It has recently been suggested that successive intense lightning-electromagnetic pulse (EMP) events could cause significant large-scale changes to the properties of the nighttime lower ionosphere. In order to examine this quantitatively, data on lightning detected over the United States are combined with the output from a simulation code. During the course of a night strong lightning-EMP events can lead to significant (∼100% or even greater) increases in the electron density of the lower ionosphere, with the largest increases at ∼90 km altitude. Regions with significant decreases in electron density are also possible. It is shown that changes in the electron temperature of the lower ionosphere are unlikely to be significant. The time required to produce large-scale changes of ionospheric electron density above an active thunderstorm may explain the observation of a thunderstorm “modification time” before red sprite activity is initiated.