Instability in weather patterns and more extreme weather events are one of the consequences of global warming, often refered to as global weirding.

There could be a reason for such a structure. Financial institutions often still have very old esoteric hardware, such as mainframes. (Even if not actively used, they still need to keep it functional to access archived data.)

Just, because you can program and compile something without errors, doesn't mean it will run correctly.
Nested functions, calling a function within a function, which calls a function, etc... is a very common way of doing things.
However, some CPUs have a limit on how deep such a stack can go, going as low as to only 4 or 8 stacks deep. (Ex: Arduino.)
Very old hardware doesn't even have such a stacking possibility that allows nested functions.

>A leap year is a year that has an extra day. This extra day is added at the end of February: February 29th. It's added every four years because Earth does not actually take exactly 365 days to orbit the Sun, but closer to 365 1/4 days. If not for leap years, we'd gradually shift our calendar relative to Earth's orbit.

FYI: In the beginning, the leap day was actually added in between days in February and all patron saints coming after that leap day moved one day over.
Later, the leap day was indeed added at the end of February.

The Fast Fourier Transform is an algoritm to do a Fourier Transform.

A Fourier Transform transforms data between de time and frequency domain, which gives anoter perspective on the data.

Imagine a simple trafic light:

In the time domain, the traffic light is first Green, then Orange, then Red in a repeating cycle.

At any given moment in time, you only see a single color, so looking in the time domain doesn't tell you anything about which and how many colors exist in the sequence.

By transforming this sequence into a frequency domain (the frequency/wavelength of the light), you will see 3 distinct peaks corresponding to Green, Orange, and Red.
In the frequency domain, you can imediately tell which collors exists, but you don't have any information about the sequence in time for these colors.

FFT is used for about everything:
In sound it detects individual notes in chords.
In images, it is used to detect all colors and intensities.
In data, it is used to detect patterns.
Etc...

Publicly funded research is freely accessible in most countries. The US is just not one of those countries.

Hint: You can always get an abstract of the research paper, which identifies the author(s). Contact the author(s), and they will almost always provide you the researchpaper for free. (They are allowed to do this.)