This post is part of a continuing series on the “Representative Concentration Pathways” (RCPs), presented by the Intergovernmental Panel on Climate Change as possible trajectories of atmospheric concentrations of green house gases (GHGs) over the next century. The RCPs start with a target GHG concentration, on the basis of which they estimate global temperature over various time periods. The scariest is RCP8.5, which projects a mean global temperature rise of 3.7°C by 2100. Each RCP is associated with development paths that could plausibly lead to the projected GHG concentrations. For instance, RCP8.5 assumes fairly robust human population growth rate this century, culminating in a global population of 12 billion by 2100. The current UN estimate for 2100 is a global population of 11.2 billion.

That extra 800 million matters, because other projected developments are based in part on assumptions about population trends. For instance, according to RCP8.5, cultivated land is expected to expand 16% above 2000 levels by 2080, largely in response to global population pressures. If the population estimate is off-base, so is the projected global increase in cultivated land (a net contributor to emissions). But, then the global expansion in cultivated land envisioned by RCP8.5 makes little sense anyway, given that global land use for agriculture peaked in 1998 and has slowly declined since - despite continued population growth of almost 1.5 billion people since then.

In terms of predicting climate change and its effects, it’s essential to get population projections right.  And in terms of climate change mitigation, the fewer humans the better. Per O’Neill et al, every 1% decrease in global population would mean a 1% decrease in emissions. And the best way to reduce our numbers is continued development throughout the world. Unfortunately, some climate activists see development as the problem and not part of the solution. As Dean Spears puts it:

“…many of the present-day high-emissions populations have comparatively lower fertility. Human development—such as improving health, education, and women’s social status—could encourage parents to freely choose to reduce population size while improving average well-being, and could therefore be an exception to the standard claim that development is in conflict with climate policy.”


Spears, D. (2015) Smaller human population in 2100 could importantly reduce the risk of climate catastrophe. PNAS vol. 112 no. 18 E2270, doi: 10.1073/pnas.1501763112

O’Neill BC, et al. (2012) Demographic change and carbon dioxide emissions. Lancet 380(9837):157164