WASHINGTON — More people will be exposed to floods, droughts, heat waves and other extreme weather associated with climate change over the next century than previously thought, according to a new report in the British medical journal The Lancet.

The report, published online Monday, analyzes the health effects of recent episodes of severe weather that scientists have linked to climate change. It provides estimates of the number of people who are likely to experience the effects of climate change in coming decades, based on projections of population and demographic changes.

The report estimates that the exposure of people to extreme rainfall will more than quadruple and the exposure of people to drought will triple compared to the 1990s. In the same time span, the exposure of the older people to heat waves is expected to go up by a factor of 12, according to Peter Cox, one of the authors, who is a professor of climate-system dynamics at the University of Exeter in Britain.

Climate projections typically are expressed as averages over large areas, including vast expanses, like oceans, where people do not live. The report calculates the risk to people by overlaying areas of the highest risk for climate events with expected human population increases. It also takes into account aging populations — for example, heat waves pose a greater health risk to old people.