Arguments
At a glance - How do human CO2 emissions compare to natural CO2 emissions?
Posted on 26 September 2023 by John Mason, BaerbelW
On February 14, 2023 we announced our Rebuttal Update Project. This included an ask for feedback about the added "At a glance" section in the updated basic rebuttal versions. This weekly blog post series highlights this new section of one of the updated basic rebuttal versions and serves as a "bump" for our ask. This week features "How do human CO2 emissions compare to natural CO2 emissions?". More will follow in the upcoming weeks. Please follow the Further Reading link at the bottom to read the full rebuttal and to join the discussion in the comment thread there.
At a glance
Have you heard of Earth's carbon cycle? Not everyone has, but it's one of the most important features of our planet. It involves the movement of carbon through life, the air, the oceans, soils and rocks. The carbon cycle is constant, eternal and everywhere. It's also a vital temperature control-mechanism.
There are two key components to the carbon cycle, a fast part and a slow part. The fast carbon cycle involves the seasonal movement of carbon through the air, life and shallow waters. A significant amount of carbon dioxide is exchanged between the atmosphere and oceans every year, but the fast carbon cycle's most important participants are plants. Many plants take in carbon dioxide for photosynthesis in the growing season then return the CO2 back to the atmosphere during the winter, when foliage dies and decays.
As a consequence of the role of plants, a very noticeable feature of the fast carbon cycle is that it causes carbon dioxide levels to fluctuate in a regular, seasonal pattern. It's like a heartbeat, the pulse of the Northern Hemisphere's growing season. That's where more of Earth's land surface is situated. In the Northern Hemisphere winter, many plants are either dead or dormant and carbon dioxide levels rise. The reverse happens in the spring and early summer when the growing season is at its height.
In this way, despite the vast amounts of carbon involved, a kind of seasonal balance is preserved. Those seasonal plant-based peaks and troughs and air-water exchanges cancel each other out. Well, that used to be the case. Due to that seasonal balance, annual changes in carbon dioxide levels form regular, symmetric wobbles on an upward slope. The upward slope represents our addition of carbon dioxide to the atmosphere through fossil fuel burning.
Fossil fuels are geological carbon reservoirs. As such, they are part of the slow carbon cycle. The slow carbon cycle takes place over geological time-scales so normally it's not noticeable on a day to day basis. In the slow carbon cycle, carbon is released by geological processes such as volcanism. It is also locked up long-term in reservoirs like the oceans, limestone, coal, oil or gas. For example, the "37,400 billion tons of 'suspended' carbon" referred to in the myth at the top of this page is in fact dissolved inorganic carbon in the deep oceans.
Globally, the mixing of the deep ocean waters and those nearer the surface is a slow business. It takes place over many thousands of years. As a consequence, 75% of all carbon attributable to the emissions of the industrial age remains in the upper 1,000 m of the oceans. It has not had time to mix yet.
Fluctuations in Earth's slow carbon cycle are the regulating mechanism of the greenhouse effect. The slow carbon cycle therefore acts as a planetary thermostat, a control-knob that regulates global temperatures over millions of years.
Now, imagine the following scenario. You come across an unfamiliar item of machinery that performs a vital role, for example life support in a hospital. It has a complicated control panel of knobs and dials. Do you think it is a good idea to start randomly turning the knobs this way and that, to see what happens? No. Yet that is precisely what we are doing by burning Earth's fossil fuel reserves. We are tinkering with the controls of Earth's slow carbon cycle, mostly without knowing what the knobs do - and that is despite over a century of science informing us precisely what will happen.
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For a person making $50,000/yr, and for whom expenses balance with income, a windfall of receiving an additional $2000/yr is a big deal, even though $2000 compared to $50,000 is a small amount. The same is true when comparing human emissions to the otherwise balanced natural carbon sources and sinks.
Look at human-added CO2 as borrowing debt on a fossil fuel credit card. When we were young (>200 years ago), we had 280ppm CO2 in the bank and life was good. We had just received a fossil fuels credit card (which created debt in CO2) with seemingly no limit. Our family started to grow and we needed more energy so we borrowed a little fossil fuel. It seemed like we were borrowing almost nothing and thought there was almost no CO2 to worry about.
But as the years went by and the balance, grew we became shocked at how much it had grown. At first there was only about 1 billion of us and we had 280 ppm CO2 in the bank. We needed a little extra and started borrowing, just a little, less than 1ppm per year. But as our family grew (population), we borrowed more and more fossil fuels because suddenly we needed the extra energy to support our growing family (population) using our fossil fuels credit card (fossil fuels from 300 million years ago, and our 100 million CO2 account credit limit of stored solar energy).
By the time we reached 1960 our family (population) had grown to about 3 billion and our fossil fuel credit borrowing had increased to about (315-280= 35ppm) of CO2 debt, still not too bad. But then things started going crazy and our family was increasing by another billion every 12-13 years. All of a sudden in 2023 our family was 8 billion and we had borrowed so much fossil fuel our CO2 debt reached (421-280=141 ppm). We owed more than half of our entire starting bank account of 280ppm, and were borrowing more than ever at 3ppm with no end in sight. Instead of retiring in comfort in our old age, we are going to have to work until our dying day.
It all seemed like such a small amount of borrowing at the start, but now, we are in big trouble! We will never be able to pay off that CO2 credit card debt?
rip71749@2, to add to the impact of your analogy, coming out of the last glacial cycle CO2 rose at a rate of 1 ppm/100 yrs for 1000s of years. So even the relatively slow increase you note of 1 ppm/year during the early years following the Industrial Revolution were, in geological terms, extremely rapid.
I'm a statistician, not a natural scientist, so I cannot say it matters, but I wonder whether we should call attention to the thickness of the atmosphere &, in particular, of the troposphere (bottom 8 to 14 km of atmosphere). Given that thickness, an IR ray passing upwards thru CO2, CH4, & other GHGS at even very low (but increasing) concentrations would have diminishing chances of escaping to outer space. Is that a valid argument?
amhartley... What you stating isn't a good explanation of how the greenhouse effect works. Andrew Dessler has a good explanation here.
Essentially, the GHE functions due to the effective IR emission altitude. Adding greenhouse gases causing that emission altitude to rise and the thermal gradient determines the temperature rise at the surface.
A single photon of IR of right wavelength leaving earths surface doesnt travel very far at all. I think mean path length before absorption is less than 10m at 400ppm (too lazy to calculate). As the Dessler video shows, what matters is the height where IR can escape to space.
I am fond of Chris Colose explanation.
There is also a less arriditic explanation here. it is a bit longer but there are some drawings that help the simple minds like mine.
Enter "I Misunderstood the Greenhouse Effect. Here's How It Works"; and the name of the author: "Sabine Hossenfelder" in the search bar and you will find it on Utube.
[RH] Link added. She's saying exactly the same thing as Dessler in the PhD version.
CORK @7 :
What is your phrase "less arriditic explanation" [unquote] ??
. . . "arriditic" is not in my English or German dictionaries.
Hossenfelder is a German woman, but speaks fluent English and also supplies some humorous quips in her many Youtube videos. She is a mainstream scientist ~ not a denialist crank, nor harridanitic at all.
( Or am I misunderstanding your dry humor, CORK ? )
On the GHE, you find a better explanation at SkS , really.
amhartley @ 4:
You've had a few answers that might help. I'll add the following.
You mention "thickness of the atmosphere". When discussing radiation transfer (absorption in this case), it is not the physical distance that matters. It is the number of molecules of the absorbing gas that affects the probability of radiation absorption. You can pack the same number of molecules into a short physical distance, or spread them over a larger distance, and the absorption characteristics will remain the same. In radiation transfer, you will see the term "optical thickness" or "optical depth". to distinguish this from physical distance.
This post on Beer's Law gives an illustration of this.
...but yes, IR radiation emitted at low altitudes will be unlikely to reach space directly. But at each level, the atmosphere also emits IR radiation, and the further up you go, the more likely it is to reach space directly. Understanding the greenhouse effect writ large requires looking at both absorption and emission, and at all levels.
There is more discussion of this on the Beer's Law post I linked to above, but a useful resource online is MODTRAN. You can play around there with a full atmospheric IR radiation transfer model that includes all these effects.