Greenhouse gases are chemicals in the atmosphere that trap some of the heat radiated by Earth’s surface. These gases absorb and radiate heat in the infrared spectrum, preventing it from escape. This process is called the greenhouse effect. Without the presence of these gases, temperatures would be much warmer.
Over the past century, the concentrations of the main greenhouse gases – carbon dioxide, methane, nitrous oxide and hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) – have increased significantly. The increase is mostly the result of human activities and is responsible for the current global temperature rise.
The atmospheric concentrations of these gases have been monitored worldwide for decades, starting with the pioneering work of Charles Keeling in 1958. This work is complemented by measurements of “old” air preserved in bubbles in ice cores and the analysis of carbon isotopes in tree rings. These observations, combined with climate models, show unequivocally that the concentrations of these gases have risen significantly.
This is mainly due to the combustion of fossil fuels for energy and transport and to land clearing, and to agricultural practices that release methane and nitrous oxide. The largest source of direct human-caused CO2 emissions is coal and oil burning, followed by natural gas extraction and solid waste. The most significant methane sources are wetlands, tundra and ocean bottom sediments, and termites and other animals that decompose organic matter. Nitrous oxide levels are increased by fertilizer use and run-off from soils.
The warming impact of these long-lived greenhouse gases is measured by their ‘Global Warming Potential’ or GWP. These values are calculated using a combination of their atmospheric abundance and their ability to absorb and radiate infrared radiation. A GWP of one is equivalent to that of carbon dioxide, and gases with higher GWPs have a larger warming impact.
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