Introduction to the Greenhouse Effect: It's All About The Energy Balance
Updated: May 18
What do Fourier, Foote and Tyndall have in common?
They were all born during the industrial revolution and are all involved in the discovery of the Greenhouse Effect.
In the early 1800's, Fourier calculated that, based on its size, distance from the sun etc., our planet should be colder than it actually was if it was only warmed by the effects of incoming solar radiation ☀
Something else had to warm the planet.
Could the atmosphere act as an insulator?
“Yes!” Foote and Tyndall both independently answered in the 1850’s. They showed that gases like CO2 and water vapour absorb heat.
How does the greenhouse effect work? 🌶
Earth's temperature is a balance between how much energy comes in versus how much energy goes out.
Energy enters our atmosphere as visible light, the surface absorbs some of the light’s energy and re-radiates it as infrared waves, which we feel as heat. i.e. (visible light in) 💡 ➡ 🌍 ➡ ♨ (infrared out). If that doesn't sound intuitive, think about how warm rocks can get on a sunny summer day: they take sunlight and re-emit this energy as heat.
Oxygen and nitrogen (2 major components of the atmosphere) are simple molecules that interact with few wavelengths, but GreenHouse Gases (GHGs) like CO2 and CH4 are more complex molecules that have more ways to stretch/bend/twist and thus more ways to interact with a wider range of wavelengths, including the infrareds (these gases don’t interact with visible light however).
When GHGs interact with infrareds, they re-emit the energy back in all directions, meaning that some of the energy will go out to space, and some will be projected back towards Earth (i.e., more energy gets trapped).
TL;DR: Greenhouse gases compromise the energy balance. Energy enters easily and struggles to get out because GHGs act as a one-way door: they don't interact with visible light, but they do with infrareds. More energy in and less energy out of the climate system means colder upper atmosphere (less heat manages to escape into upper layers of the atmosphere) and Earth's overall temperature increases.
⚠ This is not a “theory”. It’s been known for 300 years, has been experimentally verified, and is observed today using instruments in space (e.g. see: https://www.climateresear.ch/t/interview-with-camille-viatte-from-latmos-discussion-on-atmospheric-composition-and-iasi/22 where we talk about the IASI instrument that is onboard ESA-operated MetOp satellites).
Ok, so why are scientists still debating over safe levels of greenhouse gas concentrations in the atmosphere? 🤷♀️
Because models are a simplification of the real world!
A model is nothing more than a set of assumptions and equations parameterized with an uncertainty parameter that is used to factor our ignorance of certain phenomenon in the system being modelled. Differences in results may occur from one model to another based on the assumptions made (especially around the possible values of the uncertainty parameter). Different assumptions drive different results.
This all stems from a well-known issue in probability theory called The Bertrand Paradox, which relates to the Principle of Indifference and Uncertainty Quantification 📊
For more info, check Prof. Myles Allen's lecture https://www.gresham.ac.uk/watch-now/atmospheric-zero.