Historical warnings
Th e fi rst warning that excess carbon dioxide in our atmosphere might cause global warming came in 1859 when John Tyndall discovered that carbon dioxide blocks infrared radiation. He noted that carbon dioxide in the Earth’s atmosphere could therefore block infrared radiation from escaping and thus cause warming of the planet. 8
5 Colin Macilwain , “ A Touch of the Random ,” Science 344 ( 2014 ): 1221–3 .
6 Paul Voosen , “ Th e Weather Master ,” Science 356 ( 2017 ): 128–31 .
7 Anthony Del Genio, “Will a Warmer World Be Stormier?” Earthzine (2011). Online: http://
earthzine.org/2011/04/16/will-a-warmer-world-be-stormier/ (accessed January 6, 2016).
8 Steve Graham, “John Tyndall (1820–1893),” NASA Earth Observatory (1999). Online: http://
earthobservatory.nasa.gov/Features/Tyndall/ (accessed January 7, 2016); American Institute of Physics, “Th e Carbon Dioxide Greenhouse Eff ect,” Th e Discovery of Global Warming (2015). Online:
https://www.aip.org/history/climate/co2.htm (accessed January 7, 2016); John Tyndall , “ On the Absorption and Radiation of Heat by Gases and Vapours ,” Philosophical Magazine ser. 4, 22 ( 1861 ):
169–94, 273–85 ; John Tyndall , “ On Radiation through the Earth’s Atmosphere ,” Philosophical Magazine ser. 4, 25 ( 1863 ): 200–6 .
In 1896, Svante Arrhenius made the first calculations of just how much warmer the Earth might get. He predicted that if the amount of carbon dioxide in our atmosphere was doubled, then the average temperature of the Earth would rise by 5 or 6 degrees Celsius. 9 At the time, this was not seen as a concern because he estimated that, at the then current rates of burning fossil fuels, it would take some 3,000 years to double the carbon dioxide in the atmosphere. Little did he know how quickly the next generation of humans would dig up and burn those fossil fuels.
Fossil fuels
Fossil fuels are not really fossils in the sense of being the preserved skeletal remnants of prehistoric plants or animals. Th ey are, however, the remnants of prehistoric life preserved in a much diff erent way.
Th e earliest life on Earth was blue-green algae and cyanobacteria. It came into being on an early Earth that had an atmosphere containing a great deal of carbon dioxide but no oxygen. Th at early life, and the higher-order plants that followed, removed most of the carbon dioxide and replaced it with oxygen. Th is eventually led to the present-day atmosphere that is about 20 percent oxygen and much less than 1 percent carbon dioxide. Th is very small amount of carbon dioxide is quite suffi cient to provide a substantial amount of natural greenhouse warming. Without it, the Earth would be a much colder place than it is now.
As early plant life fl ourished and died, its remnants were laid down in layers of carbon that became what we now call fossil fuels. Land-based plants were buried underground where they became coal, and ocean-dwelling plankton were buried beneath the sea fl oor where they became oil and natural gas. 10 In these processes, the carbon they took out of the atmosphere while alive was stored away underground when they died, where it remained until the industrial age. We are now digging up that carbon and, by virtue of burning it, returning it to the atmosphere. In the process of burning, we are reattaching oxygen to the carbon in the fossil fuels and thus re-creating the carbon dioxide, which we then release into the atmosphere.
Human activities
Carbon dioxide is by far the most prevalent greenhouse gas that we put into our atmosphere. Th e most prodigious source is the coal that we burn for electricity,
9 Svante Arrhenius , “ On the Infl uence of Carbonic Acid in the Air upon the Temperature of the
Ground ,” Philosophical Magazine 41 ( 1896 ): 237–76 ; Svante Arrhenius , “ On the Infl uence of Carbonic Acid in the Air upon Temperature of the Earth ,” Astronomical Society of the Pacifi c 9 . 54 ( 1897 ): 14 .
10 Smithsonian Institution, “My, How You’ve Changed,” Change Is in the Air (2006). Online: http://
forces.si.edu/atmosphere/02_02_00.html (accessed January 5, 2016); Octave Levenspiel , Th omas Fitzgerald , and Donald Pettit , “ Earth’s Atmosphere before the Age of Dinosaurs ,” Chemical Innovation 30 . 12 ( 2000 ): 50–5 . Online : http://pubs.acs.org/subscribe/archive/ci/30/i12/html/12learn.html (accessed January 5, 2016) ; California Energy Commission, “Fossil Fuels—Coal, Oil, Natural Gas,” Energy Quest (2012). Online: http://www.energyquest.ca.gov/story/chapter08.html (accessed January 5, 2016).
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heating, and industrial processes. 11 To produce a given amount of useful energy, coal releases roughly twice as much carbon dioxide than does natural gas; however, leakage of natural gas directly into the atmosphere can make natural gas as potent a source of greenhouse gases as coal. 12 Oil is slightly less of a greenhouse gas source, releasing roughly 25 percent less carbon dioxide to produce a given amount of energy than does coal. 13 Th e next largest sources of carbon dioxide are land use activities, most notably industrial agriculture and the burning of forests. 14
Th e amount of carbon dioxide in our atmosphere is substantially higher now than it has been for at least the past 800,000 years. Th is is revealed by records found in ice cores drilled from deep in Antarctic glaciers. As layers of ice are laid down year by year, water and bubbles of air are frozen into each layer, creating a record that can be read. Th e bubbles of air allow direct measurement of the historical atmospheric composition, and the ratios of isotopes in the water can be used to infer the past temperatures. 15 Such analyses have now been successfully conducted for the past 800,000 years of Earth history, and they show that the amount of carbon dioxide in the atmosphere and the Earth’s average temperature have gone up and down in lockstep over the last nine ice ages. 16
Examining this record in detail shows how our atmosphere has historically responded to small changes in temperature with large feedback loops that accelerate the temperature change. Th e temperature changes leading to ice ages are initiated by the interplay of periodic variations in the Earth’s orbit around the Sun with the tilt of the Earth’s rotation axis with respect to its orbit. Th is interplay leads to periodic small changes in the solar heating upon the Earth in cycles of roughly 100,000 years, known as Milankovitch cycles.
A multiplicity of factors then leads to feedback loops that drive the climate system into an ice age or into a warm interglacial period. Among the most prominent of these factors are the absorption of carbon dioxide by the ocean 17 and the refl ection of sunlight by glaciers. 18
11 US EPA, “Overview of Greenhouse Gases”; National Research Council , Advancing the Science of
Climate Change ( Washington, DC : Th e National Academies Press , 2010 ) ; US Department of State ,
Fourth Climate Action Report to the UN Framework Convention on Climate Change: Projected Greenhouse Gas Emissions ( Washington, DC : US Department of State , 2007 ) ; US EPA, Global Greenhouse Gas Emissions Data (2016). Online: http://www3.epa.gov/climatechange/ghgemissions /global.html (accessed January 5, 2016); IPCC, “Summary for Policymakers.”
12 See Union of Concerned Scientists, “Environmental Impacts of Natural Gas.” Online: http://www
.ucsusa.org/clean_energy/our-energy-choices/coal-and-other-fossil-fuels/environmental-impacts -of-natural-gas.html (accessed October 5, 2016).
13 US Energy Information Administration, “How Much Carbon Dioxide Is Produced When Diff erent
Fuels Are Burned?” (2016). Online: https://www.eia.gov/tools/faqs/faq.cfm?id=73&t=11 (accessed January 14, 2016).
14 US EPA, Global Greenhouse Gas .
17 N.J. Shackleton , “ Th e 100,000-year Ice-age Cycle Identifi ed and Found to Lag Temperature, Carbon
Dioxide, and Orbital Eccentricity ,” Science 289 ( 2000 ): 1897–902 .
15 Amy Dusto, “Climate at the Core,” NOAA (2014). Online: https://www.climate.gov/news-features
/climate-tech/climate-core-how-scientists-study-ice-cores-reveal-earth%E2U80%99s-climate (accessed January 14, 2016); Holli Riebeek, “Paleoclimatology: Th e Ice Core Record,” NASA Earth Observatory (2005). Online: http://earthobservatory.nasa.gov/Features/Paleoclimatology
An ice age is triggered when the orbital cycles lead to times of decreased solar heating and therefore lower temperatures. Th e oceans respond by absorbing more than their usual amount of carbon dioxide from the atmosphere, thus reducing the amount of greenhouse warming and consequently driving temperatures down even further. Ice sheets begin to grow. Since ice refl ects back into space more sunlight than does bare ground or water, the Earth absorbs even less sunlight and thus cools even faster. Th e result is an ice age.
Conversely, when the orbital eff ects lead to periods of increased solar heating, warmer temperatures decrease the amount of carbon dioxide absorbed by the ocean.
Th is places more carbon dioxide into the atmosphere, enhancing the greenhouse eff ect and raising the temperature. Ice melts, exposing bare ground and water that absorbs more sunlight than did the ice, thus accelerating the warming. Th e result is a warm interglacial period such as the one in which we are now living.
Th ere are two important points to note in Figure P.1 , which shows the rise and fall of temperatures and carbon dioxide concentration over the past nine ice ages, spanning a period of 800,000 years. First, historically, changes in temperature have always preceded changes in carbon dioxide concentration for the reasons described earlier. Second, before the industrial age, the carbon dioxide concentration never exceeded 300 parts per million.
Modern direct measurements of carbon dioxide concentrations in the atmosphere, which began in the 1950s, are displayed in the upper right of Figure P.1 . Th ey show carbon dioxide concentrations rising rapidly toward 400 parts per million (as of
Thousands of years before 1950 respectively, derived from ice core measurements covering the past 800,000 years. Th e arrows mark 100,000-year cycles of triggers due to orbital variations. Superimposed at the upper right are modern-day measurements of CO 2 (the Keeling curve) showing CO 2 approaching 400 ppm as of 2010. From Daniel Harris, “Charles David Keeling and the Story of Atmospheric CO 2 Measurements,” Anal. Chem. 82.19 (2010): 7865–70.
Prologue 13
2010). In 2015, carbon dioxide concentrations passed the threshold of 400 parts per million. 19 If we continue to add carbon dioxide to the atmosphere at the present rates, the concentration could exceed 600 parts per million by mid-century. 20 Th is would be the doubling of carbon dioxide that Arrhenius thought would not be reached for at least another 3,000 years.
So we, the people of planet Earth, are conducting a massive experiment with our atmosphere. We are forcing the amount of carbon dioxide to levels far beyond that found in past natural cycles, and we are doing so in such a way that the carbon dioxide increases are leading temperature changes rather than following them. Th e logical conclusion is that rising temperatures are sure to follow and that those temperatures will exceed any experienced over the past 800,000 years.