February 16, 2015. At its most basic, the theory that human activity can affect the climate has two parts: 1. Changes in Earth’s atmosphere can affect surface temperature; and 2. Human activity can alter the makeup of Earth’s atmosphere. This post provides an overview of the science behind both principles, relying on scientific reports and a number of sources that collect and report data on the link between atmospheric gasses and climate. This post focuses on carbon dioxide (CO2) as one of the most significant contributors to warming; other “greenhouse” gasses include methane, nitrous oxide and fluorinated gasses.
How the atmosphere affects temperature; the history of the “greenhouse effect”. The scientific theory that Earth’s atmosphere affects the planet’s surface temperature — the “greenhouse effect” — goes back nearly 200 years. As early as the 1820s, French scientist Jean Baptiste Fourier theorized that gasses surrounding the Earth retained heat, allowing the planet to warm more than the sun’s influence alone could explain. British physicist John Tyndall did some of the earliest experimental work to prove the relationship, demonstrating that water vapor and carbon dioxide hold more heat than oxygen and nitrogen. In 1861, Tyndall published the results in a paper titled On the Absorption and Radiation of Heat by Gases and Vapours, and on the Physical Connexion of Radiation, Absorption, and Conduction. In 1896, Swedish scientist Svante Arrhenius published a paper that for the first time quantified the relationship between CO2 in the atmosphere and Earth’s surface temperature.
Tabletop experiments: A number of educational and scientific websites provide instructions on how to do your own table-top experiment demonstrating how changes in the atmosphere affect temperature. For a demonstration, see this BBC video.
Sources: Discovery of Global Warming website (maintained by Spencer Weart and hosted by the American Institute of Physics); the National Aeronautic and Space Administration (NASA); the National Oceanic and Atmospheric Administration (NOAA); the University of York’s Tyndall correspondence website; and the Tyndall Centre for Climate Change Research. For an overview of the history of climate science carried forward through the 20th century, see a post by John Mason on the Skeptical Science website.
Trends in Atmospheric CO2. Scientists have been taking monthly measurements of CO2 at the Mauna Loa Observatory (Hawaii) since 1958. The chart below shows the trend line.
The red line plots the CO2 measurements; the black line represents the seasonally adjusted CO2 level. In 2014, CO2 levels measured at Mauna Loa reached 400 parts per million for the first time since modern record-keeping began. Research indicates that current CO2 levels are the highest in hundreds of thousands of years. Or as science writer Andrew Freedman put it more colorfully in an article for Climate Central:
The last time there was this much carbon dioxide (CO2) in the Earth’s atmosphere, modern humans didn’t exist. Megatoothed sharks prowled the oceans, the world’s seas were up to 100 feet higher than they are today, and the global average surface temperature was up to 11°F warmer than it is now.
Most of the increase in atmospheric CO2 has occurred since the beginning of the Industrial Revolution (in the late 1700s) when atmospheric levels were around 280 parts per million and the rate of change has increased in the last 50 years. The upward curve in CO2 looks very similar to the upward curve in mean global temperature since 1960 shown in the previous post:
Questions about human activity and increased CO2 levels
Haven’t CO2 levels on Earth been higher in the past? Yes, but the highest levels occurred around 500 million years ago when Earth was a very different place. The last time CO2 levels were similar to those being measured now was about 7,000 years ago. CO2 levels fell over several intervening centuries; then the curve reversed and the rate of increase accelerated within the last 50 years.
How do we know human activity has caused the recent increase in CO2? Scientists have looked at the relationship in several different ways. Two indications of human influence:
1. Mathematical accounting for the conversion of carbon to CO2. CO2 comes from both natural processes and human activity. People convert carbon to CO2 by burning fossil fuels and by clearing and burning forested areas. Scientists can calculate both the amount of CO2 produced by human activity (which has greatly increased in the last 150 years) and the capacity of oceans and forests to absorb CO2. Excess CO2 — the difference between the amount produced and the amount taken up by the oceans or by plant life — goes into the atmosphere. Atmospheric CO2 significantly increased as CO2 emissions from industry and energy generation spiked, indicating a large human contribution. Human activity also overwhelms CO2 increases associated with natural sources like volcanic eruptions.
2. Studying the atomic “fingerprints” of atmospheric CO2. Not all carbon atoms are created the same. Elements like carbon can occur in different forms (called “isotopes”) based on the number of neutrons in each atom. Carbon occurs as three isotopes — 14C (radioactive and least common), 13C (about 1% of carbon isotopes) and 12C (the most common). Fossil fuels like oil and coal contain no 14C because the radioactivity has long since decayed. Both plants and fossil fuels tend to have a low ratio of 13C to 12C. Scientists have found that the mix of atmospheric CO2 has become “lighter” in the last 150 years. An increase in carbon associated with plant-based fossil fuels seems to have changed the ratio of “light” carbon to “heavy” carbon in the atmosphere. The change has tracked the significant increases in CO2 emissions from combustion of fossil fuels for industrial purposes and electricity generation.
What kinds of human activities contribute to atmospheric CO2? Based on reporting of greenhouse gas emissions, the U.S. Environmental Protection Agency has created a chart showing the most significant sources.
Sources: “How do we know that recent CO2 increases are due to human activities?”, www.realclimate.org, (December 22, 2004); Andrew Freedmen: “The Last Time CO2 was this High, Humans Didn’t Exist”, www.climatecentral.org, (May 2, 2013); World Meteorological Organization: 2013 Global Greenhouse Gas Report; NOAA Earth Science Research Laboratory, Global Monitoring Division website; Scripps CO2 Program website; “Sources of Greenhouse Gas Emissions”, U.S Environmental Protection Agency website; NASA:Vital Signs of the Planet: Carbon Dioxide; Caitlyn Kennedy: “Earth’s Hottest Topic is Just Hearing Up”, www.climate.gov (2009).