Science of the Anthropocene - ACS Publications - American Chemical

Chapter 4

Science of the Anthropocene

Downloaded by UNIV OF FLORIDA on October 31, 2017 | http://pubs.acs.org Publication Date (Web): October 23, 2017 | doi: 10.1021/bk-2017-1247.ch004

Nicole M. DeLuca Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, United States *E-mail: [email protected].

The age of the Anthropocene is upon us, with humans affecting the earth’s climate and environment more than ever before. As a student at the United Nations Framework Convention on Climate Change (UNFCCC) in Doha, Qatar and Warsaw, Poland, I experienced firsthand how important it is to educate my peers and the public on the science of climate change. This chapter aims to provide a basic understanding of some of the key concepts of climate science, including how greenhouse gases warm the earth’s surface, how natural processes affect climate, and how the observed changes in temperature today differ from times of change in the past. Evidence that humans are responsible for today’s changes in climate includes a shift in atmospheric carbon isotopes and results from climate models that simulate various climate scenarios. Our modern human ancestors did not experience the climate conditions that the earth is currently careening towards. In order to preserve the habitability of the earth for our species’ survival, immediate actions must be taken to reduce the damaging effects of human activities. Educating students and the public about climate science is a vital step in calling for policies that can make these necessary actions a reality.

© 2017 American Chemical Society Peterman et al.; Climate Change Literacy and Education The Science and Perspectives from the Global Stage Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Many countries throughout the world are actively engaged in reducing their contribution to climate change. However, my journey to the United Nations Framework Convention on Climate Change (UNFCCC) as a student representative of the American Chemical Society, and subsequently as a mentor to new student representatives, brought me to two distant countries where I would never have expected to go to for such a purpose. I attended the UNFCCC 18th and 19th Conference of Parties (COP) in Doha, Qatar and Warsaw, Poland, respectively. Both of these host countries were under scrutiny for their relationships to fossil fuels, which are a major source of greenhouse gas emissions. Qatar is an oil-rich country with the largest per capita CO2 emissions of any country in the world (1). Meanwhile, Poland depends on coal for power production, with about 85% of its electricity being generated at coal-powered plants as of 2013 (2). Along with many of the other conference delegates and attendees, I was skeptical as to why these countries would host a conference that aims to mitigate greenhouse gas emissions in order to keep warming of the earth below 2°C. The attitude from my fellow youth attendees was particularly agitated when they learned that Warsaw would be hosting the World Coal Association’s International Coal and Climate Summit, which aims to continue using fossil fuels as an energy source, during the same week as the COP. In these host cities, I did not get the impression that the local people were very concerned about climate change. As I spoke to some of these local people, many times on public transportation to and from the conference, I realized that they viewed the international conference and our presence in their city as a nuisance instead of as an opportunity to improve their country’s relationship to the environment. In hindsight, I think this bizarre and unexpected feeling of disdain from the local people that I experienced may have been a learned attitude from their leaders’ resistance to make changes needed to fight climate change. As a student at the COP, I learned what an important role the world’s youth has in combating climate change. We do not yet have the legislative authority that is needed to make global changes in the way that people interact with the environment, but we do have the responsibility and power to educate the public, our elders, and most importantly our peers. By leading the effort to educate others, we can make a meaningful difference in countries like Qatar and Poland and change the ways that people view their impacts on the earth. Throughout this chapter, I hope to give you a basic understanding of several key aspects of the scientific background of a time now being referred to as the “Anthropocene” and how we know that humans are responsible. In August 2016, the world experienced its 16th consecutive hottest month on record (3). The need for climate change awareness, education, and action is dire!

What Is the Anthropocene? A working group of scientists at the International Geological Congress recently voted that the world has entered a new geologic time period, departing from the Holocene epoch that began almost 12,000 years ago at the end of the 50 Peterman et al.; Climate Change Literacy and Education The Science and Perspectives from the Global Stage Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


last ice age (4). This new geologic unit was named the “Anthropocene” in the early 2000s by Nobel Prize-winning chemist, Paul Crutzen (5). Crutzen proposed that the Anthropocene epoch is distinct from the Holocene epoch because of the escalating human influence on the global environment and repercussions on the climate system (5). Numerous geologic units throughout Earth’s 4.6 billion-year existence have been recognized by the scientific community, and each unit describes a period of time when rocks and sediments that were forming reflect a unique phase in the Earth’s history. Some geologic units are divided by changes in atmospheric composition or types of organisms living on the Earth, while other geologic units are separated by catastrophic events such as asteroid impacts, volcanic eruptions, and mass extinctions. Has humankind become the next “asteroid” that divides geologic time? Although a preliminary decision has been made that the Anthropocene indeed is a new geologic epoch to describe modern anthropogenic changes to the climate and environment, it will not be an official geologic unit until the Executive Committee of the International Union of Geological Sciences ratifies its official inclusion in the Geologic Time Scale. In order to do this, a distinct point in time in the rock record that marks the beginning of the Anthropocene epoch and the end of the Holocene epoch must be identified and agreed upon (4). One potential marker for the beginning of the Anthropocene goes back to thousands of years ago when ancient human civilizations began to engage in agriculture, which changed the physical and chemical composition of the landscape. Another potential beginning of the new epoch could be the Industrial Revolution in the 1800s, when carbon dioxide began to be pumped into the atmosphere at an unprecedented rate. The prevalent proposal from the working group of scientists that recently voted on the existence of the Anthropocene is that the marker should be the start of the Atomic Age in the 1950s, when large quantities of radioactive elements were introduced into the environment from nuclear bomb testing and human impacts on the climate accelerated (4). Even if a marker is never agreed upon and the Anthropocene does not become an official unit of the Geologic Time Scale, the concept of this new age has ignited awareness in the media and the public about the extent of what humans are doing to their only home in the solar system. How could our day-to-day actions as a single species affect the earth’s climate in such a substantial way that could bring about the dawning of a new age? The answer lies within the science behind greenhouse gases and their effects on the earth’s climate system.

The Role of Greenhouse Gases Earth’s atmospheric gases are largely transparent to the sun’s incoming light at visible wavelengths. This allows solar energy to easily pass through the atmosphere and be absorbed by the earth’s surface, which then reemits some of this energy at a longer wavelength called thermal infrared. As the emitted thermal infrared energy reaches the atmosphere, much of it is able to pass through it back 51 Peterman et al.; Climate Change Literacy and Education The Science and Perspectives from the Global Stage Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


into space. The range of wavelengths where the earth’s emitted energy is able to pass through the atmosphere is called the atmospheric window. Greenhouse gases in the atmosphere absorb outgoing thermal infrared energy and reemit it back to the earth’s surface, thereby “closing” some of the atmospheric window that would otherwise allow that energy to pass through. This is known as the “greenhouse effect,” and it is a natural phenomenon that keeps global temperatures relatively stable so that our planet is habitable. However, the enhancement of this phenomenon that is produced by an increase in the amount of greenhouse gases in the atmosphere closes more of the atmospheric window, which causes the planet to warm (6). Carbon Dioxide The amount of carbon dioxide (CO2) in the Earth’s atmosphere is the key factor in the climate change discussion concerning greenhouse gases. The International Panel on Climate Change’s (IPCC’s) Fifth Assessment Report (AR5), released in 2013, identified CO2 as the primary contributor to the radiative forcing from 1750-2011 (7). Radiative forcing is a term you may frequently hear being used by climate scientists. It essentially describes the influence of a forcing, such as a greenhouse gas or solar energy, on the Earth’s energy budget. This energy budget can be calculated, and an imbalance of the energy budget affects whether the planet warms or cools. Since the beginning of the Industrial Revolution in the early 1800s, the energy budget has tipped towards warming because of large amounts of CO2 being released into the atmosphere. Finding ways to reduce the amount of CO2 that humans emit in the industrial age is our biggest hurdle in combating the warming climate. Anthropogenic CO2 emissions come from two major sources: the burning of fossil fuels and land-use changes. The largest source of anthropogenic CO2 emissions is the burning of fossil fuels, which became largely practiced around the world following the Industrial Revolution. Types of fossils fuels include coal, oil, and natural gas. All of these carbon-based natural resources take millions of years to form after organic material is buried and subject to high temperatures and pressures under the surface of the Earth. When these fossil fuels are extracted and combusted for energy production, transportation, or industrial processes, carbon is released from that chemical reaction in the form of CO2 gas. Land-use changes also contribute to the rise in atmospheric CO2 when forested lands are cleared for development or agriculture. Trees and soils in forests capture and store CO2 from the atmosphere as they mature, which is known as a “carbon sink.” Decreasing the amount of forested land on the Earth diminishes one of the world’s major carbon sinks, which increases the quantity of CO2 being accumulated in the atmosphere. Many nations have realized the detrimental impacts of deforestation and have developed programs to restore forested lands. However, some areas in the tropics, where this sink is particularly important, have seen continued deforestation. Scientists first realized the degree to which the use of fossil fuels and deforestation was changing our atmosphere’s CO2 composition through a record 52 Peterman et al.; Climate Change Literacy and Education The Science and Perspectives from the Global Stage Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


called the Keeling Curve. The Keeling Curve is an empirical, or observed, record of the continuously rising atmospheric CO2 concentrations over the last several decades. It is the longest continuous record of atmospheric CO2 concentrations, measured at the pristine mountaintop of Mauna Loa in Hawaii. Because it is far from the influence of any industrial or other anthropogenic influences, the CO2 record at Mauna Loa remains our most reliable tracker of the state of our atmosphere’s changing composition through the 21st Century. Since Charles Keeling’s first measurement in 1958, the average atmospheric CO2 concentration has increased more than 80 parts per million (ppm, or 0.008 %). Compared to the atmospheric CO2 record acquired from ice cores over the past 800,000 years, the Mauna Loa measurements from the last 60 years show a huge upward spike in CO2 concentration (8–10). It is clear that there is natural variability in atmospheric CO2 over time, however the level to which the CO2 concentration is currently climbing is “off the chart” (Figure 1). Carbon dioxide is the most important anthropogenic greenhouse gas to understand due to its large contribution to modern climate change. However, other greenhouse gases emitted by human activities also play a role. Increases in atmospheric methane and nitrous oxide concentrations also contribute to climate change.

Figure 1. 800,000-year record of atmospheric CO2 concentrations acquired from ice cores before 1958 and the Keeling Curve measurements at Mauna Loa after 1958. The horizontal line at CO2 concentration of 400 ppm indicates the present atmospheric level, which is significantly higher than any concentration seen in the last 800,000 years. Sources: Scripps Institution of Oceanography (8), MacFarling et al. 2006 (9), and Lüthi et al. 2008 (10). Methane Methane (CH4) accounts for the second-largest proportion of global greenhouse gas emissions after CO2 (11). CH4, however, is a more potent greenhouse gas than CO2, which means that it is better able to absorb the outgoing thermal infrared radiation that enhances the greenhouse effect. Given the same 53 Peterman et al.; Climate Change Literacy and Education The Science and Perspectives from the Global Stage Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


amount of CH4 and CO2 in the atmosphere, the CH4 will warm the Earth about 28 times more than the CO2 over a 100-year period (12). Like CO2, the concentration of CH4 in the atmosphere has rapidly climbed since the Industrial Revolution, indicating humans’ role in its emissions (Figure 2) (13). Methane is produced mainly through anaerobic decay processes of organic materials in wetlands. However, CH4 is also produced through agriculture, landfill use, and the extraction and use of natural gas, coal, and oil. Anthropogenic emissions of CH4 currently exceed natural emissions by 50-65%, but warming temperatures could cause natural wetland emissions to increase further (14). The melting of permafrost in northern latitudes due to rising global temperatures is also a potential major emitter of methane. In a type of climate cycle known as a positive “feedback,” permafrost warms and melts, allowing buried plants that were frozen to begin decaying and releasing CH4. The additional CH4 in the atmosphere further enhances the greenhouse effect’s warming of the earth’s surface and melts more permafrost, so the cycle continues. Atmospheric methane concentrations actually seemed to plateau for about a decade from the 1990s to early 2000s, but by 2007 they began to increase again (14). Some studies suggest that methane released from the melting of permafrost in the Arctic could have caused the return in the rise of the methane concentrations in 2007 (15).

Nitrous Oxide Nitrous oxide (N2O) is considered the third most important greenhouse gas emitted by humans. It accounts for a much smaller proportion of global emissions than the gases previously mentioned, but it is about 265 times more efficient at absorbing outgoing radiation than CO2 over a 100-year period (12). The lifetime of atmospheric N2O is also estimated to be longer than CO2 and CH4 (12). This means that any N2O emitted today will remain in the atmosphere longer than the other greenhouse gases mentioned, so the effects of N2O will be felt long after it is actually emitted. The concentrations of this greenhouse gas have also rapidly increased since the beginning of the 19th Century (Figure 2) (13). Natural sources of N2O mainly include microbial processes in soils and the oceans. It is produced in anthropogenic activities such as the agricultural use of fertilizers, industrial processes, and wastewater management. The increased reliance on nitrogen-containing synthetic fertilizers for agriculture to feed an exponentially growing global population was found to be the main driver of the upward spike in atmospheric N2O (16). Greenhouse gases in the atmosphere are currently at levels that the Earth has not experienced for at least 800,000 years (8–10). To give some perspective, the modern human species, homo sapiens, has only existed for about 200,000 years (17). That means that our modern human ancestors have never experienced a world with the greenhouse gas concentrations that we observe today and predict for the future! We have already begun to face challenges in adapting to this new climate and its impacts on the land and oceans.

54 Peterman et al.; Climate Change Literacy and Education The Science and Perspectives from the Global Stage Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 2. Atmospheric concentrations of three major greenhouse gases – CO2, CH4, and N2O – over the past 2000 years. Source: IPCC Fourth Assessment Report. The Physical Science Basis. Ch. 2: Changes in atmospheric constituents and in radiative forcing (13).

Rising Temperatures In the previous section, we looked at the rapid increase in the concentrations of major greenhouse gases over the past 200 years. We know that greenhouse gases absorb thermal radiation emitted from the Earth and remit energy back down to the surface. How has the relatively recent change in their concentrations affected global temperatures? A link between the rise in greenhouse gases in the Industrial era and global temperatures, which had been previously suggested in scientific studies, was reported again by Mann and Jones in the early 2000s (18). The Mann and Jones study was one of many to draw the same conclusion—anthropogenic emissions are dramatically raising global temperatures. The Mann and Jones study included a plot showing global temperature anomaly over the past two millennia, which was dubbed the “Hockey Stick” (18). Temperature anomaly is a commonly used metric to evaluate temperature changes in climate science. It is calculated as the difference between the average temperature at a fixed interval of time, commonly a 30-year instrumental reference period from 1961-1990, and the temperature at any given time. Therefore, during 1961-1991 on these plots the temperature anomaly would be zero. Any temperatures on the plot below this average temperature would have a negative value, and any temperatures greater than it would have a positive value. 55 Peterman et al.; Climate Change Literacy and Education The Science and Perspectives from the Global Stage Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Mann and Jones developed a record of global mean surface temperature anomalies over the last 2,000 years by combining proxies of temperature in the past, such as ice cores, tree rings, and sediment cores, with instrumental temperature data closer to present-day (18). To many, the resulting graph of global temperature anomaly seemed to look like a hockey stick because there was relatively little change in temperature anomaly up until about 1900, when it suddenly made a large spike upwards. Remember: This is the same timing as the spike in CO2 we saw in Figure 2, the greenhouse gases record. Did this graph show that anthropogenic greenhouse emissions and the observed rising global temperatures were linked? Scientists certainly thought so, but skeptics began to criticize the statistical methods used in the study and used this as a ploy to say that anthropogenic climate change is not real. They also argued that the current temperature rise could be due to a natural fluctuation that has caused temperatures on Earth to rise many times before. However, the key to the scientists’ argument that climate change is being caused by humans was not based on how much the temperature had risen. Instead, they debated that one of the major differences between the past and today was how fast the temperature had risen. In 2013, a new study in the journal Science by Marcott et al. reported another record of global mean surface temperature anomaly, this time spanning the last 11,300 years (19). This period spans the most recent geologic epoch (the Holocene), including the more recent Anthropocene time period. Like the Mann and Jones study in 2003, this new team combined paleoclimate proxies of global mean surface temperature with more recent instrumental data to develop their record of temperature anomaly based on a reference period from 4500-5500 years before the present. The Marcott et al. study determined that temperatures in the Holocene reached a maximum at about 7,000 years ago and began to steadily decline afterward (19). This decline was due to orbital variations that change the Earth’s position in relation to the sun, which we will talk more about in the next section. They found that the decade spanning 2000-2009 was warmer than about 72% of the past 11,000 years (19). The study also noted that global temperatures today have not yet exceeded the Holocene’s maximum temperature, but they are predicted to do so by the end of the 21st Century (19, 20). The most striking feature of this new global temperature anomaly record was the same sudden spike upwards around the start of the 20th Century that was seen in the scrutinized Mann and Jones study, correlating with the rise in greenhouse gas concentrations. In this longer record, it could clearly be seen that the global mean surface temperature has risen and fallen naturally throughout the Holocene. However, these natural variations have taken thousands of years to influence the temperature. Meanwhile, the warming over merely the past 100 years was shown to be occurring at a pace that is unprecedented since the last ice age. The recent warming actually seems to be reversing the long-term cooling trend that has been occurring for the last several thousand years (21). Based on many model projections, the IPCC reports that temperatures will continue to rise at this rapid rate unless drastic human emission reduction goals are met (22). 56 Peterman et al.; Climate Change Literacy and Education The Science and Perspectives from the Global Stage Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

Natural Drivers of Climate Change Changes in the Earth’s climate throughout most of the Holocene and previously in Earth’s history are attributed to natural processes. Two major natural drivers of the earth’s climate are changes in solar energy and volcanic eruptions. While these natural variations do affect climate and global temperatures, neither of them can explain all of the rapid warming or changes in climate we have experienced in the past century.


Solar Energy The sun is the earth’s primary source of energy, and it does not give off a constant amount of energy every year. The number of sunspots, or dark regions visible on the sun, is correlated with the amount of radiation that the sun is emitting into space—some of which will ultimately reach the earth. In years with higher numbers of sunspots, the sun is typically more active and emitting more energy. These years are known as Solar Maxima years. In contrast, Solar Minima years occur when there are relatively few sunspots and the sun is emitting less energy. The sunspots cycle between Solar Maxima and Solar Minima was discovered by Samuel Heinrich Schwabe in 1843, and occurs roughly every 11 years (23). These solar energy cycles can also undergo trends over longer timescales when more or fewer sunspots are counted within each Solar Maxima or Solar Minima (Figure 3) (24). This can affect the average amount of energy the earth is receiving over several decades or centuries. Like greenhouse gases, the amount of energy received also influences the earth’s energy balance and therefore the temperature of the surface. As seen in Figure 3, a colder time period in the 17th Century known as the “Little Ice Age” is likely due to an abnormal lack of sunspots known as the Maunder Minimum. Could changes in solar energy explain today’s warming? Sunspot numbers and solar energy output increased throughout the Industrial Revolution until 1958, when they reached a maximum (Figure 3). Since this maximum, however, sunspot numbers have declined with each successive sunspot cycle. If warming in the 20th Century were solely due to the amount of solar energy being received by the sun, global temperature should have begun to decrease after the 1958 maximum. Instead, temperatures have continued to climb. During longer periods of high solar energy, the global surface temperature only increases by about 0.1°C (25). Between 1980 and 2000, however, the temperature increased at a rate of 0.16°C per decade (20). Changes in solar energy output only account for approximately 10% of the temperature increase observed in the past century, so this natural driver does not explain the rise in temperatures we have observed (25). Another way that changes in incoming solar energy can affect the earth’s climate is through three types of orbital variations, called Milankovitch Cycles (26). The earth’s orbit around the sun changes shape (known as eccentricity) in cycles of approximately 100,000 years. The orbit’s shape varies from being roughly circular to being more elongated. When the orbit is elongated, the Earth is farther away from the sun at more times during the year, which generally causes cooling. 57 Peterman et al.; Climate Change Literacy and Education The Science and Perspectives from the Global Stage Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 3. Yearly averaged number of sunspots from the 17th Century to the present, showing a time of decreased solar activity called the Maunder Minimum and a sunspot maximum in 1958. A downward trend in sunspot numbers has been observed since this maximum. Source: NASA/ Marshall Space Flight Center (24). In approximately 41,000 year cycles, the degree to which the earth tilts on its axis changes. The degree of axial tilt today is 23.5°, and this degree causes the seasons we experience throughout the Northern and Southern Hemispheres. The seasons occur because the most intense solar radiation hits the earth’s surface at different latitudes throughout the year. Changes in the degree of axial tilt during obliquity cause more dramatic seasonal differences when there is a larger degree of tilt or smaller differences between seasons when there is a lesser degree of tilt. In approximately 26,000 year cycles, the earth experiences a “wobble” on its axis, known as a precession, that is much like a spinning top’s wobble as it slows down. During a precession, the Northern and Southern Hemispheres are pointed toward the sun at different times of the year than they do today. This causes both hemispheres’ seasons to occur at different times of the year. Ice ages typically occur when the Northern Hemisphere is pointed away from the sun during the time in the Earth’s orbit when it is farther from the sun. The Milankovitch cycles ultimately interact with each other to determine times of glaciation and deglaciation. The two most recent periods between ice ages, called interglacials, lasted about 10,000 years, which is about the length of the current, warm Holocene epoch (27). Additionally, the Northern Hemisphere is currently in a phase of reduced solar energy due to a decreased axial tilt. Both of these factors suggest that the Earth is due for another glacial period (27). However, the Holocene temperature anomaly record in the Marcott et al. study shows that the slow cooling trend that started about 7,000 years ago was disrupted by the sharp rise in global temperatures in the 20th Century (19). Another recent study suggested that the increase of CO2 from human activities since the Industrial Revolution could postpone the next ice age for over 50,000 years, effectively skipping over an entire glacial cycle (28). Volcanic Events In 1816, the New England region of the United States experienced an extraordinary snowfall event. This event was not unusual because of a record-breaking snowfall amount, but because the snow fell in the middle of 58 Peterman et al.; Climate Change Literacy and Education The Science and Perspectives from the Global Stage Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


June. Along with snow in New England, the summer of 1816 brought unusually cold and stormy weather to many parts of the Northern Hemisphere (29). This phenomenon caused significant economic losses and famine from crop damage (30). The year of 1816 came to be known as “the year without a summer,” and global temperatures were about 2°C below average (31). Scientists now know that an explosive volcanic eruption in Indonesia in April of 1815, more than a year earlier, caused this strange climate anomaly in the summer of 1816. Volcanic eruptions spew rock particles and gases—such as sulfur dioxide (SO2) and CO2—high into the atmosphere. If SO2 reaches the stratosphere, it can react with water vapor and spread out to form a “blanket” of sulfate aerosols that scatters sunlight away from the earth’s surface (21). This causes a temporary net cooling effect of global surface temperatures and could alter precipitation patterns. These aerosols can remain in the stratosphere for months or years because they are above the level from which rain falls that would wash them out of the lower atmosphere more quickly (30). Volcanic events have also interacted briefly with the rapid warming occurring in the 20th Century (Figure 4). Most recently, the eruption of Mt. Pinatubo in 1991 led to the third-coldest and wettest summer in the United States in the last 77 years (30). The timing and severity of explosive volcanic events is unpredictable, and the largest effects on climate occur within the two years following an eruption (21). The effect that future eruptions will have on radiative forcing and global surface temperatures depend on the amount of SO2 ejected into the stratosphere. These cooling effects are only measured in months or years, while anthropogenic emissions will warm the earth for centuries to come if they are not curbed. Volcanic eruptions affect the earth’s climate and surface temperatures, but not enough to counteract the warming trend observed over the last century.

How Do We Know Humans Are Responsible? Climate scientists overwhelmingly agree that climate change is happening and that humans are the cause (32). Much like we trust doctors, experts in medicine, to diagnose and treat human illnesses, the public should trust climate scientists, experts in the earth’s systems, to diagnose the earth’s “illness.” The correlation between rapidly rising greenhouse gas emissions after the Industrial Revolution and the parallel rise in global surface temperatures was only the first clue that human activities are causing this dramatic change. As scientists build more evidence about how the climate is changing, the conclusion that humans are responsible are becomes more and more clear. Carbon Isotopes One way to determine that the increased levels of atmospheric CO2 are coming from human activities is to look at a particular tracer within the atoms of the CO2 molecule itself. This tracer is known as an isotope. Isotopes of an element are atoms with the same number of protons, which identify the element, and different 59 Peterman et al.; Climate Change Literacy and Education The Science and Perspectives from the Global Stage Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


numbers of neutrons within the nucleus of the atom. This gives atoms of the same element different masses, which is denoted as a superscript number and then the element’s symbol (e.g. 12C means that the carbon atom has 6 protons and 6 neutrons, whereas 13C means that it has 6 protons and 7 neutrons). Many elements, like carbon, naturally have small proportions of stable isotopes. Various physical and biological processes distribute certain isotopes of an element to different places on the earth. For example, during photosynthesis plants prefer to take the lighter isotopes of carbon (12C) out of the atmosphere instead of the heavier isotope (13C). This results in plant materials having lower ratios of 13C to 12C than the atmosphere. Scientists have noticed that since the Industrial Revolution, carbon isotopes in atmospheric CO2 have become surprisingly lighter and lighter (33). They determined that prior to the 1800s, the carbon isotope ratio of 13C to 12C in the atmosphere had stayed relatively stable due to natural processes, even during times of gaseous CO2 emissions from volcanic eruptions (33). What could be causing this dramatic decrease in the modern atmosphere’s carbon isotope signature? Scientists realized that fossil fuels are geologically transformed ancient plant materials with low ratios of 13C to 12C. As fossil fuels are extracted and combusted, their carbon is reacted into CO2 with their specific carbon isotope signature. The declining isotope ratio of atmospheric CO2 actually reflects the burning of fossil fuels! This discovery became another piece of evidence that the observed increasing levels of CO2 in the atmosphere are due to human activities and not natural processes. Climate Models Scientists have been developing models to simulate the climate system since the late 1800s, even before they could do so computationally. These models are developed using known physical, biological, and chemical principles and help scientists understand what has driven the climate system in the past and what drives it today. Before projecting a climate model to the future to make predictions, the models are carefully evaluated for their ability to replicate past known conditions. Scientists use climate models to simulate different climate conditions. These models provide more evidence that humans are the cause of modern climate change. Figure 4 shows the comparison between two types of climate models—one accounting for only natural forcings of climate change, like solar variations and volcanic aerosols (b), and one accounting for both natural and anthropogenic forcings (34). The models simulating only natural forcings clearly diverge from the observed temperature anomaly curve (black line). Instead, they show that the global average surface temperatures should actually be slightly decreasing after about 1960 due to natural processes (34). Meanwhile, the models that include both natural and anthropogenic forcings (a), like greenhouse gas emissions, successfully capture the continued warming trend after 1960 (34). This climate model experiment provides yet another piece of evidence that human activities are causing the continuously increasing global surface temperatures. 60 Peterman et al.; Climate Change Literacy and Education The Science and Perspectives from the Global Stage Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 4. Two types of climate model simulations for temperature anomaly: a) having both natural and anthropogenic drivers of climate change and b) having only natural drivers. Thin lines represent multiple model runs, with the mean indicated as a bold line in the respective color. The black bold line shows the observed temperature record, which closely correlates with the model that simulates both natural and anthropogenic drivers of climate change. Source: IPCC Fourth Assessment Report. The Physical Science Basis. Understanding and Attributing Climate Change (34). During the earth’s 4.6 billion-year history, it has experienced many different climates and atmospheric CO2 levels. The global climate has ranged from major ice ages, with ice covering the entire earth, to periods with no polar ice at all. During the Mesozoic era when dinosaurs inhabited the land and seas, atmospheric CO2 levels are thought to have been much higher than the present with much 61 Peterman et al.; Climate Change Literacy and Education The Science and Perspectives from the Global Stage Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

warmer global temperatures. However, the rate at which the changes in climate are occurring today is unprecedented since before the evolution of modern humans. There is no doubt that the earth as a planet will survive our impacts on its climate and environment; it has rebounded after major catastrophes before, albeit over millions of years. However, the modern human species must now begin to adapt to conditions it has never experienced before. The consequences of changing our climate and environment so drastically is not that it will destroy our planet, but that it will destroy the habitability of the planet for our fragile human species.


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