A natural disaster is any extreme event in which an Earth process interacts with human society in a deadly and/or destructive manner. The death toll that results from a particular disaster therefore depends upon both the physical characteristics of the event (its magnitude, duration, speed of onset, and geographical extent) and the vulnerability of the society it affects (population density, demographics, infrastructure, preparedness, etc.). Natural disasters include geological events such as earthquakes and volcanic eruptions, atmospheric events, such as hurricanes, thunderstorms, blizzards, and heat waves, hydrological events such as storm surges, floods, and droughts, and biological events such as epidemics and insect infestations. However, natural disasters often defy precise categorization, since a series of events may normally occur in close association. For example, hurricanes (also known as cyclones or typhoons) are often associated with coastal and inland flooding, the generation of tornadoes, and possible mudslides, fires, and subsequent disease outbreaks, all of which may be more deadly than hurricane winds themselves.
The difficulty in categorizing disasters is not the only problem in determining which “type” of event is the most deadly. Death tolls are often unreliable, particularly for historical disasters, and for those that occur in less developed parts of the world. Furthermore, the historical record provides a fairly small number of disasters from which to draw definite conclusions, given the inherent infrequency of disaster and the relatively short period of accurate record-keeping. Finally, disasters such as disease outbreaks and climate change may kill enormous numbers of people, but often over such long periods of time that it is difficult to characterize them as “events” in the way that we might think of a hurricane or an earthquake.
Given those caveats, disease epidemics have killed by far the largest number of people of any disaster “type” during the past two millennia. Smallpox is responsible for an estimated 300 million deaths during the 20th Century alone, while a similar number of deaths are attributed to the Bubonic Plague epidemics that occurred in the 6th Century, the 14th through the 18th Centuries, and the mid-19th through the mid-20th Centuries. The Spanish flu pandemic of 1918-1919 killed an estimated 50-100 million people worldwide in only two years.
Drought, which also operates over a fairly long time scale, has been (partially) responsible for famines that have killed several million people during distinct historical episodes. Drought-related famine has killed over one million, and as many as five million, people on at least five occasions since 1900 (in China, India, and the U.S.S.R.).
As for short-term events, flooding is responsible for the largest recorded death tolls. Three floods on China’s Hwang He (Yellow) River in 1931, 1887, and 1938 were responsible for as many as 3.7, 2.0, and 0.9 million deaths respectively. Additionally, flooding is the most common cause of death during tropical storms (hurricanes, typhoons, and cyclones) including the 1970 Bhola Cyclone that killed nearly 500,000 people in Bangladesh.
Earthquakes have also killed an enormous number of people throughout recorded history. By far the deadliest earthquake killed approximately 830,00 in Shaanxi, China in 1556 when entire cities built into the sides of loess cliffs were destroyed as the cliffs collapsed. The 2004 Southeast Asian tsunami, which killed an estimated 230,000 people, was also the result of an earthquake in the Indian Ocean.
Although direct evidence is scarce, it has been speculated that the deadliest natural disaster in human history may have been the eruption of a supervolcano (Toba), in present day Indonesia. This eruption may have caused over 90% of the human race (an estimated 9 million people) to die off, due largely to dramatic climate change and reduction in global plant photosynthesis, which were the result of large volumes of volcanic ash that entered the atmosphere.
It is possible that a natural disaster in the future, such as a meteorite impact, or the eruption of a supervolcano, could wipe out the entire human population, or a significant portion of it. On the other hand, it is much more likely that higher frequency events such as epidemics, droughts, floods, and earthquakes will continue to account for the highest human death tolls in the foreseeable future.