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 CO_2. 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.

Englishman seated on jaw of megatooth shark

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).