Global population projections fundamentally shape energy and climate transition scenarios through 2100, yet most models rely on outdated demographic assumptions that overestimate future growth.

World population grew from 2.5 billion in 1950 to over 8 billion today, a trajectory that anchored modern assumptions about food systems, energy demand, and urban infrastructure. However, fertility rates have declined sharply across developed and developing nations. The United Nations Population Division now projects global population will peak before mid-century and decline thereafter, contradicting the exponential growth models embedded in many climate scenarios.

This demographic shift carries profound implications for emissions pathways. Higher population projections automatically inflate baseline energy demand, which inflates the decarbonization target required to meet climate goals. A scenario modeling 11 billion people in 2100 requires fundamentally different renewable energy deployment than one assuming 9 billion or fewer.

Several major climate modeling frameworks, including those used by the Intergovernmental Panel on Climate Change (IPCC), employ population variants in their shared socioeconomic pathways (SSPs). Yet many policy discussions cite scenarios anchored to older UN medium-variant projections that assume slower fertility decline. The UN's 2019 and 2022 revisions substantially lowered 2100 population estimates, but this hasn't uniformly propagated through climate impact assessments.

The denominator matters for per-capita metrics too. Decoupling carbon emissions from GDP growth means little if population assumptions artificially inflate absolute demand baselines. Countries planning infrastructure through 2050 need accurate demographic inputs to avoid overbuilding energy systems or underestimating efficiency gains.

Developing nations with younger populations face different trajectories than wealthy aging societies. Sub-Saharan Africa's fertility rates remain above replacement in many countries, while East Asia and Europe already experience natural population decline. Transition scenarios that treat population