The Paul Pittman series – Part 2: Teleconnections
Rainforest Partnership is releasing a series of blog posts in celebration of the work of Paul Pittman, a valued member of our team who sadly passed away earlier this year. Paul was researching the connection between deforestation and climate change, an issue which is rarely reported on. To learn more about Paul and his work, please visit the first blog post in the series here.
The link between deforestation and climate change is usually discussed in terms of greenhouse gas emissions. For example, I described in the last blog post how carbon is stored in living trees and released into the atmosphere as CO2 when forests are cleared. But greenhouse gas emissions are only part of the puzzle. In his research, Paul describes how the clearing of tropical rainforests would have a direct impact on weather all over the world. These links between local rainforest conditions and global weather are called teleconnections and were the focus of Paul’s work.
tel·e·con·nec·tion /teləkəˈnekSHən/ n.
- a causal connection or correlation between meteorological or other environmental phenomena that occur a long distance apart
(In other words: The link between what happens in one part of the world and how it affects the weather elsewhere)
The global climate is a complex system, the dynamics of which are still poorly understood. In an effort to make Paul’s research more accessible to those of us without multiple postgraduate degrees, I will attempt to explain the concepts behind his work. Disclaimer: I don’t have multiple postgraduate degrees. I have an undergraduate degree in biosciences and an interest in climate change and conservation. That said, I have spent the last couple of months studying Paul’s essays and I think he had an important message to share so I’ll do my best to pass it on.
Firstly, it’s important to understand how global weather works, as this provides the framework for the teleconnections Paul outlines in his research. There are four key components that link rainforests to global climate: Convection currents, Hadley cells, Jet streams, and the El Niño Southern Oscillation (ENSO).
Convection currents are pillars of rising air that are responsible for transporting warm, moist air from the rainforest, high up into the atmosphere. As this warm, damp air rises it cools and condenses, causing some of the moisture to fall back down as rain. Because of this, rainforests are said to be ‘coupled’ to their local climate, as they directly generate the local weather conditions. Convection currents exist all over the world but they are particularly strong over tropical rainforests.
Some of the warm, moist air transported from the forest floor by convection currents, reaches the atmosphere and changes direction. Instead of continuing to move upward, it starts to travel north or south away from the equator in a process is known as ‘divergence’. This air starts to cool and shed water, forming rain and clouds as it travels. It eventually descends again roughly 32,000km north or south of where it began, in the sub-tropics.
Image © Arizona Board of Regents / ASU Ask A Biologist. licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. Source: https://askabiologist.asu.edu/explore/desert
The result is a persistent pocket of dry air at the surface of the sub-tropical region, which over time has established the planet’s deserts. A region of high pressure forms, which forces air back towards the equator, creating the planet’s easterly trade winds and closing a giant loop known as a ‘Hadley cell’.
Hadley cells are at least partially responsible for forming the sub-tropical jet streams. Jet streams are probably the most instantly recognisable component of the global climate system because they drive the weather patterns you see on the weekly forecast. The sub-tropical jet streams circumnavigate the globe at roughly 30° north and south of the equator. They can be described as giant, fast-flowing ‘rivers’ of air in the sky, with currents that meander, twist, split and re-join. They affect high and low pressure fronts, rainfall and temperature all over the world.
The ENSO is a weather phenomenon over the Pacific Ocean, driven by the easterly trade winds, which in turn are established by Hadley cells.
Under normal conditions the easterly trade winds drive ocean currents from the west coast of America across the Pacific. This causes warm water to pool on the western side of the Pacific Ocean around Southeast Asia and Australia. As the warm water pools on this side of the ocean, cool water rises on the opposite side to replace it, in a process called ‘upwelling’.
Air rises over the warm water pool, flows back across the Pacific towards the American continent, and sinks again over the cold water pool. This reinforces the easterly trade winds and creates a big loop known as a ‘Walker cell’. This process maintains the normal climactic conditions on the west coast of America, Southeast Asia and Australia.
However, if the trade winds weaken for any reason then the whole process grinds to a halt. Warm water is no longer forced towards the West Pacific, and cold upwelling stops, which in turn shuts down the Walker cell circulation. This is known as an El Niño, and it is characterised by extreme weather events and global high temperatures. El Niño events occur fairly regularly, roughly every two to seven years.
This short video by the MET Office does an excellent job of explaining how the ENSO works:
So, there’s your crash course in global weather! It’s a lot of information to absorb, but the take home point is that rainforests, convection currents, Hadley cells, jet streams and the ENSO are all interconnected. This means that if deforestation causes a change to convection currents in the tropics, the effects could be felt by changes to weather all over the world. In Paul’s own words:
“Tropical rainforests not only actively participate in global weather patterns, they play an indispensable role in driving them”