When you lay on the beach and soak up the sun, you don’t see the invisible infrared waves that your body is absorbing, but you can feel them as warmth. Certain gases have the ability to absorb this infrared radiation in the same way: as heat energy. Gases that trap heat in the atmosphere are known as greenhouse gases, perhaps for the way that Earth’s atmosphere functions similarly to a greenhouse.
Water vapor is the most abundant greenhouse gas. Since air temperature determines evaporation rates (much like the heat setting on a stove determines how much water boils and evaporates out of a pot), it remains unchanged from human behavior. As of 2021, carbon dioxide (CO2) was the most significant greenhouse gas, making up 76% of the global greenhouse gas emissions from human activity. Methane and nitrous oxide accounted for another 22%, and fluorinated gases were responsible for 2%.
In the natural world, Earth’s carbon dioxide budget is balanced by sources and sinks. Sources of carbon dioxide include volcano emissions, decay of organic matter, and respiration by organisms that are aerobic (meaning they use oxygen). Humans are aerobic organisms- we inhale oxygen and breathe out carbon dioxide. Carbon sinks include land plants, which take up CO2 during photosynthesis, as well as oceanic processes. The “solubility pump” takes place when atmospheric carbon is dissolved into surface waters of the ocean and sinks. The “biological pump” refers to phytoplankton removing CO2 from the atmosphere to make organic matter and form their shell. The “microbial carbon pump” involves bacteria, which absorb carbon and transport it to the deep ocean. So if Earth has the carbon dioxide budget balanced with sources and sinks, why is the planet warming?
Anthropogenic activity (the influence of humans in nature) has become a new carbon source for Earth, and a large one at that. Fossil fuels (oil, coal, and natural gas) are burned for transportation, heating, and electricity production. This process accounts for 87% of total global CO2 emissions. China, the United States, and the European Union are the three largest CO2 emitters in the world on an absolute basis. Since the industrial revolution, carbon dioxide levels have risen so rapidly that the natural sinks cannot keep up.
Source: Boden et al., U.S. Department of Energy