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At a glance - How do we know more CO2 is causing warming?

Posted on 25 July 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 we know more CO2 is causing warming?". 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

To make a statement like, "minor greenhouse gases such as CO2 have little effect", is to ignore 160 years of science history. So let's look at who figured out the heat-trapping properties of carbon dioxide and when.

Experiments involving various gas mixtures had demonstrated the heat-trapping properties of water vapour, CO2 and methane in the 1850s. But those effects were yet to be quantified - there were no meaningful numbers. It was to be another 40 years before that happened.

Swedish scientist Svante Arrhenius (1859-1927) was the person who crunched the numbers. The results were presented in a remarkable paper, "On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground", in 1896.

The many calculations in the 1896 paper include estimates of the amounts of CO2 increase or decrease required to drive the climate into a different state. One example used was the Hothouse climate of the Cenozoic, around 50 million years ago. Another was the glaciations of the last few hundred millennia.

To get a temperature rise of 8-9°C in the Arctic, Arrhenius calculated that CO2 levels would have to increase by 2.5 to 3 times 1890s levels. To lower the temperature 4–5°C to return to glacial conditions, he calculated a drop in CO2 was needed of 0.62-0.55 times 1890s levels.

We know CO2 levels in the 1890s from ice-core data. They were around 295 ppm. Let's do the sums. A reduction factor of 0.55 to 0.62 on 295 ppm gives 162.2-183.9 ppm. Modern ice-core measurements representing the past 800,000 years show that in glacial periods, CO2 levels fell to 170-180 ppm.

What we now know due to additional research since 1896 when Arrhenius worked on this, is that CO2 was an essential 'amplifying feedback'. That means changes triggered by long term, cyclic variations in Earth's orbit cause warming or cooling and CO2 release or entrapment in turn. Those changes in CO2 levels affected the strength of Earth's greenhouse effect. Changes in the strength of the greenhouse effect then completed the job of pushing conditions from interglacial to glacial - or vice-versa.

Arrhenius also made an important point regarding water vapour: "From observations made during balloon voyages, we know also that the distribution of the aqueous vapour may be very irregular, and different from the ideal mean distribution." This statement holds true today: water vapour is a greenhouse gas but because water exists in gas, liquid and solid forms in the atmosphere, it is continually cycling in and out of the air. It is distributed in a highly uneven fashion and is uncommon in the upper atmosphere. That's where it differs from CO2.

Once CO2 is up there, it's up there for a long time. As a consequence it has a pretty even distribution: 'well-mixed' is the term. As Arrhenius quantified all that time ago, once it's up there it constantly absorbs and re-radiates heat in all directions. That's why dumping 44 billion tons of it into our atmosphere in just one year (2019 - IPCC Sixth Assessment Report 2022) is a really bad idea.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above


Click for Further details

In case you'd like to explore more of our recently updated rebuttals, here are the links to all of them:

Myths with link to rebuttal Short URLs
Ice age predicted in the 1970s sks.to/1970s
It hasn't warmed since 1998 sks.to/1998
Antarctica is gaining ice sks.to/antarctica
CRU emails suggest conspiracy sks.to/climategate
What evidence is there for the hockey stick sks.to/hockey
CO2 lags temperature sks.to/lag
Climate's changed before sks.to/past
It's the sun sks.to/sun
Temperature records are unreliable sks.to/temp
The greenhouse effect and the 2nd law of thermodynamics sks.to/thermo
We're heading into an ice age sks.to/iceage
Positives and negatives of global warming sks.to/impacts
Global cooling - Is global warming still happening? sks.to/cooling
How reliable are climate models? sks.to/model
Can animals and plants adapt to global warming? sks.to/species
What's the link between cosmic rays and climate change? sks.to/cosmic
Is Al Gore's An Inconvenient Truth accurate? sks.to/gore
Are glaciers growing or retreating? sks.to/glacier
Ocean acidification: global warming's evil twin sks.to/acid
The human fingerprint in global warming sks.to/agw
Empirical evidence that humans are causing global warming sks.to/evidence
How do we know more CO2 is causing warming? sks.to/greenhouse

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Comments

Comments 1 to 4:

  1. Perhaps you do nto think it worth mentioning in this brief the following early work in the area of CO2 and climate change. This includes de Saussure's demonstration of the focusability and transmission of "obscure chaleur" (dark heat) and the measurement of atmospheric heat trapping as a function of altitude with an insulated, dark interior, double glazed cubic foot box he transported from seal level to Alpine peaks: Fourier's mention of human industrial pollution's heat trapping potential in 1827 (which refers to de Saussure), and Arrhenius' 1896 paper with the first computed (single equation, single flat layer) atmospheric model, that sought to explain recently discovered evidence of ice ages by calulating the effect of halving, and also up to tripling the then current concentration of CO2 (about 295pmm) month by month at 10 degree latitude intervals to display the effects on changes of seasonal solar inputs. I think at lleast the last of these is worthmentioning in the brief.

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  2. Walsculer - see my comment below the following piece, in response to the very similar comment you posted there a few days ago:

    At a glance - Empirical evidence that humans are causing global warming

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  3. I am wondering if the information I supplied separately, including the pages from de Saussure's work, has been posted somewhere...

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  4. walschuler @3 - Sorry, we didn't yet have time to take a close(r) look. We'll be in touch directly once we can pick it up again.

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