efforts, but also on Earth itself for reclamation of deserts and devastated landscapes.