Global warming is at the forefront of many environmental discussions, so it is important to understand how it works. Understanding the basics can aid in interpreting research and analyzing the validity of sources. 

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The greenhouse gas effect is both what keeps the Earth at a livable temperature and what aids in a warming climate -- this process is what you see in the diagram above. Light enters our atmosphere as short-wave solar radiation. Where it goes from there is determined by a variety of factors. 

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Solar radiation can either be absorbed by the Earth's surface as heat (and be used for photosynthesis) or it can be reflected back into space. The solar radiation that is absorbed by the Earth is then emitted as long-wave infrared radiation, which can be sent into space or radiated back down to Earth's surface. A variety of atmospheric and surface features influence where the different types of radiation travel.

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Greenhouse gases are the most prominent of the factors that effect global warming. Greenhouse gases are those that prevent a large portion of heat (infrared radiation) from escaping into space, therefore warming up the atmosphere and, subsequently, Earth's surface. A certain amount is necessary to keep the Earth at a livable temperature, but if the level is too high you get global, warming. Global warming is a process that has occurred naturally over the course of Earth's history but at a much slower rate than what is occurring today. Global warming is happening at an unprecedented high rate due to human activity, which will not allow natural (and human) systems ample time to adjust. Water vapor is the most abundant greenhouse gas and a key step in the water cycle. Carbon dioxide is the most talked about because humans have greatly increased its presence in the atmosphere from deforestation, fossil fuel burning, and more. It can also enter the atmosphere from natural processes, like volcano eruptions and respiration. Methane is a very powerful greenhouse gas, but less abundant than carbon dioxide. Some of the main sources are landfill decomposition and agriculture. Nitrous oxide is also powerful and produced by activities like fossil fuel and biomass burning as well as the use of fertilizers. Chlorofluorocarbons are a greenhouse gas that comes from a variety of industrial and consumer sources, however they were a key culprit in the depletion of the ozone layer and are now heavily regulated.

 

Clouds can let solar radiation pass through them, and also reflect it back into space. The amount of radiation that passes through is determined by the thickness of the cloud (often relative to height from the surface of Earth). Lower clouds are optically thick and reflect most of the solar radiation back into space. These clouds help to cool the Earth. Higher clouds are much thinner and tend to let most of the incoming solar radiation pass through them on the way to the surface. These high clouds also tend to trap infrared radiation and radiate it back to the surface. The combination of these things lead to a warming of the Earth. Deep, convective clouds are really tall clouds that don't lead to an increase in warming or cooling. This is because their effects balance out. At the top of the cloud, much of the incoming solar radiation is reflected back into space. At the bottom of the cloud, infrared radiation is absorbed and radiated back to Earth's surface.

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Ice reflects solar radiation back into space, due to its high albedo (a measure of reflectiveness). A common positive feedback loop (a cycle of events that destabilizes a system) related to climate change is the ice-albedo loop. We'll walk through how this feedback loop works with polar ice. As temperatures increase, ice melts and decreases in surface area. This exposes more of the dark ocean surface. Less solar radiation is reflected because of the decreasing ice and more is absorbed by the dark ocean water. This leads to an increase in temperature and even more ice melts. This feedback loop destabilizes the system because it leads to a continual loss of ice rather than a cycle of depletion and formation.

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Aerosols are a broad category of particles that are suspended in the atmosphere and have complicated effects on climate. They can come from natural sources like sea salt and dust or from anthropogenic sources like the burning of fossil fuels and biomass. The majority of aerosols in the atmosphere have a high reflectivity and cooling effect by reflecting the majority of solar radiation that hits them back into space. Darker aerosols, like black carbon, absorb incoming solar radiation and heat the atmosphere. They can also absorb and radiate infrared radiation from the earth. Aerosols can also affect the way clouds form, but there is still more research to be done to determine the exact effects and mechanisms. Many aerosols are pollutants and negatively impact human health.

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Sources:

Jenkins, A. (2014, September 27). Just 5 questions: Aerosols – Climate Change: Vital Signs of the Planet. Retrieved May 2, 2020, from https://climate.nasa.gov/news/215/just-5-questions-aerosols/

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Borunda, A. (2019, April 9). Aerosols, explained. Retrieved May 2, 2020, from https://www.nationalgeographic.com/environment/global-warming/aerosols/#close

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NASA. (1999, August). Clouds and the Energy Cycle [PDF File]. Retrieved May 2, 2020, from http://www.iiserpune.ac.in/~p.subramanian/Clouds_energy_cycle.pdf

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The Causes of Climate Change. (2020, April 30). Retrieved May 2, 2020, from https://climate.nasa.gov/causes/

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Voiland, A. (2010, November 2). Aerosols: Tiny Particles, Big Impact. Retrieved May 2, 2020, from https://earthobservatory.nasa.gov/features/Aerosols/page1.php