• Question: How do we know that the extra carbon in the atmosphere comes from the burning of fossil fuels and not some other source?

    Asked by anon-257313 on 17 Jun 2020.
    • Photo: Peter Bentham

      Peter Bentham answered on 17 Jun 2020: last edited 17 Jun 2020 12:12 pm

      Great question Geoff!

      It isn’t easy to show that climate change is caused by the greenhouse gases that humans emit because nature emits around 20 times more CO2 into the atmosphere than humans do, but it’s the new carbon that humans introduce that changes the total amount of carbon in the system – which is a key driver of climate change. Different methods are used, but here are two key ones.

      Using carbon-14 (14C), a radioactive isotope of carbon which is found naturally in atmospheric CO2, however it is not present in the fossil fuels (oil, gas, or coal) that humans extract from underground. This is because fossil fuels were formed millions of years ago, so that all of the radioactive 14C has decayed away. So CO2 emitted by burning these fuels does not contain any 14C. By measuring levels of 14C and total CO2 over time, it is found that the vast majority of excess CO2 does not contain 14C and therefore must be linked to fossil fuels.

      The other method measures changes in amount of oxygen in the atmosphere. It is used to determine whether any extra CO2 is from burning fossil fuels, or if it is instead from other sources like volcanic eruptions or cow farts. When a fossil fuel is burned, oxygen is consume in order to give off energy, along with water vapor and CO2. So oxygen levels are expected to decrease when humans burn fossils fuels, and when measuring levels over time it is found that they do, in amounts consistent with the changes in measured carbon dioxide levels.

      Hope that makes sense.

      Useful source

    • Photo: Laura Hunt

      Laura Hunt answered on 17 Jun 2020:

      Hi Geoff, thank you for your question. It’s a good question – and one that is important to understand when thinking about climate change.

      There are 2 main ways we know that this ‘extra’ carbon is in the atmosphere due to human activity.

      1. We can use a method called ‘carbon’ accounting . We basically add up all of the carbon released by burning of fossil fuels by humans, which we know from inventories of coal and other fuels being burnt, and add up other ‘natural’ sources of carbon emissions such as emission of carbon dioxide from forest fires. We then take these carbon emissions away from the amount of carbon that is stored, year on year, by the natural environment – for example, by the ocean. We know the amount of carbon ‘captured’ by earth’s natural processes from previous science experiments, which have created clever models we can use to do this. We then can calculate the amount of carbon released into the atmosphere that is ‘extra’ because of fossil fuels, and other human sources. We can double check this against the measurements of carbon dioxide in the atmosphere, which can be used to calculate the increasing volume of carbon in the atmosphere. This can be quite a challenging process because of something called ‘positive feeddback loops’ – this means that as the earth warms due to increasing carbon dioxide trapping solar energy, more carbon is released from the earth’s natural carbon stores, for example, as the earth warms, permafrost melts, and emits methane, another greenhouse gas.Therefore, not all of the ‘extra’ carbon dioxide in the atmosphere is simply due to fossil fuel burning, but due to ‘knock on’ processes.

      2. There are a few different ‘forms’ of carbon that exist (carbon atoms can have slightly different masses which we can measure) – we can think about this as carbon having different ‘fingerprints’. The carbon that is locked up in fossil fuels, and forms carbon dixoide when burnt is REALLY old (fossil fuels were formed millions of years ago) and been subjected to lots of different processes to turn from plant material to coal/oil/gas. This means that carbon dioxide that is in the atmosphere because of humans burning fossil fuels has a different ‘fingerprint’ to carbon dioxide that is in the atmosphere ‘naturally’. We can measure the fingerprint of carbon dioixde in the atmosphere and see how this changes over time, and can only be explained due to the emission of carbon from burning fossil fuels.

      I hope this makes sense – please let me know if there is anything further you’d like to ask.

      Laura 🙂

    • Photo: Emma Markham

      Emma Markham answered on 19 Jun 2020:

      Hi Geoff,

      Great question. Global warming is a really complex problem, but luckily we have been measuring temperature, CO2 concentrations and many other measurements for over 100 years, so we can look at changes and trends over time.

      As countries become more industrial they increase their use of fossil fuels. Increases in these emissions correlate with increases of CO2 in the atmosphere. As nations develop they also tend to change their diet to be more ‘western’ and increase the amount of meat eaten, and so this increase the amount of livestock. Livestock produces a significant amount of Green House Gas emissions, which contributes to global warming.

      But there are lots of carbon soaks or places which sequester carbon, like the ocean, peat bogs, rainforests, etc. so as atmospheric levels rise more carbon is being stored in these carbon soaks, but this cannot absorb all the increase. So, initial increases in fossil fuel use might not make a big change in the atmosphere, as the atmosphere is a large volume of gas, and some of the carbon will be removed as part of the normal carbon cycle and stored in other forms. But over time the levels slowly increase, causing global temperature rises, releasing carbon from many places where it is stored (the ocean can store less carbon as it get warmer, peat bogs dry out and release their carbon into the atmosphere, etc).

      I think it doesn’t matter so much where the carbon is coming from, as it is part of a cycle, but we want to shift the balance so that more carbon it being stored and not in the atmosphere where it is contributing to global warming.

      I hope that helps answer your question,
      Emma

    • Photo: Mary Gagen

      Mary Gagen answered on 25 Jun 2020:

      Great question! there’s som fab answers here already so I wanted to add my answer as someone who works on palaeo (past) climate changes. Basically we cannot mathematically explain (using climate models) the changes in climate that have happened over the last centuries without including human-caused carbon emissions. Changes in the sun, and changes in land use, do not explain the mean rise in global temperatures that we have measured and observed. We can only explain the changes we have witnessed when we add the carbon emissions into climate model simulations. They are the smoking gun.

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