How do we know that CO2 is increasing in the atmosphere?
High precision measurements of atmospheric CO2 made by many scientific organizations around the world show that its average global concentration in 2008 was about 385 ppm and more than 100 ppm higher than its value at the beginning of the industrial revolution in the 18th century. Almost all of these measurements have been made by high precision non dispersive infra red gas analysers 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 direct 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 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 atmospheric CO2 data collected by organizations around the world 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 has 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.
But how do we know that this increase in CO2 really comes from fossil fuel combustion? Well it turns out that carbon contains isotopic “markers” which are very like fingerprints (see below). The distinctive isotopic character of the carbon contained in fossil fuels is transferred to the carbon in the CO2 released into the atmosphere when the fuels are burned and can easily be found using modern analytical instruments. This isotopic marker or fingerprint has been measured at many locations world wide and is smoking gun proof of the fossil fuel origins of increasing atmospheric CO2 since the industrial revolution. Also when fossil fuels are burned oxygen is removed from the atmosphere and atmospheric oxygen measurements made at a world wide network of sites confirm that atmospheric oxygen has been decreasing in parallel with increasing atmospheric CO2.
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 trapped 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 10,000 years until the 18th century when it began to rise. As of 2008, the level had risen to about 385 ppm, an increase approaching 40%. 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.