Key points from article :
The fertility rate - the average number of children a woman gives birth to - is falling.
23 nations are expected to see their populations halve by 2100.
It is driven by more women in education and work, greater access to contraception.
A smaller population would reduce carbon emissions as well as deforestation.
"That would be true except for the inverted age structure (more old people than young people) and all the negative consequences" - Christopher Murray, Researcher.
Researchers warn against undoing the progress on women's education and access to contraception.
Study says population of sub-Saharan Africa is expected to treble in size by 2100.
Nations with higher childhood mortality also need a higher fertility rate.
"If predictions are even half accurate, migration will become a necessity for all nations and not an option" - Prof Ibrahim Abubakar.
Research by University of Washington's Institute for Health Metrics and Evaluation published in the Lancet.
