Climate change evidence

(Hossenfelder, 2024, 4:01; Keeling & Graven, 2021, p. 96)

I was interested to watch a video on what evidence there is to show clearly that climate is happening. Sabine Hossenfelder, a German physicist and science vlogger, explained this beautifully in a single clear video.

She outlined five evidence statements, as follows:

1. Firstly, "we can measure [how carbon dioxide molecules absorb and emit light] in the laboratory", so we know it "is really good at absorbing light in the infrared. This is what makes carbon dioxide good at keeping our planet, and other planets, warm" (Hossenfelder, 2024, 1:27)
2. Next, we have measured "how much carbon dioxide [there] is in the atmosphere, [which has] been increasing ever since" recordings began (1:55). That "gives us a [probable] cause [for] warming: it’s the carbon dioxide" (2:02)
3. Then, there's our oceans. They "take up carbon dioxide from the air and that lowers their pH value, so it makes them more acidic" (2:08). Other potential causes of acidification have been ruled out, so "it’s probably the carbon dioxide" (2:27)
4. Now we get to the key point, making the leap that the "additional carbon dioxide in the atmosphere actually comes from fossil fuels" (2:33). To begin, there are "three different isotopes of carbon: carbon 12; carbon 13; and carbon 14" (2:44) which plants absorb in differing ratios. "Carbon 14 is radioactive and decays with a half-life of about 6000 years. It’s really good for dating organic" matter (2:58), but, "because nuclear bomb tests in the 1960s pumped a lot of carbon 14 into the atmosphere (3:13), we don't know "what the baseline is" (3:16) to discount that from what we want to measure. We could measure carbon in the atmosphere using Carbon 13, but it is heavier so the plant uptake is lower than Carbon 12, which plants like. The "carbon 13 to carbon 12 ratio in plants is lower than it is in the atmosphere" (3:36). OK, so if we buried "a lot of dead plants underground, [we would] bury a lot of carbon 12 with them, and if [we then dug] them up and burn[ed] them, that carbon 12 goes back into the atmosphere and lowers the carbon 13 to carbon 12 ratio" (3:42). And that is what is happening: "The Carbon13 ratio has been dropping" (3:56), which "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" (Hossenfelder, 2024, 4:01)
5. Lastly, there "is stratospheric cooling. Carbon dioxide warms the earth’s surface, but high up in the atmosphere it actually has a cooling effect. In [...] 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" (Hossenfelder, 2024, 4:29)

The video below shows the graphs which support each statement. A brief, but interesting watch.

The image is taken from the video, but appears in Keeling & Graven's paper (2021, p. 96).

Sam

References:

Hossenfelder, S. (2024, January 26). How do we know climate change is caused by humans? [video]. YouTube. https://youtu.be/J1KGnCj_cfM

Keeling, R. F., & Graven, H. D. (2021). Insights from time series of atmospheric carbon dioxide and related tracers. Annual Review of Environment and Resources, 46, 85-110. https://doi.org/10.1146/annurev-environ-012220-125406