January 9, 2026

Josiah Kephart, faculty member at the Urban Health Collaborative (UHC) and Usama Bilal, Co-Director of the UHC, have recently published a dual commentary in Nature Medicine. The article, Disentangling Climate Hazards from Demographic Change in Climate-Health Projections, was also the focus of a keynote presentation made by Kephart at last year’s Max Planck Institute for Demographic Research conference in Germany covering climate change, environmental hazards, and population dynamics.

When researchers project the future impacts of climate change on public health, they argue, they often conflate three distinct drivers of risk:

1. Changes in climate hazards (such as increased frequency of heat waves),
2. Changes in the total number of people exposed (population growth) and,
3. Changes in the underlying vulnerability of people exposed (such as increased population aging).

This is important because increases in climate hazards and increases in population vulnerability due to population growth and population aging are happening simultaneously across the globe. Older adults are disproportionately vulnerable to the health impacts of climate hazards, like heat and natural disasters, and the growing numbers of older adults makes the population more vulnerable to climate hazards on its own, apart from the increases in hazards we see from climate change.

The field of climate epidemiology, they warn, needs to move beyond projections that ignore or obscure the influence of population dynamics by not clearly separating changes in the climate and changes in the population. Therefore:

“when climate-health projections explicitly or implicitly bundle changing climate hazards, population growth, and population aging together under the banner of ‘climate change,’ the result is an opaque estimate that tells us whether burdens will rise (or fall) but not why.”

A common example they point to is projections that emphasize absolute measures of health, such as the total number of deaths attributable to a single climate hazard (like, say, rising global temperatures) in 2050 or 2100. In this case, they assert, “the number of people impacted will invariably be higher due to population growth alone, leaving the impacts of climate change itself obscured.”

To ameliorate this, Bilal and Kephart propose four recommendations for environmental health researchers:

1. Decompose scenarios into climate-only, demography-only, and combined estimates;
2. Report relative metrics alongside absolute counts;
3. Provide age-standardized rates to make aging effects explicit; and
4. Write abstracts and manuscripts that plainly communicate these distinctions

As the context behind climate and population dynamics changes rapidly throughout the world, Kephart and Bilal stress that the credibility and utility of climate–health projections hinge on “accurately attributing what is climate and what is demography.”

Read The Full Commentary