![[../assets/global_warming_19th_cent.webp]] *DALL-E Prompt - an anthropomorphized earth dressed up as a 19th century scholar, sweating profusely, wide aspect ratio* Global warming is [defined by the Intergovernmental Panel on Climate Change (IPCC)](https://www.ipcc.ch/sr15/chapter/spm/) as *"An increase in combined surface air and sea surface temperatures averaged over the globe and over a 30-year period. Unless otherwise specified, warming is expressed relative to the period 1850-1900, used as an approximation of pre-industrial temperatures."* While this precise definition sets the stage for understanding the scale and significance of the phenomenon, it also leaves room for an exploration. An exploration into the history of *global warming* as seen by scientists at a time when knowledge did not travel at the speed of social media. While there are excellent pieces of literature that summarize the history of climate science, I found myself drawn to the original research—the first-hand accounts of scientists who pieced together this complex story. *What were their methods? How did they make sense of the data at hand? What debates and challenges did they face along the way?* My primary motivation was to ensure that my understanding is built directly upon foundational, primary knowledge rather than relying solely on interpretations of secondary sources. In this way, I believed I could bring my *frame of reference* as close as possible to the originals. That said, I must acknowledge the limits of my understanding when it comes to the comprehension of complex formulae, esoteric scientific principles, and academic language that at times can feel a bit too daunting. Though I do not work in climate science research on a daily basis, I think of my position on the matter to be uniquely useful - a *beginner with a clean slate and sufficient curiosity to learn*. My hope as I write this piece is two fold. First, I am looking to trace the journeys of early climate science pioneers and understanding *"Earth's warming"*, in a way that makes sense to me. Second, to share this journey with you, the reader, as a narrative that is both engaging and accessible. If this article inspires even a small spark of curiosity or motivates you to start your own research, I will consider it a success! --- Before I begin, I would like to call your attention to an [article published first on June, 2020 by John Mason that offers, in incredible detail a clear, historical account of all climate science research](https://skepticalscience.com/history-climate-science.html). If there is one source I have revisited and used as a "guiding light" for my exposition on the matter, it is this. The article is richly detailed, featuring thorough explanations and even a [timeline chart](https://static.skepticalscience.com/graphics/ClimateScienceMilestones_TwoCenturies_1024w.jpg) that illustrates the major milestones in climate science over two centuries. While Mason’s article covers the evolution of climate science from the late 19th century through the 2000s, my focus in this piece is more specific. I delve into the foundational period, exploring developments only up to the mid-20th century. For readers interested in the broader story, I wholeheartedly encourage you to explore Mason's work—it’s an invaluable resource for understanding the full trajectory of climate science. --- ### 1824 - Fourier's Discovery of the Greenhouse Effect In his 1824 publication, *[M´emoire sur les Temp´eratures du Globe Terrestre et des Espaces Plan´etaires (Translated as Temperatures of the Terrestrial Spheres by R. T. Pierrehumbert )](https://geosci.uchicago.edu/~rtp1/papers/Fourier1827Trans.pdf)*, Jean-Baptiste Joseph Fourier reasoned that *the earth would have the same freezing cold temperature of interplanetary sky if it were not for Earth's interior heat and the continual action of solar radiation*. Fourier, renowned for his analytical theory of heat and the Fourier series, calculated tEarth's expected temperature given its distance from the Sun. However, his calculations revealed a discrepancy: Earth’s actual temperature was warmer than expected. He hypothesized that the atmosphere played a critical role in this discrepancy. He believed that there had to be "something" in the atmosphere that allowed solar radiation to enter more easily than it allowed heat to escape. In the below diagram, I represent what this might look like as an illustration. ![[../assets/fourier_find.png]] **Figure 1.** Fourier's hypothesis that the atmosphere prevents extreme cold temperatures of the Earth (Credits: Author) We now understand that the atmosphere acts like an insulating blanket. The Sun’s energy, in the form of solar radiation, heats Earth’s surface. While some of this heat escapes back into space, much of it remains trapped by the atmosphere, maintaining a temperature that can sustain life. Over time, some heat is lost, but it is more difficult for heat from Earth to escape than for solar radiation to enter, due to the specific properties of the atmosphere. Fourier's research is widely regarded as the [first every discovery of the **greenhouse effect** , though he never used that exact term](http://mpe.dimacs.rutgers.edu/2013/01/19/the-discovery-of-global-warming/). These insights laid down by him paved the way for further enquiry into the matter of earth's temperature. --- ### 1856 - Eunice Foote's Discovery of the Effect of Carbon in Atmospheric Warming Eunice Foote was a prominent suffragette and an amateur scientist from America. In 1856, she published the results of an experiment in the American Journal of Science & Arts as the *[Circumstance's affecting the heat of the sun's rays](https://davidmorrow.net/s/foote_circumstances-affecting-heat-suns-rays_1856.pdf)*. Aimed at understanding how heat is absorbed by different components in the atmosphere, her experiment has more recently gained traction as one of the *first attempts* to understand the effect carbon dioxide has in heat absorption. Her experiment highlighted three key observations - 1. The denser the air, the higher the rise in temperature 2. The more the presence of moisture in the air, the higher the rise in temperature 3. The presence of carbon in the air magnifies the effect of sun's temperature. On the basis of her findings, she posited that *if more carbon is mixed into the air than what was present historically, it could lead to a much warmer atmosphere*. This insight, though simple in its articulation, foreshadowed modern understandings of greenhouse gases and their impact on global temperatures. ![[../assets/eunice_foote_carbon_air.png]] **Figure 2.** Screenshot from Eunice Foote's Paper While we now understand the importance of her findings, at the time it was not at the forefront of scientific discourse. In fact, her observations were lost to history while those conducted by John Tyndall on the same matter, only 3 years after Foote, earned him a place of reputation in the field of climate science (see next section for more on this). Over the years, several attempts have been made to understand why this might have been the case. A common explanation is that [Eunice Foote's work could not gain the prominence it deserved because she was an American woman and an amateur scientist](https://royalsocietypublishing.org/doi/10.1098/rsnr.2018.0066). In the mid-19th century, women had few platforms for academic recognition, and scientific exchange between American and European researchers was limited. It is also probably important to note that Foote's publication was only 2 pages long. Her experimental setup, as described in the paper is quite simple. The lack of details makes it hard to understand how controlled the conditions of her experiment were. And [they probably were not](https://www.climatechangenews.com/2016/09/02/the-woman-who-identified-the-greenhouse-effect-years-before-tyndall/). There is also no original schematic diagram to help visualise her experimental setup. Despite these historical limitations, Eunice Foote is now recognized as the first to identify the heating effects of greenhouse gases like water vapor and carbon dioxide. While her observations were overlooked for decades, modern researchers have worked to restore her place in history. Scholars such as [Raymond Sorenson](https://www.searchanddiscovery.com/documents/2011/70092sorenson/ndx_sorenson) and [Katherine Hayhoe](https://www.facebook.com/katharine.hayhoe/posts/in-1856-mrs-eunice-foote-presented-the-results-of-a-simple-experiment-at-the-ann/1743186032572943/) have championed her legacy, bringing her contributions into the spotlight and ensuring they receive the recognition they deserve. --- ### 1859 - John Tyndall's Sophisticated Experiment John Tyndall, an Irish physicist with wide-ranging scientific interests, made a significant contribution to our understanding of atmospheric science in 1859 when he began investigating the absorption and radiation of heat by gases. His findings were published as [On the Absorption and Radiation of Heat by Gases and Vapours, and on the Physical Connexion of Radiation , Absoprtion , and Conduction](https://royalsocietypublishing.org/doi/pdf/10.1098/rstl.1861.0001). In an incredible amount of detail, Michael de Podesta examines Tyndall's work on his blog [Protons for Breakfast](https://protonsforbreakfast.wordpress.com/2023/09/04/tyndall-1/). As a series of paragraphs, Michael provides a secondary account of Tyndall's experimental setup. As an attempt to understand this experimental setup to a high level of detail, in the figure below, I overlay some points (in blue) over Michael's annotations over Tyndall's original diagram. ![[../assets/tyndall_expt.png]] **Figure 3**. Annotate schematic of Tyndall's experimental setup ([Source (I have annotated in blue for my reference)](https://protonsforbreakfast.wordpress.com/2023/09/04/tyndall-1/)) The setup consists of a heat source (the Leslie cube), a hollow pipe through which gases are passed, a thermopile detector to convert heat energy to electrical energy and a galvanometer to measure the electric current, thus detecting the temperature change. This setup is in stark contrast to that of Eunice Foote's experimental setup (which was basically a couple of glass tubes). Another key difference here is that while Eunice Foote's heat source was solar radiation (that includes both visible and invisible (infrared) radiation), the [Leslie cube only emanates Infrared (IR) radiation](https://www.youtube.com/watch?v=LFwio38EK9s). This is crucial as the greenhouse effect as we know it now is [predominantly centered around IR rather than energy in the visible range](https://gml.noaa.gov/outreach/behind_the_scenes/whymeasure.html). Through this experiment, Tyndall measured the absorptive powers of gases like Oxygen, Nitrogen, Hydrogen, Water vapour, Carbon dioxide and Ozone. He discovered that while these gases were transparent to visible light, some of them were opaque to IR. And of those that were opaque to IR, there was an order - Water vapour absorbed and radiated most heat, followed by carbon dioxide and ozone. While Tyndall’s primary goal was not to study climate change, his work provided crucial evidence for the relationship between atmospheric gases and Earth's temperature. His findings reinforced earlier theories, such as Fourier’s, by demonstrating that specific gases trap heat in the atmosphere, affecting the planet’s thermal balance. In his 1861 paper, the famous [Bakerian lecture](https://web.gps.caltech.edu/~vijay/Papers/Spectroscopy/tyndall-1861.pdf), he has used the word *"climate"* four times, acknowledging the broader implications of his research. Below, I have highlighted these occurrences and offered some condensed explanations on his thoughts on the same. These references suggest that while his focus was on the physical properties of gases, he was aware of the potential significance of his findings for understanding Earth's climate system. Although Tyndall’s work was not initially framed within a climate change context, it became a cornerstone for later studies of the greenhouse effect and atmospheric warming. ![[../assets/tyndall_climate.png]] **Figure 4**. Annotated screenshot from Tyndall's paper --- ### 1896 - Svante Arrhenius Discovers the Effect of Carbon Dioxide on the Atmosphere Svante Arrhenius was a Swedish scientist who won the [Nobel Prize in Chemistry in 1903](https://www.nobelprize.org/prizes/chemistry/1903/summary/) for his work in quantifying the electrical conductivity of solutions. However, he also had another profound impact in the scientific world. This was his attempt to understand the effect of carbon dioxide in regulating Earth's surface temperature. In 1896, he published the [world's first paper on carbon dioxide and global warming](https://blogs.bl.uk/science/2016/12/the-first-paper-on-carbon-dioxide-and-global-warming.html). In the paper, we see Arrhenius take inspiration from the works of Fourier and Tyndall among others. Below is an excerpt from the paper annotated by me to call specific attention to these past interpretations that Arrhenius started from. The greenhouse effect, gases that cause it and the spectrum of sunlight that is affected by these gases were strong points that formed the basis of Arrhenius's enquiry. ![[../assets/arrhenius_lit_survey.png]] **Figure 5**. Annotated screenshot from Arrhenius' paper Only a decade ago, Samuel P Langley had used his invention, the bolometer to calculate the amount of infrared radiation (or simply, heat) received on the earth from the moon. Inadvertently, this data collection also revealed the absorption of infrared radiation by atmospheric components, such as water vapour and CO2. This data turned out to be crucial for [Arrhenius as he was able to re-purpose it and augment it with estimations for his own inquiry into the role of CO2 in trapping heat](https://climatephysics.substack.com/i/145167286/every-model-needs-empirical-data). In Table VII of the paper, Arrhenius calculates the mean alteration of temperature as a consequence of varying quantity of carbonic acid (CO2) in the air. He takes the present mean value of carbonic acid in the air as $K=1$ . In the following diagram, I have made an attempt to annotate these results for simplicity. ![[../assets/arrhenius_results.png]] **Figure 6**. Annotated screenshot from Arrhenius' paper On the basis of these measurements, Arrhenius observed the following - *If $K$ reduces to 0.67, then the fall in temperature (let's say $\Delta t$) is nearly the same as the temperature increase if $K$ is multiplied by a factor of 1.5.* This leads to his famous rule - *"If the quantity of carbonic acid increases in geometric progression, the augmentation of temperature will increase nearly in arithmetic progression"*. It is also to be noted that he explicitly states that this "rule" naturally holds good in the part investigated. I am not too sure what that means, but it is clear that he thought he was able to quantify a relation between atmospheric CO2 content and the surface temperature of the earth, but also believed there were certain underlying assumptions that had to be true if this were to be correct. After over a decade of study into this matter, Arrhenius went on to publish the much read popular science book *Worlds in the Making* in 1908. Through this he introduced the ["hot-house theory," explaining that the presence of CO2 in earth's temperature raises surface temperatures by about 30°C more than what it should have been](https://wattsupwiththat.com/2009/04/13/6995/). Deriving ideas from Fourier, Pouillet, and Tyndall, Arrhenius had arguably set in motion the use of the *greenhouse* as an analogy for the earth. It is also to be noted that at the time, Arrhenius[ described the use of fossil fuels as a *good thing* to ensure the earth stays warm](https://blogs.bl.uk/science/2016/12/the-first-paper-on-carbon-dioxide-and-global-warming.html) and does not fall into another ice age. It seems that he, like many other of his contemporaries grossly underestimated the scale at which fossil fuels would begin to be used. --- ### 1901 - Knut Ångström Refutes Arrhenius Knut Ångström was a fellow Swede who was skeptical (like several others at the time) of Arrhenius' claims around the relationship between CO2 and earth's warming. While Ångström acknowledges the effect of water vapour on absorbing solar and terrestrial radiation, he was not as convinced when it came to carbon dioxide. Through his work in 1901, *[About the importance of water vapour and carbon dioxide during the absorption of the Earth's atmosphere](https://www.justproveco2.com/papers/Angstrom1900English.pdf)* (this is a translated English version), he attempted to put forth his own findings that made his position on the matter clear. Amongst the claims he looked to refute, was Arrhenius' theory. Via a simple experiment involving a couple of glass tubes, a thermocouple and a galvanometer, he and his assistant tried to understand absorption of infrared radiation. Of the points raised in this experiment, the most important one was Ångström's claim that *CO2 could be saturated and be unable to absorb any more heat than it already has absorbed*. ![[../assets/angstrom_expt.png]] **Figure 7**. Ångström believed that CO2 gets saturated as he observed via his glass tube experiment Ångström further argued that Arrhenius relied on **imprecise data** from Langley's measurements of weak lunar radiation, which he claimed produced an "unclean spectrum" due to overlapping absorption bands from CO₂ and water vapour. He also suggested that CO₂ **absorption is limited to specific bands** and could not account for the broad absorption range proposed by Arrhenius. --- ### 1906 - Arrhenius Fires Back In 1906, Arrhenius struck back when he [responded to Knut Ångström's criticisms by defending the validity of his methods and the broader implications of his findings on CO₂'s role in Earth's climate](https://friendsofscience.org/assets/documents/Arrhenius%201906%2C%20final.pdf). He pointed out different studies to indicate the narrow focus of Ångström's experiment and how, the latter *"did not consider it necessary to investigate"* Arrhenius' finds in detail. While he acknowledged that Langley’s lunar radiation data had limitations, he countered Ångström’s claim that water vapour had a greater role to play than carbon dioxide by suggesting that while water vapour concentrations are high in the lower layers of atmosphere, this reduces with higher layers. He **criticised Ångström’s narrow focus on laboratory measurements**, which did not account for atmospheric dynamics, such as variations in temperature, pressure, and altitude, nor the complex interactions between water vapour and CO2. While recognising the need for improved data and methods, Arrhenius maintained that his conclusions about CO2’s significant impact on Earth’s temperature were robust. However his rebuttal was read less than his original paper and Ångström's criticism. So, the carbon dioixe theory of atmosphere temperature went to sleep! --- ### 1931 - Hulburt's View on the Relationship between Atmospheric Layers and Heat Absorption In 1931, Edward Olson Hulburt revived the now, largely dormant body of work on Earth's temperature regulation with his publication [*The Temperature of the Lower Atmosphere of the Earth*](https://journals.aps.org/pr/abstract/10.1103/PhysRev.38.1876). In the final section, titled _"The Carbon Dioxide Theory of the Ice Ages,"_ Hulburt explores the relationship between atmospheric carbon dioxide and global temperatures—a topic deeply influenced by the earlier theories of John Tyndall and Svante Arrhenius, but one where he diverged from the conclusions of Knut Ångström. This section is particularly noteworthy as it underscores Hulburt's general acceptance of the greenhouse effect while questioning Ångström's observations. Through his calculations, Hulburt proposed that doubling or halving atmospheric carbon dioxide could result in surface temperature changes of up to 4°C. This finding aligned with evidence from ice age cycles, leading him to support the idea that *removal of carbon dioxide could trigger an ice age.* Through the following illustration, I attempt to explain his physical basis for such a conclusion. ![[../assets/hulburt_convection_radiation.png]] **Figure 8**. Hulburt's view on the greenhouse effect Unlike earlier studies that preceded him, Hulburt expanded the understanding of atmospheric heat transfer beyond radiation alone, emphasizing the role of altitude. He noted that below 12 km above sea level, the atmosphere is dominated by convective heat transfer due to the higher density of air, which facilitates free air movement. In this lower region—the troposphere—hot air rises after being heated by Earth's surface radiation. However, beyond the troposphere, in the thinner air of the stratosphere, radiation becomes the dominant mode of heat exchange, as convection is less effective. In this upper region, the thin air offers _less resistance_ to escaping heat. Intuitively, one might assume that this allows heat to dissipate into space, but Hulburt reasons that this is not the case. At these altitudes, heat exists primarily as infrared radiation, which is readily absorbed by carbon dioxide and water vapour. This absorption prevents heat from escaping entirely, trapping it in the atmosphere. Hulburt also highlighted that while water vapour plays a significant role at lower altitudes, carbon dioxide dominates heat absorption in the upper layers. In the words of Hulburt, *"the radiative region controls to a considerable extent the temperatures in the lower lying convective region"*. An increase in carbon dioxide in the upper atmosphere reduces the amount of infrared radiation escaping to space, causing these higher layers to warm. If the upper layers become as warm as the lower layers, the temperature gradient necessary for convection is disrupted. Without this imbalance, hot air from the lower atmosphere ceases to rise effectively, leading to further warming near the surface. This feedback mechanism, Hulburt argued, would ultimately raise global average surface temperatures. Hulburt’s work, while not addressing human activity, provided a critical foundation for understanding the relationship between atmospheric carbon dioxide and global temperatures. His calculations demonstrated the sensitivity of Earth's climate to changes in CO₂ concentrations, illustrating how natural fluctuations could drive significant shifts in surface temperatures. It was a British engineer who took the next logical step at connecting the dots to human activity... --- ### 1938 - The Callendar Effect Guy Stewart Callendar was an English engineer who was a war researcher with contributions to the second world war. Post the war, in his [own free time he would conduct research into climate change](https://theconversation.com/a-mild-mannered-biker-triggered-a-huge-debate-over-humans-role-in-climate-change-in-the-early-20th-century-170954). In 1938, he published his paper, [*The artificial production of carbon dioxide and its influence on temperature*](https://rmets.onlinelibrary.wiley.com/doi/10.1002/qj.49706427503) which has gone down in history as the first time when human activity's impact on global temperature was explicitly analysed. Callendar analyzed global temperature records and CO₂ measurements, linking the observed warming over the previous 50 years to the industrial-era rise in atmospheric carbon dioxide. For the first time, climate data was systematically analyzed to suggest a direct connection between human activity and global temperature increases. This groundbreaking insight—now called the **Callendar Effect**—illustrated how greenhouse gases like CO₂ trap heat, preventing it from escaping into space. ![[../assets/callendar_effect.png]] **Figure 9**. Screenshot from Callendar's paper ([Source](https://rmets.onlinelibrary.wiley.com/doi/10.1002/qj.49706427503)) What makes Callendar’s work even more remarkable is that his meticulous calculations and graphs were produced without the aid of modern computing tools. Astonishingly, subsequent analysis has confirmed that his predictions—made entirely by hand—closely align with results from today’s sophisticated climate models. As per this analysis performed 75 years later, [Callendar's estimates were very, very close to what has now been estimated with more sophisticated models](https://www.met.reading.ac.uk/~ed/POSTER_callendar_A4.pdf). However, in the conclusion to his paper, he takes the same positive stance towards fossil fuels as Arrhenius did about 40 years ago. This perspective, however, likely [reflected his inability to foresee the dramatic acceleration of fossil fuel consumption](https://theconversation.com/a-mild-mannered-biker-triggered-a-huge-debate-over-humans-role-in-climate-change-in-the-early-20th-century-170954) in the following decades. This does make sense as Callendar’s then projection of a 0.39ºC temperature rise by the 21st century has been far exceeded, as current estimates show a rise of over 1.2ºC. ![[../assets/callendar_conclusion.png]] **Figure 10**. Annotated screenshot of Callendar's paper --- This moment represents a pivotal turning point—the first instance where concrete, data-driven evidence linked human activity to global temperature changes. Since Callendar’s groundbreaking work, climate science has advanced dramatically, uncovering deeper insights and sparking greater controversies. These developments have shaped not just scientific understanding but also global policy and public discourse. However, exploring those advancements and the debates they’ve ignited is a story for another time—perhaps the subject of a future article. --- ### Conclusion The story of early climate science is one of curiosity, persistence, and a gradual evolution from qualitative reasoning to quantitative evidence. Starting with Fourier's visionary ideas about Earth's atmosphere acting as a "heat trap," we see how Eunice Foote's groundbreaking experiments laid the groundwork for understanding the heating effects of carbon dioxide and water vapour. Despite her contributions being overlooked for decades, Foote's insights remain pivotal in the historical narrative. The work of Tyndall brought rigour and sophistication to the experimental methods, demonstrating the selective absorption of infrared radiation by atmospheric gases. Arrhenius's bold mathematical estimates connected carbon dioxide levels to surface temperature changes. Hulburt's exploration of atmospheric layers introduced a nuanced understanding of how heat moves through convection and radiation, while Ångström's skepticism pushed the field to refine its models and data. Finally, Callendar’s meticulous data analysis marked a turning point. By linking industrial emissions to rising global temperatures with real-world evidence, he gave the scientific community—and humanity—its first strong evidence of anthropogenic global warming. His work bridged the gap between theoretical exploration and concrete observation, setting the stage for the modern understanding of climate science. This piece offers a glimpse into the intellectual journey of early climate pioneers—how their curiosity about the world translated into hypotheses, experiments, and sometimes debates. Each step built upon the previous, with contributions adding depth and refinement to our understanding. These scientists may not have fully grasped the magnitude of what they were discovering, nor did they anticipate the scale of human activity's impact on Earth's climate. Yet their efforts laid the foundation for what we now know about global warming. What stands out to me is how many of these early researchers—Fourier, Tyndall, Arrhenius, and even Callendar—framed their discoveries as neutral or even beneficial in some contexts. This highlights an important lesson: the trajectory of science depends not only on the quality of discovery but also on the lens through which society views it. #### Note from the Author Much of the article is written from a neutral standpoint, acknowledging that climate science has evolved significantly over time. I recognize that some findings discussed here may have been challenged or even disproved by later research. However, I believe it is important to honor the efforts of the pioneers who laid the groundwork for our understanding of climate science. Their work, regardless of its present-day accuracy, represents critical milestones in humanity's ongoing effort to make sense of the world. I approach this subject with humility, aware that there are countless perspectives and that no single viewpoint is definitive. To me, the goal isn’t to find the "correct" perspective, but rather to discover perspectives that are useful in understanding and addressing the complex challenges we face. I remain open to learning more, being corrected (even if brutally so), and evolving my understanding as I continue this journey.