You may have heard of the butterfly effect theory, where a butterfly in the Amazon flaps its wings and there are tornados in Texas.
It may sound strange, but it is only half a joke.
Small changes in one part of the world can affect the rest.
Imagine you are in a room on a hot day and you turn on a fan.
At first, it will only move the air directly in front of it but over time, the air in the room will mix and the whole space will be affected.
The fan affects what is closest to it most, but it influences the whole room a little.
In the same way, disturbances in one part of the Earth's atmosphere are felt in another.
An individual insect might be too small to literally have a big global impact, but the topic of this explainer — the Walker Circulation —most certainly does.
What is the Walker Circulation?
Before we get into the Walker Circulation, here are some things you need to know:
The Walker Circulation describes the phenomena where air travels across the Pacific Ocean and rises when it gets to Indonesia.
The air then heads back across the Pacific before descending upon Central America, completing the cycle. This is the neutral phase — it is what happens most of the time.
The La Nina phase is when the normal circulation has had too many Weetbix.
The air is drawn more strongly across the Pacific, so there is more moist rising air on the Australian side of the Pacific, leading to rainy conditions over the east of Australia.
The El Nino phase is when the Walker Circulation is weakened or reversed, so air descends on the Australian side of the Pacific, preventing lift and resulting in warm, dry conditions for Australia.
ENSO is the result of a complex give and take between the ocean and the atmosphere over the Pacific.
For Australia, El Nino generally leads to drier and hotter conditions, while La Nina usually leads to cooler and wetter conditions.
How weather impacts health, economies and conflicts
Small changes in temperature and pressure over the Pacific have flow-on effects all over the world.
The connection between ENSO and its impact on rainfall and temperature, leading to floods, fires, droughts and cyclones, is well documented.
But not all of ENSO's effects are as well known.
Prehistoric El Nino events have been found to correlate with temple construction and abandonment in Peru between about 5,800and 2,800 years ago.
However, societies getting into trouble because of ENSO is not confined to the ancient past.
Research out of the University of Cambridge suggests Australia, Chile, Indonesia, India, Japan, New Zealand and South Africa suffer drops in economic activity in response to El Nino.
For other places such as the United States and Europe, they say El Nino can have an enhancing effect on economic activity.
The relationship between economics and climate is complex,but at its core is a link between consistent rainfall and agricultural productivity.
ENSO also has modern-day impacts on health.
For example, El Nino has been linked to significant effects on air pollution in Eastern China.
During the 2015 El Nino, the stable, descending air from the reversed Walker Circulation significantly enhanced smog build-up over the already heavily polluted area of Beijing.
These negative impacts can lead to violent outcomes.
A study published in Nature found ENSO may have had a role in 21 per cent of all civil conflicts between 1950 and 2004, and that new civil conflicts in the tropics were twice as likely to arise during El Nino years than La Nina.
Researchers suggest the economic stress by brought about by El Nino, as well as stress from El Nino-induced natural disasters, can stress the human psyche,which can sometimes lead to aggressive behaviour.
It's not all about ENSO
It is important to note that not all of the world's, or even Australia's, weather is driven by ENSO alone.
One of the main effects associated with El Nino is increased fire potential in Australia, and yes, Ash Wednesday (1983) was associated with an El Nino.
But Black Saturday (2009) happened at the end of a La Nina, and the October 2013 fires in the Blue Mountains happened during a neutral phase.
ENSO is only one of many climate drivers that affect Australia.
The Indian Ocean Dipole is the other main year-to-year driver, but the Pacific Decadal Oscillation, the Southern Annular Mode, and the Madden Julian Oscillation, among others, play a part.
These drivers are happening on top of the backdrop of climate change, with seasonal changes and individual systems in the foreground.
It is a mixed up world out there with lots of butterflies.ABC