Question: I have heard that extreme storm events can be caused simply by a butterfly flapping its wings somewhere in a distant location. Is it true that such a small disturbance in the air in one location can result in such a large catastrophic event in another separate location? If so how can we know this is possible, and how is this even possible?

Answer: 
[*Predictability: Does the flap of a butterfly's wings in Brazil set off a tornado in Texas?*](http://eaps4.mit.edu/research/Lorenz/Butterfly_1972.pdf)

That was the title of Edward Lorenz's invited talk at the 139th meeting of the American Association of the Advancement of Science held in 1972. This is the origin of the term "butterfly effect". The catchy title suggests that the answer must be "Yes!" Why else ask that question? The bulk of the talk says the answer is "Nobody knows." "Nobody knows" doesn't jibe well with a sensationalistic, unscientific press. That a butterfly in Brazil *might* trigger a tornado in Texas does. 

Lorenz had discovered in 1961 that early 1960s weather simulations were incredibly sensitive to initial conditions. Did this mean the weather itself is incredibly sensitive to minute changes? That the answer to this question is also "yes" marked a very important discovery. Weather and climate are the quintessential chaotic systems. Lorenz's work marked the start of modern chaos theory. His seminal 1963 paper, [*Deterministic nonperiodic flow*](http://journals.ametsoc.org/doi/abs/10.1175/1520-0469%281963%29020%3C0130%3ADNF%3E2.0.CO%3B2), has been cited 13479 times, per Google scholar. (In comparison, his 1972 talk has "only" been cited 345 times.) The vast majority of those 13479 citations came after his 1972 AAAS talk. Sometimes it takes a catchy title to catch the attention of a scientist.

Taking Lorenz's talk literally, asking whether a flap of a butterfly's wings in Brazil truly can set off a tornado in Texas, misses the point of his talk and of his work. The key point is that weather is chaotic. The accuracy of a detailed weather forecast fourteen days from now is rather low because that two week interval is well beyond the relevant Lyapunov timescale for such detailed predictions.

What about that butterfly? It's wing flap is a very small perturbation. It's rather difficult to say that that flap caused anything of significance to happen because the relevant timescale for such infinitesimally small perturbations is very short.
The causes of a single particular extreme weather event, like a tornado, may never be fully understood, especially if it is a chaotic system. The causes or contributing factors to the number of tornados expected for a particular atmospheric condition is much more fully understood and is certainly not chaotic. In that sense, butterflies do not cause tornados.