Carbon Dioxide and the Greenhouse Effect
The greenhouse effect is the process that keeps our planet warm. If it weren't for the greenhouse effect, Earth would be a frozen ball of snow, and totally uninhabitable. Without the greenhouse effect, Earth's average temperature would be around 0°F (-18°C). It is a process which makes life as we know it possible. Unfortunately, too much greenhouse gas in the atmosphere intensifies the greenhouse effect, and has led to the warming of the planet.
The atmosphere is filled with gases, the most abundant of which are things like Nitrogen, Oxygen, and Argon. Some of these gases have important properties relating to longwave radiation, or what we call heat. Radiation from the Sun reaches the earth in a short wavelength (shortwave radiation, or visible light), that easily passes through most components of the atmosphere. It passes through greenhouse gas, through Nitrogen and Oxygen, and through most clouds. There are a few things in the atmosphere that can reflect the Sun's rays back into space (like very high, cold clouds), but most of it passes untouched to the Earth's surface. Here, radiation from the Sun is absorbed by the surface. What amount is absorbed depends on the type of surface; oceans tend to reflect solar radiation back to space, snow and ice are obviously good reflectors. But much of it is absorbed, and then reemitted into the air as longwave radiation (sometimes referred to as infrared radiation or thermal radiation).
Longwave radiation is what controls the air temperature, and creates the temperature we feel. Some longwave radiation travels out of the atmosphere into space, but much of it is absorbed again by greenhouse gases, which reemit the radiation in all directions and warms the atmosphere further. This essentially traps the heat in the Earth's atmosphere, and as more greenhouse gas is added, the temperature increases, as well.
The existence of the greenhouse effect is well-known and experimentally proven science. It was first hypothesized by French physicist Joseph Fourier in 1827, and was proven by experiments later that century in 1859.
Carbon dioxide (CO2) is a greenhouse gas that is emitted naturally and in human activities like the burning of fossil fuel. Natural sources of CO2 include dying plants, plants going dormant in the fall, weathering of things that contain carbon (like mountains or rocks), and volcanic eruptions (though this source is very small). Nature also includes CO2 sinks (where carbon dioxide is removed from the atmosphere), such as ocean absorption and the respiration of plants. However, the largest source of CO2 emissions are human activities that require the burning of fossil fuels like coal, oil, and gas in power plants, cars, and industrial facilities.
In 1938, an English engineer named Guy Stewart Callendar hypothesized that rising carbon dioxide levels in the atmosphere was linked to warming. This idea wasn't considered plausible until the 1960s, when Dr. Charles Keeling began measuring the level of CO2 in the atmosphere from the Manoa Loa Observatory in Hawaii, and found that it was rising fast. Researchers began to study how carbon dioxide is linked to global temperature, and how it forced global climate change in the past. They found that CO2 and temperature are tightly linked, and realized the quickly rising level of the gas could seriously impact today's climate, as well.
Today, more than 34 billion tons of carbon dioxide are released each year. The major players in carbon dioxide emissions tend to be first-world, industrialized countries. China and the United States are the top two emitters, followed by the E.U., India, and Russia. China, the U.S., and the E.U. emit more carbon dioxide than all other countries combined.
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Water vapor occurs naturally in the atmosphere as water evaporates. It's what creates weather: rain and snow are both products of how much water vapor is in the air. While water vapor is the most abundant greenhouse gas in the atmosphere, the amount in the atmosphere is directly related to temperature. As air temperature increases, the amount of water vapor the atmosphere can hold goes up, as well. This is an important feedback effect: more warming leads to more water vapor, and more water vapor leads to more warming.
It was suggested that this feedback effect could introduce a "runaway greenhouse" here on Earth, not unlike what happened to the planet Venus. It is thought that Venus's possible global ocean began to evaporate quickly as the Sun grew brighter. As the water evaporated into water vapor, it could have kicked Venus's greenhouse effect into overdrive, such that all the water on the surface of the planet boiled away into the atmosphere, leaving the average temperature of the planet a scorching 460°C (860°F). However, the IPCC has stated that "a runaway greenhouse effect—analogous to Venus—appears to have virtually no chance of being induced by anthropogenic [human] activities."
Methane is a potent greenhouse gas with 25 times the warming power of carbon dioxide. Natural gas is 87% methane by volume. Natural sources of methane are controlled by precipitation and temperature; it comes from wetlands, termites, hydrates, wildfires, and wild animals. Man-made sources of methane include landfills, natural gas systems, coal mining, livestock, wastewater treatment, and rice cultivation.
Scientists think that methane played a large role in previous, natural, global warming events. Methane is released when permafrost and ocean hydrates melt and allow the gas to escape into the atmosphere. As the ground warms, methane will either be released directly into the atmosphere or bacteria will break it down into carbon dioxide, which will then be released. If areas of thawed permafrost exist at depth between frozen layers, it's possible that microbial activities will continue unabated, even during the winter, to create new methane from organic material. There is evidence that this is already occurring around Siberia's lakes.
Nitrous oxide is a clear, colorless gas with a slightly sweet odor. It's best known as the "laughing gas," used in surgery and dentistry. N2O has a very long atmospheric lifetime (~120 years) with 310 times the global warming power of carbon dioxide. N2O is produced naturally by a variety of biological sources in soil and water. Man-made sources of N2O include agriculture, burning of fossil fuel, and the production of adipic acid (used in controlled-release medications and in some foods as a flavorant and gelling aid.) and nitric acid (used for rocket fuel, chemistry, and woodworking).
According to the IPCC, global average atmospheric concentrations of N2O increased 16% from 1750 to 1998, which equates to a 16% increase for the period. In the last two decades, atmospheric concentrations of N2O continue to increase at a rate of 0.25% per year. There has been significant multi-year variance in the observed growth of N2O concentrations, and the reasons for these trends are not yet fully understood.