The Science Behind Global Warming

With the summer starting, I’ve begun to dive deeper into the intersection between climate change and science. My aspirations for this summer were to take some classes online so that I could learn more about the climate crisis and how science might offer solutions. While browsing through courses on Coursera, I came across one that caught my attention. It was called Global Warming I: The Science and Modeling of Climate Change.  I decided to give it a try, and even after just a few days, it’s completely changed the way I think about global warming.

One of the first topics that we covered was the Stefan–Boltzmann Law, which states that the total energy flux is equal to ϵσT^4, where ϵ is the emissivity (usually equal to 1), σ is the Stefan-Boltzmann constant (5.67 x 10^-8 W/m²·K⁴), and T is the temperature in Kelvin. The incoming sunlight energy flux can be modeled with the expression L(1-a)/4, where L is the solar constant and a is the albedo (reflectivity). At equilibrium, the Earth balances incoming sunlight and outgoing infrared radiation, meaning we can set these two to be equal: L(1-a)/4=ϵσT^4. We can then solve this equation to determine the temperature T at Earth’s surface.

Things get much more interesting when we consider the Greenhouse Effect. Certain gases in the atmosphere trap outgoing infrared radiation, re-emitting some of it back toward the surface. This added “blanket” of warmth raises the surface temperature to be higher than just a naked planet. I learned to calculate that each gas layer essentially increases the surface temperature by a factor of 2^(¼) ≈ 1.189, with each layer elevating the temperature exponentially.

But which specific molecules in our atmosphere contribute to these gas layers? Which of them are considered greenhouse gases? For a gas to trap heat, it has to vibrate in a way that it creates a fluctuating electric field, or in other words, an oscillating dipole. Totally symmetric molecules, such as O2, N2, and H2, are incapable of doing this, so they aren’t greenhouse gases. Water vapor (H2O), on the other hand, is asymmetric, so it is a greenhouse gas. But even some symmetric molecules like carbon dioxide (CO2) or methane (CH4) can bend or stretch to be asymmetric, which makes them greenhouse gases as well. In fact, climate change on Earth is mainly caused by the bending of carbon dioxide molecules in our atmosphere. If we can discover a way to prevent this bending, we would have found a way to mitigate global warming significantly.

Bending and stretching of a CO2 molecule

I never expected to find physics and chemistry in this field, but I find the way it explains the climate system is just fascinating.