The terms like “once-in-100 years drought”, “a 50-year flood” and “once-in-30 years wildfire” to describe how likely an event occurs are based on statistics, on the premise that the world works as if it is static (note the similarity of the words “static” and “statistics”).

But the world is not like that. It is changing (dynamic). So the yesterday “once-in-100 years storm” may become more frequent to “once-in-30 years storm” today due to climate change.

For example, given a distribution of river discharges for the years 1950 to 2000, can this distribution be used to predict how often a certain river discharge will be exceeded in the years 2000 to 2050? The answer is yes, provided that the environmental conditions do not change. If the environmental conditions do change, such as alterations in the infrastructure of the river’s watershed or in the rainfall pattern due to climatic changes, the prediction on the basis of the historical record is subject to a systematic error.

from Wikipedia.

Other good explanations are Teaching recurrence intervals and Two streams, two stories… How Humans Alter Floods and Streams.

This post is my comment to this news (in Chinese).