Part I. How Does Global Warming Work?

In my post “How Do You Know That?” I aim to explain how science actually works, how scientific evidence is evaluated and conclusions reached. I describe three criteria: First, scientific ideas must be tested against data. Second, they must be tested against well-established scientific theory. Third, they must be evaluated by peer review. When you read or hear a phrase that starts, “Scientists say…,” ask yourself whether the claim meets all three of these conditions. If you cannot trace the statement back to a peer-reviewed publication in a reputable journal where the evidence is laid out and related to established scientific theory, then you should be suspicious of the statement.

Let’s apply this process to global warming, and specifically to the claim made by almost all climate scientists (97 to 99 % of them [1]) that global warming is occurring and is anthropogenic (human-caused). I will use endnotes to cite peer-reviewed sources in the mainstream scientific literature so that if you wish you can look them up to see where the evidence comes from.

So let’s jump in. How do we know that global warming is caused by humans? There are many lines of evidence, but I will focus on the three that seem to me to be the most important:

  • The mechanism of global warming, the “greenhouse effect,” is well-understood and based on established science.
  • Data from measurements show that the planet is warming rapidly as a result of greenhouse gases generated by human activity.
  • Computer models of the climate confirm that global warming (as well as sea-level rise, ocean acidification, melting of ice sheets, and changes in weather) result from increasing levels of greenhouse gases.

I originally thought I could cover these three lines of evidence in one post, but I soon realized that a single article could not do them justice. This will have to be a three-part series, and even then, I can barely scratch the surface on each one.

In this post, I make the case for the first point: the mechanism of global warming follows from basic physics. It passes the test described in the first paragraph above: scientific claims must be tested against well-established scientific theory. In fact, the process by which the greenhouse effect works has been understood for well over 100 years [2] and follows directly from the fundamental properties of how molecules interact with light [3]. According to basic, well-established science, the greenhouse effect must occur.

The greenhouse effect in brief

Here’s a concise description of how it works: Sunlight warms the surface of the earth. The earth radiates energy back into the atmosphere as infrared light (not visible to our eyes). Molecules such as water (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and others, absorb infrared light through the vibrations of their atoms [4]. Some of that energy is re-emitted out into space, but some is trapped in the atmosphere.

This process is analogous to trapping heat inside a greenhouse, so these gases (and some others) are known as greenhouse gases [5]. As more greenhouse gases are dumped into the atmosphere, more heat is trapped and the earth, including land, oceans, and atmosphere, warm up. Greenhouse gases are well-mixed throughout the atmosphere, so this process is global. Rising CO2 levels must result in increased temperatures, i.e. global warming. And they do.

In fact, the greenhouse effect is vital for making earth habitable. Water vapor is a natural greenhouse gas, and without it the earth would be quite a bit colder and not a very hospitable place to live. We need the greenhouse effect! The problem is that high levels of greenhouse gases lead to too much warming. We cannot reduce the amount of water vapor in the atmosphere because water evaporates naturally from oceans, lakes, and rivers. As increased CO2 levels warm the planet, more water evaporates, leading to more water vapor in the atmosphere, and causing an increased greenhouse effect from water vapor. This is an example of positive feedback. Water vapor amplifies the effect of rising CO2 levels. The result is that the average global temperature increases about twice as much, according to estimates [6], as it would due to the increase in CO2 alone.

The greenhouse gas driving climate change is CO2, and the amount of CO2 in the atmosphere has increased sharply over the last 100 years [7],[8] (see Figure). We know this from analysis of CO2 in ice cores drilled from ice sheets, for example in the Antarctic [8]. These ice cores contain pockets of air that were trapped when the ice was deposited. Ice deeper in the ice sheet was formed longer ago and so contains a record of air (including its CO2 content) from that time. The record goes back to the most recent ice age. Direct measurements of CO2 levels are available from 1958 on [9]. The amounts of methane and nitrous oxide have also increased and contribute to warming, but CO2 is the predominant cause of global warming [10].

Figure: Atmospheric CO2 concentration in parts-per-million over the past ca. 2000 years. CO2 concentrations were determined from ice cores from the Law Dome Antarctic ice dome. Based on Figure 2.4b in endnote [8]. Data available for download here.

As the Figure shows, CO2 levels have increased sharply since around 1900, around the time when burning of fossil fuels also increased sharply. In fact, the rate of increase is much faster now than at any time over the past 800,000 years [8]. The evidence is also clear that the increase comes from burning fossil fuels [11]. I’ll summarize that evidence below under the heading “How can we be sure that rising CO2 levels come from burning fossil fuels?”.

If that’s enough science for you, feel free to skip ahead to “Let’s Summarize.” If you would like a more detailed account of the basic science of the greenhouse effect, read on.

The basic science of the greenhouse effect

Most sunlight comes to us as visible light (light that we can see with our eyes), together with some ultraviolet light (high-energy light with shorter wavelengths than visible light) and some near-infrared light (light with slightly longer wavelengths than visible light). When sunlight strikes the surface of the earth, some of the light from the sun is simply reflected back into space while some is absorbed by the earth’s surface and warms it. Warm objects (the earth’s surface in this case) emit light energy in a process called “thermal radiation” or “blackbody radiation.” (The name comes because black objects absorb light most efficiently.)

The process of light absorption followed by thermal, infrared radiation is different from reflection. Reflected light simply bounces off the surface as it would from a mirror without a change in its wavelength. In blackbody radiation, the earth’s surface absorbs sunlight, warming it. The earth’s surface re-emits thermal radiation as longer-wavelength infrared light, not visible to our eyes. (Night vision scopes detect thermal infrared light, so that warmer objects like human bodies show up.)

At low altitudes greenhouse gases absorb much of the infrared radiation emitted from the surface of the earth. They re-emit radiation in all directions, some back toward the earth and some up towards higher levels of the atmosphere, where the radiation is absorbed again by greenhouse gases at higher altitude. In this way, energy is relayed molecule-by-molecule towards the upper atmosphere. With higher altitude, the temperature decreases, since warming (mostly by convection) comes from the surface of the earth, which is farther away. (The region of the atmosphere where temperature decreases with higher altitude is called the troposphere and extends up to elevations of 20,000 to 60,000 feet, depending on the latitude.) In each layer of the atmosphere greenhouse gases absorb infrared radiation and then re-emit it in all directions. At higher altitudes, less of the radiation emitted upwards is reabsorbed because the number of greenhouse gas molecules per unit of volume decreases as the pressure decreases. As a result more of the radiation escapes into space.

Now imagine what happens when the amount of greenhouse gas (e.g. CO2) increases. Then, there are more greenhouse gas molecules at higher elevation to reabsorb radiation, and the average altitude from which radiation makes it out into space without being reabsorbed is higher. The temperature was originally colder at this slightly higher altitude, so not enough radiates out into space to balance the energy incoming to the earth until the temperature increases at this higher altitude. The result is that from that elevation down, temperatures are higher than they would have been without the increased amount of greenhouse gas [12]. The temperature of the atmosphere and the surface of the earth are both increased. This is global warming.

How can we be sure that rising CO2 levels come from burning fossil fuels?

The graph showing the sharp increase of CO2 since around 1900 may be enough to convince you that rising CO2 in the atmosphere is human-caused, but there is further strong evidence that the rising CO2 is anthropogenic, which I will briefly summarize here. (For more details, see endnote [11].) First, CO2 levels have increased at a rate which cannot be accounted for from other sources. Second, higher CO2 concentrations measured at an observatory in Hawaii compared to one at the South Pole can be explained by the fact that more fossil fuel is burned in the Northern than in the Southern Hemisphere. Third, the concentration of oxygen in the atmosphere has been declining as CO2 levels rise, and that decrease has occurred at a rate consistent with the loss of oxygen due to burning fossil fuels. Finally (and this one is a little more technical), the ratio of CO2 containing the carbon-13 isotope (13C) relative to CO2 containing the carbon-12 isotope (12C) has been declining [13]. This is relevant because fossil fuels are richer in 12C (and poorer in 13C) compared to CO2 in the atmosphere, so as fossil fuels are burned, more CO2 with 12C is introduced into the atmosphere relative to CO2 with 13C, leading to the decrease in the ratio of 13C to 12C in atmospheric CO2. Other potential sources of CO2 would not have this isotopic fingerprint.

So four independent lines of evidence agree that the rocketing CO2 concentrations come predominantly from burning coal, oil, and gas, with more minor contributions from other factors such as cement production [14].

Let’s summarize

The greenhouse effect is the result of the fundamental (and well-understood) interaction of molecules with light. There is really no doubt about the greenhouse effect. It’s built into the basic physics of the atmosphere and must occur. Increased levels of CO2 will inevitably lead to warming of the atmosphere. The question is how much warming.

The amount of CO2 in the atmosphere has risen very rapidly. Expected consequences include rising temperatures, increasing ocean heat content, increasing severity of storms, rising sea levels, ocean acidification, melting of sea ice and glaciers, and extinctions [15]. The resultant rise in temperatures is much too rapid for living species to adapt to the changes that result.

But how do we know that warming is actually occurring, that sea levels are in fact rising, that sea ice is really melting, etc., and that those phenomena have a human cause? That is the topic of my next post. Stay tuned!

See also the post Climate Injustice by my friend Dr. Steve Pardini on the impact of climate change on poor and marginalized populations.

Suggestions for further reading

John Houghton, Global Warming. The Complete Briefing, 5th ed. (Cambridge University Press, 2015). This is an excellent textbook on the basic science of global warming. The magazine Christianity Today has an interesting article on Sir John Houghton.

Eli Tziperman, Global Warming Science. A Quantitative Introduction to Climate Change and its Consequences (Princeton University Press, 2022). Another good textbook, a little more mathematical than Houghton’s book.

An excellent source of data related to climate change is the Intergovernmental Panel on Climate Change (IPCC), which has released its 6th Assessment Report. It is easily accessed online. The report is based on hundreds of peer-reviewed publications, and the report itself is peer-reviewed. A good place to start is the “Summary for Policymakers” in the AR6 Synthesis Report: Climate Change 2023. The full report contains references to data sources and original publications.

Endnotes


[1]. J. Cook, et al., “Consensus on consensus: a synthesis of consensus estimates on human-caused global warming,” Environmental Research Letters 11, 048002 (2016).

[2]. S. Arrhenius, “On the influence of carbonic acid upon the temperature of the ground,”. London, Edinburgh, and Dublin Philosophical Magazine 41, 237- 276 (1896). Evidence pointing to the effect of greenhouse gases goes back even further. See R. Jackson, “Eunice Foote, John Tyndall and a question of priority,” Notes and Records: the Royal Society Journal of the History of Science 74(1) 105-118 (2020).

[3]. For more details about the greenhouse effect, see, for example, the textbook Global Warming, by Sir John Houghton (5th ed., Cambridge University Press, 2015) and E. Tziperman, Global Warming Science (Princeton University Press, 2022).

[4]. The classic reference text on infrared light absorption is G. Herzberg, Molecular Spectra and Molecular Structure. Volume II: Infrared and Raman Spectra of Polyatomic Molecules (Van Nostrand, 1945).

[5]. You may wonder whether the two major components of the atmosphere, nitrogen gas (N2—about 78% of the atmosphere) and oxygen gas (O2—about 21%) contribute to the greenhouse effect. The answer is no, because these molecules do not absorb infrared light for reasons related to their molecular symmetry. See any introductory textbook on molecular spectroscopy, for example, Jeanne McHale, Molecular Spectroscopy, 2nd ed. (CRC Press, 2017).

[6]. S. Manabe and R.T. Wetherald, “Thermal equilibrium of the atmosphere with a given distribution of relative humidity,” Journal of the Atmospheric Sciences 24(3) 241-259 (1967); R. Colman and B.J. Soden “Water vapor and lapse rate feedbacks in the climate system,” Reviews of Modern Physics 93(4) 045002 (2021); A. A. Lacis, G. A. Schmidt, D. Rind, R. A. Ruedy, “Atmospheric CO2: principal control knob governing earth’s temperature,” Science 330 356-359 (2010); See also Eli Tziperman, Global Warming Science (Princeton University Press, 2022), pg. 27.

[7]. A. A. Lacis, G. A. Schmidt, D. Rind, R. A. Ruedy, “Atmospheric CO2: principal control knob governing earth’s temperature,” Science 330 356-359 (2010).

[8]. For CO2 levels today compared to the past see IPCC, 2021: Climate Change: The Physical Science Basis, Chapter 2, Section 2.2.3, available online at https://www.ipcc.ch/report/sixth-assessment-report-working-group-i/. The full citation is: S.K. Gulev, P.W. Thorne, J. Ahn, F.J. Dentener, C.M. Domingues, S. Gerland, D. Gong, D.S. Kaufman, H.C. Nnamchi, J. Quaas, J.A. Rivera, S. Sathyendranath, S.L. Smith, B. Trewin, K. von Schuckmann, and R.S. Vose, 2021: “Changing State of the Climate System,” in Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [the 6th Assessment Report from the Intergovernmental Panel on Climate Change: IPCC, 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [V. Masson-Delmotte, P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA (2021), Section 2.2.3.

[9]. IPCC, 2021: Climate Change: The Physical Science Basis, pg. 303.

[10]. Methane and nitrous oxide levels are summarized at IPCC, 2021: Climate Change: The Physical Science Basis, Chapter 2, available online at https://www.ipcc.ch/report/sixth-assessment-report-working-group-i/.

[11]. IPCC, 2021: Climate Change: The Physical Science Basis, Chapter 5, Sections 5.2.1-5.2.2, pp. 687-691, available online at https://www.ipcc.ch/report/sixth-assessment-report-working-group-i/. The full citation is: J.G. Canadell, P.M.S. Monteiro, M.H. Costa, L. Cotrim da Cunha, P.M. Cox, A.V. Eliseev, S. Henson, M. Ishii, S. Jaccard, C. Koven, A. Lohila, P.K. Patra, S. Piao, J. Rogelj, S. Syampungani, S. Zaehle, and K. Zickfeld, “Global Carbon and other Biogeochemical Cycles and Feedbacks,” in Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, (2021), Section 5.2.1-5.2.2, pp 687-691, available online at https://www.ipcc.ch/report/sixth-assessment-report-working-group-i/.

[12]. R. Colman and B.J. Soden, “Water vapor and lapse rate feedbacks in the climate system,” Reviews of Modern Physics 93(4), 45002 (2021).

[13]. Isotopes are forms of the same element that differ in their number of neutrons in the nucleus, and therefore differ in their atomic mass. Isotopes of the same element have mostly identical chemical behavior, but not completely. In the case of carbon, the lighter isotope of carbon (12C) is taken up by photosynthesis at a higher rate than the heavier isotopes of carbon (13C and 14C) because of the difference in mass. This phenomenon is called the kinetic isotope effect. As a result, plant material, including fossil fuels, is richer in 12C relative to 13C than the ratio in the atmosphere. See endnote 11.

[14]. The formation of cement releases CO2 (about 4% of the total CO2 emissions, but about half of the CO2 released from cement manufacture is slowly reabsorbed). For sources of for CO2 emission see IPCC, 2021: Climate Change: The Physical Science Basis, Chapter 5, available online at https://www.ipcc.ch/report/sixth-assessment-report-working-group-i/. Full citation in endnote 11.

[15]. Consequences of climate change are analyzed in Chapters 4 and 8-12 of Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.), Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, (2021).


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