A recent article  which presented “Extinction Rebellion” in Sweden, claimed that the Roger Hallam’s statement that 6 billion people will die this century was unscientific based on assessments by a various scientists . According to my information, there is no scientific evidence based on complex simulations, however, there are at least two leading climate scientists who made or supported a similar statement.
Actually, Professor Hans-Joachim “John” Schellnhuber, a leading climate scientist, stated in 2009 at COP15 in Copenhagen (see Schellnhuber (2015) ), that in his opinion the maximum carrying capacity for humans on planet Earth was limited to a maximum of 1000 million people if global warming continued unmitigated. This “guess-timate” was repeated by Prof. Will Steffen, another leading climate scientist, in a presentation in 2019 in Uppsala (see references below) . One may discuss the fine difference between 6 billion people being directly killed by global warming or not being born because they do not have a basis for existence. Personally, I would rather use the original, though more complex wording of Schellnhuber, which sounds less dramatic, although I understand that in certain contexts the first simplification may be acceptable. Also Prof. Johan Rockström was cited with a similar statement , although assuming a maximum carrying capacity several times higher, which nonetheless assumes that the livelihoods of many billion people will be destroyed:
“Indeed, the consequences of a 4C warmer world are so terrifying that most scientists would rather not contemplate them, let alone work out a survival strategy. … It’s difficult to see how we could accommodate eight billion people or maybe even half of that,” he says. “There will be a rich minority of people who survive with modern lifestyles, no doubt, but it will be a turbulent, conflict-ridden world.”
In my opinion, the prediction that after another 70 years of unlimited global warming only 1 billion people will be left (cf Hallam) which is often paraphrased as “climate change will kill 6 billion people”, is a long-term prediction which is affected by a complex and large network of natural and political variables which has considerable uncertainty and is based on various assumptions (“scenarios”). As far as I know, such a prediction has not been attempted based on computer-based simulations and has not been published in peer-reviewed journals. However, considering that three world-leading experts make an educated guess about the maximum carrying capacity after uncontrolled global warming, this statement is still informative to describe a plausible size of damage which may be done to humanity by global warming.
 Schellnhuber, HJ (2015). Selbstverbrennung. Bertelsmann, p. 507.
 Steffen, W. (2019-03-16). Climate Change 2019: Rising Risks; Growing Challenges. Uppsala University, Retrieved 2019-05-21 from https://bit.ly/2Isb0Ue .