Over-consumption in rich countries and rapid population growth in the poorest both need to be tackled to put society on a sustainable path, a report says.
An expert group convened by the Royal Society spent nearly two years reading evidence and writing their report.
Firm recommendations include giving all women access to family planning, moving beyond GDP as the yardstick of economic health and reducing food waste.
The report will feed into preparations for the Rio+20 summit in June.
"This is an absolutely critical period for people and the planet, with profound changes for human health and wellbeing and the natural environment," said Sir John Sulston, the report’s chairman.
"Where we go is down to human volition - it’s not pre-ordained, it’s not the act of anything outside humanity, it’s in our hands."
Sir John came to fame through heading the UK part of the Human Genome Project.
He shared the 2002 Nobel Prize for Physiology or Medicine, and now chairs the Institute for Science Ethics and Innovation at Manchester University.
Back on the table
Although the size of the Earth’s human population used to be a main ingredient of environmental debate, it has fallen off the table in recent years.
In part that was because the Earth appeared able to support more people than predictions had suggested, and partly because developing countries came to view the population issue as a smokescreen to hide Western over-consumption.
However it is now back on the table, largely because of research showing that women in the poorest nations generally want access to family planning and that people benefit from it.
The UN’s “medium” projection indicates the population peaking at just over 10 billion before the end of the century, and then starting to fall, from a current level of seven billion.