Learn more about return periods and some common misconceptions to keep in mind
After reading this article, you will learn:
What are return periods?
Return periods are a statistical measurement often used in risk analysis. They’re used to convey information about the probability of an event, such as flooding or extreme rainfall, happening in a particular location.
You’ll find return periods within your most detailed EarthScan Insights for each individual Risk Category, as well as in the CVaR Insight for Wind and Flooding Risk.
Return periods, also called recurrence intervals or repeat intervals, estimate the average time between events of a similar intensity occurring. This probability is often given in years e.g. 10-year flood, 50-year flood, and describes the average time between events of a similar magnitude. The higher the return period, the greater the intensity of the event.
The scientific community describes return periods as a “1 in 10-year event”, or a “1 in 50-year event”. Sometimes, they’re expressed as a percentage. For example, a 50-year flood event can also be described as having a 2% chance of happening in any one year. Turning this around, if there is a 10% chance of a heatwave occurring, then we say it has a 10-year return period.
What are some common misconceptions about return periods?
Although they’re widely-used metrics, return periods are easy to misinterpret.
The key thing to remember is that if an event has a 100-year return period, it could happen multiple times, or not happen at all within a given 100 years.
A 100-year return period, or a 1 in 100-year event, means there is a 1/100 or 1% chance of an event of that magnitude happening in any given year. For example, a flooding event with a 100-year return period could have occurred 4-5 times between 1920 and 2020. It’s also possible it might not have occurred at all.
Another common misconception is that if a 100-year event has just happened, another similar event won’t happen for another 99 years. However, a 100-year event could conceivably happen two years in a row. For example, if a 10 year extreme rainfall event happened in 2021, it is equally as likely that it could happen in 2022 or 2023.
Return periods remain unchanged even if an event at a similar level of intensity has occurred recently within the timeframe.
It’s also important to note that return periods do not account for any interventions or mitigating factors that may influence how much impact a climate hazard could have at a particular location. Examples of interventions and mitigations include: adding flood defences, improving ventilation or upgrading water storage facilities.
Learn more about the return periods that are available in EarthScan.