Lesson 11b: How do we know it’s fossil fuel burning?

As I was writing about carbon dioxide levels rising in the previous post, I began asking myself what evidence we have to support that the rise is caused by fossil fuel burning by us – rather than from natural causes. That set me off down different paths – which I’ll explore with you here. I’m not an expert on any of these topics, but I know how to think about things in a scientific way – so here are my explorations.

radiocarbon_sub1
Principles of carbon dating. Image from http://rses.anu.edu.au/services/anu-radiocarbon-laboratory/radiocarbon-dating-background

First, I wondered about whether the carbon dating techniques would teach us about this. Carbon dating is a technique used to work out how old wooden objects are. It works like this: In the upper atmosphere, nitrogen atoms are hit by cosmic rays and are converted into carbon-14 (carbon atoms with 6 protons and 8 neutrons). Carbon-14 is radioactive and it decays, slowly, back to nitrogen (7 protons, 7 neutrons). If you have a large number of carbon-14 atoms, then after ~5730 years, half of them have decayed back to nitrogen (that’s what a half-life means). In the atmosphere, the cosmic rays keep making new carbon-14 atoms. A growing tree will take in carbon-14 as well as the other isotopes of carbon (carbon-12 and carbon-13) from the atmosphere while it is alive. Once it dies, there is no more carbon-14 coming in from the atmosphere but the carbon-14 that is in the wood continues to decay into nitrogen. So, if a boat or a chair was made from a tree, you can tell how old it is by seeing how much carbon-14 is left in it. Every ~5730 years the amount of carbon-14 halves.

Now, fossil fuels are fuels made from fossilised wood that grew hundreds of millions of years ago. So, there have been many, many half-lives that have passed, and there is no carbon-14 left. I wondered whether, as a result of us burning fossil fuels, the amount of carbon-14 in the air is noticeably lower than it “should be”?

I read quite a few online documents and scientific papers and discovered a couple of things – first that in the early 20th century there was a noticeable “ageing” of the atmosphere – it looked older than it should have done. But then we really messed up the readings by setting off lots and lots of atomic bombs.

Hemispheric_14C_graphs_1950s_to_2010
Image from Wikipedia article. I’m not sure what the vertical axis really means because carbon-14 is never several percent of the carbon, but while I think they’ve missed off a scaling factor, or not explained what it is a percentage of, the shape tells a powerful story – atmospheric carbon-14 went up when we released nuclear bombs

However, that’s now dropping and the scientific paper I found suggests that by 2050 brand new wood might look like it grew in 1050! I’m not completely sure whether that’s based on measurement or projection making the assumption that humans are emitting fossil carbon, but it does provide some evidence that you could test.

There’s also another carbon isotope, carbon-13. This is not radioactive, so doesn’t decay. From that you can tell something about the origin of the material. Photosynthesis affects the ratio of carbon-13 to carbon-12 as it prefers one to the other (I’m massively out of my depth with this chemistry and biology, so I’ll stop there – but apparently there are two types of photosynthesis). Whereas geological processes have no such bias. Therefore, if something was ever a plant, or ate a plant, the ratio is different than if it came from rocks. As a result you can distinguish fossil fuel carbon (from 100s of millions of years old trees that had photosynthesis) from volcano carbon. And the increase in carbon dioxide in the atmosphere shows it comes from plants – but ones that are old enough for carbon-14 to decay: in other words, fossil fuels.

We attempt to track carbon dioxide from volcanoes. There is no where near enough. Even if we’re a lot wrong in that, it’s not enough.

Also the oxygen levels are decreasing at the rate you’d expect if we were burning things. And we know carbon dioxide levels are increasing in the ocean, so it’s not ocean outgassing.

Other evidence that the increase in carbon dioxide comes from us comes from a simpler source – we know how much fossil fuel we’ve dug or pumped out of the ground. Because it has a monetary value, we actually track that very carefully. Basic chemistry tells us that carbon dioxide is a combustion product when we burn fossil fuels (we can also measure that in a laboratory easily). So we can calculate how much increase we’d expect.  The increase in carbon dioxide in the atmosphere is quite a lot lower than what we’d expect from that simple calculation. That’s because the oceans and the trees have taken up a lot of our emissions. But not all. And measurements over them (e.g. by those satellites we talked about in the last lesson) show that they are now absorbing less (the oceans are “saturating” and simply can’t take any more and we’re cutting down, rather than planting, forests). The global climate budget tries to track and measure all this.

(I promise a later blog called “But dinosaurs didn’t drive SUVs” to discuss why carbon dioxide levels were much higher in their days without us).

 

 

Lesson 10: Anthropogenic Climate Change

models-observed-human-natural
Figure from the reportĀ  “Climate Change Impacts in the United States: the Third National Climate Assessment” (2014). https://nca2014.globalchange.gov/

In the last few lessons I’ve been talking about climate models and how they can model incredible complexity including energy balance, convection (circulation) in the atmosphere and oceans, and biogeochemical processes. Once we have such models we can do many things. First, the models help us ask questions and test our assumptions. They allow us to explore “what if” scenarios and understand how important certain components of the system are. Second, the models help us to predict the future and third, they allow us to understand what we can, and cannot, influence.

The figure above comes from a US government report published in 2014. It compares two runs of a climate model with observations of “global average temperature”.

The two model runs have a broad shaded area. That represents the uncertainty of the model – it indicates the range that the temperature could be in, based on multiple runs of the model (the so-called “ensemble run”) in which initial starting points (and the sizes of certain effects) are varied from run-to-run in a way that is consistent with our understanding of our lack of knowledge.

Global average temperature is not an easy thing to measure (we’ll come on to that in later lessons), but the black line is the result of our best attempt at combining the data we have. Really it should also have “uncertainty” prescribed to it – I’d prefer to see this graph with a band around the black line too. I don’t know enough about how this value is determined (I’ll try to find out and get back to you!), but my guess is that it has an uncertainty (width) of somewhere between half that of the models and the same size as the models.

The green model band describes “natural factors only”. This runs the model considering all the biogeophysical processes, and also considering the distance between the Earth and the Sun, variations in the solar cycle, volcanos erupting and releasing gases into the atmosphere, trees growing and dying, lightning-caused fires and so on. The blue model band describes “natural and human factors”. It includes all the quantities above, but also includes anthropogenic (human released) fossil fuel burning (coal, oil, gas), cement making, the release of particles in cities (smog, air pollution), refrigerant gases (CFCs and their more modern replacements), methane release in industrial-style farming and landfill waste tips), and land use changes (cities, deforestation). Note that 80% of the observed difference between the blue and green lines is due to fossil fuel burning. The other things make up a further 20% of that.

Until 1980 you can’t tell the difference between the lines. It becomes clear (now, in hindsight) around 1990. But it’s worth remembering that in 1990 our computers were a lot smaller, our climate models a lot less detailed (remember the 1987 storm that the MetOffice failed to predict – that was because the weather forecasts were a lot less reliable then – and the climate models are based on the same programs as the weather models). So while in hindsight it was around 1990 that humans became a driving force in the climate, we’ve only had the science to understand that since about 2010. We are in the very early days of our full understanding of the problem.

I’d like to keep the science and the politics separate, so I’ll write a separate note on my thoughts about this.