Greenhouse gases such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), absorb infrared radiation as it passes through the atmosphere. Greenhouse gases are measured at sites around the world.
Sunlight warms the surface of the Earth and infrared radiation expelled into outer space is, ultimately, the way the Earth cools down. If the two are out of balance, then the surface of the Earth can warm or cool. Carbon dioxide (CO2) and other greenhouse gases can absorb and release infrared radiation and alter that balance. The long-term increase in greenhouse gases in the atmosphere is the primary cause of the observed long-term increase in global temperature. The warming effect of greenhouse gases was first predicted in the 19th century.
Around 25% of anthropogenic - that is, manmade - CO2 emissions in the past decade have been absorbed by the oceans. CO2 reacts with sea water and reduces its pH, a process often referred to as acidification. The changes in water chemistry associated with this reaction have the potential to affect marine life.
A further 25% of carbon emissions are absorbed by land plants and stored in living tissue and later as dead organic matter in soils. This combined effect of the natural carbon cycle means that approximately only half of manmade emissions stay in the atmosphere.
Direct measurements of CO2 show that the concentrations have clearly increased since the 1950s, and using other sources of information, such as gas bubbles trapped in Antarctic ice, it is possible to measure greenhouse gas concentrations even further back in time. The concentration of CO2 in the atmosphere has increased by around 46% since pre-industrial times.
On top of the long-term trend, a clear seasonal cycle is to be noted. Each year the concentration of carbon dioxide in the atmosphere increases during the northern hemisphere winter and spring, whereas it decreases during the summer and early autumn as photosynthesis converts CO2 into oxygen and plants.
Greenhouse gases are measured very accurately by making use of their interactions with infrared radiation. First, the air is dried and then infrared light is passed through it. The amount of infrared light absorbed by the dry air at different frequencies is related to the amount of CO2 in the sample.
Measurements have been made at the Mauna Loa observatory in Hawaii since 1958. Nowadays a global network of stations monitors the concentration of CO2 and other greenhouse gases in the atmosphere. These measurements are combined to produce an estimate of global concentrations by the World Data Centre for Greenhouse Gases (WDCGG) in the World Meteorological Organisation’s Global Atmosphere Watch programme.
The long-term increase of CO2 in the atmosphere is due to human activities including the burning of fossil fuels, cement production and land management. Methane concentrations have increased as a result of human activities like farming, natural gas extraction and decaying rubbish in landfills. Nitrous oxide concentrations are affected by the use of fertilizers and the burning of fossil fuels.
Natural processes also impact the concentrations of these gases. Inter-annual variability in CO2 concentrations is affected by the El Niño Southern Oscillation. Methane is produced in wetlands and nitrous oxide is released by natural processes in the oceans and in soil.