How do we know that climate change is caused by humans?

[This is a transcript with links to references.]

How do we know that climate change is caused by humans? I got this question on twitter and I thought, c’mon, Google can answer that.

Well, the first hit I got on Google is a quote from a NASA website “It is undeniable that human activities have produced the atmospheric gases that have trapped more of the Sun's energy in the Earth system.” Alright, but that doesn’t really answer the question, does it? Going down the list of search results didn’t really help. I then went and asked my pal ChatGPT, which, true to character, produced a lot of words but didn’t really answer the question either.

That actually worried me. You’d think people can just look up how we know that we are responsible for climate change on the snap of a finger but turns out it’s not that simple.

To be fair, both Google and ChatGPT did a pretty good job explaining what evidence we have that the climate is indeed changing: rising temperatures, rising seawater levels, ocean acidification, declining ice covers, more extreme weather events, and so on and so forth. But you already knew that. How do we know it’s us who are responsible? Let’s have a look.

First, we know what type of light carbon dioxide molecules absorb and emit because we can measure this in the laboratory. Carbon dioxide is really good at absorbing light in the infrared. This is what makes carbon dioxide good at keeping our planet, and other planets, warm. This is the first piece of evidence: we know what carbon dioxide does.

Second piece of evidence is that we can measure how much carbon dioxide is in the atmosphere, and the fraction’s been increasing ever since the beginning of the measurements. Together with the first piece of evidence, this gives us a plausible cause of the warming: It’s the carbon dioxide.

The third piece of evidence is that oceans take up carbon dioxide from the air and that lowers their pH value, so it makes them more acidic. This is compatible with observations. Other hypothetical causes of global warming, such as an increase in the energy that comes from the sun, wouldn’t have this effect. Again, we conclude, it’s probably the carbon dioxide.

Fourth piece of evidence is that we know the additional carbon dioxide in the atmosphere actually comes from fossil fuels. We know this because there isn’t just one type of carbon, but three different ones. Those are three different isotopes of carbon: carbon 12, carbon 13, and carbon14. Plants absorb all of them in varying amounts.

Carbon 14 is radioactive and decays with a half-life of about 6000 years. It’s really good for dating organic stuff, though I recommend you leave it at home for the first dinner. Unfortunately, carbon14 dating doesn’t work well for the atmosphere, because nuclear bomb tests in the 1960s pumped a lot of carbon14 into the atmosphere, so it’s debatable what the baseline is.

This leaves carbon 13. Carbon 13 is stable, but plants don’t really like it. They prefer carbon 12 because that’s less heavy and that makes some biochemical processes run faster. This means the carbon13 to carbon12 ratio in plants is lower than it is in the atmosphere.

If you bury a lot of dead plants underground, you bury a lot of carbon 12 with them, and if you dig them up and burn them, that carbon12 goes back into the atmosphere and lowers the carbon 13 to carbon12 ratio. And this is exactly what the data say. The Carbon13 ratio has been dropping. This tells us that this carbon doesn’t come from rocks or volcanoes, it comes from plants, plants that must have stored it for a very long time. Like, you know, fossil fuels.

Fifth piece of evidence is stratospheric cooling.

Carbon dioxide warms the earth’s surface, but high up in the atmosphere it actually has a cooling effect. In a layer that’s called the stratosphere the additional carbon dioxide helps the thin air shed heat more efficiently.  It’s known as stratospheric cooling and goes back to a prediction made already in 1967. And the upper stratosphere has in fact cooled as satellite data show. If the warming of Earth’s surface was caused instead by an increase of energy coming from the sun, this would affect the surface as much as the higher layers of the atmosphere.

So, we know it’s the carbon dioxide, and we know that the carbon dioxide comes from fossil fuels, and really I didn’t make that video for you, but so you can send it to your uncle who still thinks it’s a hoax.

The quiz for this video is here.