It is not at all trivial to make precise measurements of greenhouse gas concentrations in the atmosphere, but exactly this is needed when assessing climate change impacts from human emissions. Only when present concentrations are known as accurately as possible can long-term trends be identified and seasonal variability be studied.
The three most important greenhouse gases in the atmosphere are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Several clean air measuring stations around the world, with Mauna Loa the most well known, form a global network to monitor atmospheric concentrations. One of these stations is Baring Head, close to Wellington in New Zealand. Since 1973 scientists have measured CO2 there continuously, making this the longest continuous measurement record in the Southern Hemisphere. Other gases like CH4, N2O, oxygen, carbon monoxide and ozone have been added over time to a comprehensive measurement program.
Measurements of greenhouse gases can either be made continuously by using stationary instruments that analyse the current concentration (in situ), or by collecting air samples in glass flasks or metals tanks and measuring the concentrations later back in the laboratory. CO2 at Baring Head is measured in situ, making use of the fact that CO2 absorbs infrared light. The atmospheric CO2 concentration is determined by comparing the infrared absorption of air with that of up to 12 traceable calibration gases, each with a different but precisely known CO2 concentration.
Atmospheric gas concentrations are expressed as a mixing ratio in ppm (parts per million) or ppb (parts per billion). For example, 393 ppm of CO2 means that within one million molecules of dry air there are 393 molecules of CO2. Since measurements begun in 1973, an increase of more than 65 ppm CO2 has been observed at Baring Head.
The other greenhouse gases, like CH4 and N2O are analysed at Baring Head by taking a 2 litre air sample in a glass flask, on average every 2 weeks. These samples are brought back to the laboratory in Wellington where they are connected to a gas chromatograph, which first separates the different gases and then determines their concentrations with the help of special detectors and calibration gases.
While knowing gas concentrations is vital for detecting trends and monitoring variability, more information is needed when we want to study the different sources and sinks of greenhouse gases. Isotopes are the key; they are atoms of the same element with the same number of protons but different numbers of neutrons. For example, two stable isotopes of carbon exist, 12C and 13C, with 13C being slightly heavier. Since every process in nature (like photosynthesis, respiration, burning of fossil fuels or biomass, ocean processes) will lead to a specific ratio of 12C to 13C we call it an isotopic fingerprint. So by analysing the ratio of 12C to 13C in CO2 and CH4 we can tell where these gases come from. Isotopic ratios of gases are measured by mass spectrometer machines that can distinguish between molecules of different mass. Isotopic measurements indicate that the CO2 that has gone into the atmosphere since the industrial revolution has primarily come from the burning of fossil fuels.
Another tool to investigate the sources of CO2 is the measurement of atmospheric oxygen concentration (O2). Burning of fossil fuels requires O2. For every CO2 molecule that is produced one oxygen molecule is used up, and the concentration of O2 in the atmosphere is decreasing as a result. Since these changes are minuscule compared to the total atmospheric O2 concentration (21% oxygen in the atmosphere) these measurements are very challenging. However we use an oxygen analyser and a suite of 6 calibration gases to recognise the subtle changes.
Worldwide greenhouse gas concentrations of carbon dioxide, methane and nitrous oxides have increased in the atmosphere to levels higher than in the last 800,000 years, which is shown from the analysis of gases in the EPICA ice core from East Antarctica. Measurements of atmospheric gases indicate that carbon dioxide levels have increased by 40% since pre-industrial times (pre 1750 AD). Since high greenhouse gas levels in the past have always been associated with higher global temperature, the increases we observe are concerning.
Katja talks about how the levels of CO2 and oxygen in the atmosphere are routinely measured at the Baring Head clean air monitoring station in New Zealand. The video is taken from Thin Ice.