How do we know that CO2 is increasing in the atmosphere?
High precision measurements of atmospheric CO2 made by the Scripps CO2 Program and other organizations show that its average global concentration in 2006 was more than 381 ppm; about 70 ppm higher than the first direct atmospheric measurements made in the 1950s. Records from Mauna Loa and the South Pole show nearly the same rate of rise over time; demonstrating that the rise is global in extent (see plot). Measurements from dozens of sites around the world now confirm the overall rise (NOAA/ESRL Global Monitoring Division). Almost all of these measurements have been made by high precision non-dispersive infrared gas analyzers which are calibrated using internationally agreed protocols. The standards used to calibrate the instruments are subject to rigorous international quality control procedures and peer review. The result is a series of datasets made by several laboratories in both hemispheres all confirming the current increase in atmospheric CO2.
How fast is CO2 increasing in the atmosphere and is this changing?
The rate of increase of atmospheric CO2 shows large variations from year to year. Initial high precision measurements of atmospheric CO2 showed average growth rates of less than 1 ppm/year in the 1950s and 1960s. However in the latter half of the 20th century and in the first few years of the 21st century the average growth rate had more than doubled to about 2 ppm/year.
How do we know that the CO2 increase is caused by human activities?
Industry data provides detailed figures of fossil fuels used in various sectors. This data can be used to calculate the amount of CO2 released into the atmosphere by combustion of the fuels. The emissions are more than sufficient to explain the observed increase in atmospheric CO2. Careful analysis of the atmospheric CO2 data collected by Scripps and other organizations shows that CO2 is increasing at a rate that is about 44% slower than would be expected if all the CO2 from the burning of fossil fuels stayed in the air. The real puzzle is to explain where the missing 44% of the emissions have gone. The answer is that this "missing" CO2 is absorbed by both the oceans and the terrestrial biosphere. On average over the last 50 years the oceans and the terrestrial biosphere have continued to "mop up" this amount of CO2. Whether they will continue to do this as atmospheric CO2 concentrations continue to increase is a critical question and the subject of intense international research.
Other evidence for a human cause: 1) There are no known natural sources of CO2 sufficient to account for the recent increase. 2) There are no known sinks of CO2 sufficient to have absorbed all the CO2 from fossil-fuel burning. 3) For more than 10,000 years prior to the industrial revolution, atmospheric CO2 levels were essentially constant (see below), which shows that the recent increase is not natural. 4) The increase in CO2 has been accompanied by a decrease in O2 (see Scripps O2 Program) and by changes in the ratios of the isotopes of carbon (see below) in the CO2. The O2 and isotopes changes indicate that the CO2 increase was derived from the oxidation of old organic matter - consistent with burning fossil fuel. 5) The pattern of CO2 increase since 1958 has closely mirrored that of fossil-fuel burning (see plot).
Isn't the Mauna Loa record influenced by CO2 emitted by the volcano?
If one looks at the minute-by-minute data from Mauna Loa, one finds rare occasions when the CO2 is elevated from emissions from fumaroles upwind on the mountain. The fumaroles are emitting constantly, so the timing of the events depends on wind direction and not changes in volcanic activity. These events impact only a tiny faction of the data and are easily distinguished from rest of the record. The reported version of the Mauna Loa record has been “filtered” to remove these events, as well as other certain other local effects, as described in the early publications (see Keeling 1960 Tellus paper).
How much has atmospheric CO2 changed since the industrial and agricultural revolutions?
When snow falls it traps air. In polar and other regions where the snow never melts it eventually forms ice and this air is entrained in tiny bubbles. Typically about 100 mls of air are contained in every 1 kg of ice. Thus polar ice acts as a kind of an "air museum" providing us with information on the composition of the atmosphere up to more than half a million years ago in the past. Extraction and gas analysis techniques tell us what the concentrations of CO2 were in the atmosphere before 1950. In addition they overlap the direct atmospheric measurements since the 1950s and confirm the present rate of increase.
CO2 concentrations measured from ice collected at Law Dome glacier in the Antarctic show that atmospheric CO2 has been remarkably constant at about 270 to 280 ppm over about the last 1000 years until the 18th century when it began to rise. As of 2005, the level had risen to 378 ppm, an increase of 35%. On the basis of ice core records the current CO2 concentrations are unprecedented for at least the last 650,000 years.
What are stable isotopes in atmospheric CO2 and why are these measured as well as its concentration?
CO2 molecules are made from the elements carbon and oxygen. Both carbon and oxygen contain isotopes which are atoms with the same numbers of protons but different numbers of neutrons. Because of this, the mass of each isotope is slightly different and this leads to CO2 molecules with different masses. The most common isotope of carbon, 98.9%, is carbon-12 denoted as 12C with an equal number of protons and neutrons in its nucleus. Carbon-13, denoted 13C, has 6 protons and 7 neutrons in its nucleus and is much less abundant at about 1.1%. Both physical and chemical processes in nature can discriminate against the heavier 13C atom thus changing the 13C/12C ratio of molecules of CO2.
These changes are very small but can easily be measured using modern isotope ratio mass spectrometers to 1 part in 100,000. Because different sources of atmospheric CO2 have different 13C/12C ratios isotope measurements of CO2 can be used to "fingerprint" CO2. CO2 derived from the combustion of fossil fuels, for example, has a lower ratio of 13C to 12C than carbon in the oceans or emitted by volcanoes.
What are the units for CO2 concentration, i.e. what does it mean to say that the CO2 concentration is 390 ppm?
CO2 concentrations are reported in units of micromoles of CO2 per mole of dried air, which is typically called a part-per-million, or ppm. A concentration of 390 ppm therefore means that there are 390 CO2 molecules for every million air molecules in the sample, or equivalently 390 moles of CO2 per million moles of air. Normal air contains variable amounts of water vapor. We measure the CO2 concentrations after removing the water by freeze drying. The concentration reported is therefore not sensitive to how much water vapor was initially in the sample. The concentration in ppm units is also not sensitive to the temperature or pressure of the sample. Although changes in temperature and pressure can change the density of air (via the gas law), this changes the number of moles of CO2 and air proportionally, leaving the concentration in ppm unchanged.