We know exactly how CO2 heats up the atmosphere and it is actually really easy to understand on a high level. Have a read!
CO2 is a greenhouse gas that contributes to our planet heating up. There are other greenhouse gasses, but CO2 is the most problematic one because it is very stable and hangs around the atmosphere for many hundreds of years.
We know how CO2 warms up the atmosphere; it’s fairly basic physical chemistry. When sunlight hits the earth, some of it bounces back upward as infrared waves. Humans cannot see infrared light, but we can feel it as warmth (the red lamps used in some restaurants to keep the food warm or the lamps used to keep piglets warm use infrared waves.)
The infrared waves reflect off of the earth, make their way through the atmosphere, and escape into space - unless something stands in their way. The most abundant gases in our atmosphere, nitrogen and oxygen, don’t interact with infrared waves. CO2 is different. It absorbs some of the energy of the infrared wave and then re-emits it as new infrared waves in all directions. About half of that energy is therefore bounced back towards earth, and only half escapes into space. This additional heat warms up our atmosphere.
Here’s a good explanation of how CO2 warms the atmosphere.
With great regularity, you will encounter people who will say that the increase in CO2 in the atmosphere is caused by natural processes and humans have absolutely nothing to do with it.
That is 1) plain wrong and 2) even if it were correct - which it most definitely isn’t - climate change would still be an existential threat and we would need to do something about it. But let’s focus on how we know that humans are causing that spike in temperature.
Firstly, the spike is truly a spike. It is not some gradual increase over long periods of time. Take a look:
This spike shows a huge, very sudden increase in CO2. The only source of CO2 large enough to cause that spike is fossil fuels – carbon that was sequestered for many millions of years and has now been burned and released in record time. There is no other explanation for where this CO2 could come from.
The most basic reason is that fossil fuels — the equivalent of millions of years of plant growth—are the only source of carbon dioxide large enough to raise atmospheric carbon dioxide amounts as high and as quickly as they have risen. The increase between the year 1800 and today is 70% larger than the increase that occurred when Earth climbed out of the last ice age between 17,500 and 11,500 years ago, and it occurred 100-200 times faster.
In addition, fossil fuels are the only source of carbon consistent with the isotopic fingerprint of the carbon present in today’s atmosphere. That analysis indicates it must be coming from terrestrial plant matter, and it must be very, very old. These and other lines of evidence leave no doubt that fossil fuels are the primary source of the carbon dioxide building up in Earth’s atmosphere
In summary, the mess is real and we caused it. Now we need to clean up.