Greenhouse gases are chemicals that keep heat close to Earth, rather than allowing it to radiate into space. They are emitted by human activities like burning fossil fuels and deforesting the land, but also by natural processes such as volcanic eruptions and animal respiration. Because greenhouse gases are long-lived, their concentrations build up over time.
Over the past several decades, human activity has significantly increased the levels of three major long-lived greenhouse gases in the atmosphere: carbon dioxide (CO2), methane and nitrous oxide. As a result, atmospheric concentrations of these gases are now much higher than they were in pre-industrial times.
While CO2 has a lower Global Warming Potential (GWP) than many other greenhouse gases, it is responsible for the majority of global warming that has occurred since 1750 because of its high concentration in the atmosphere. Similarly, methane has a lower GWP than other greenhouse gases, but its large rise in the atmosphere is contributing to the current warming trend.
The levels of long-lived greenhouse gases are monitored at a number of sites in the NOAA Global Greenhouse Gas Reference Network, which includes more than 50 cooperative sampling sites around the world, 20 tall towers, and routine aircraft operation locations in North America. These data are used to calculate and release the annual Greenhouse Gas Bulletin, which tracks global trends in the concentrations of the three primary long-lived greenhouse gases.
Manufacturing is another major source of greenhouse gas emissions. For example, the machines and equipment in factories to produce items such as steel, electronics, plastics and clothes use fossil fuels like coal, oil and gas. Consequently, the manufacturing sector produces significant amounts of CO2, along with other greenhouse gases like methane and nitrous oxide.