What Are Greenhouse Gases and How Do They Warm the Earth?

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As their name suggests, greenhouse gases act much like the roof of a greenhouse they trap heat on Earth.

This process happens in two steps. First, greenhouse gases let the visible and ultraviolet light in sunlight to pass through Earth's atmosphere unimpeded, and reach the Earth's surface. But then, when light strikes Earth's surface and is reflected back to the atmosphere as infrared energy, or heat, greenhouse gases absorb this heat and warm the planet.

Perhaps the most well-known man-made greenhouse gases are chlorofluorocarbons (CFCs). But not all greenhouse gases are man-made. Many are present naturally in the atmosphere, including water vapor, carbon dioxide, methane and nitrous oxide.

Human activities have increased the levels of both man-made and natural greenhouse gases. In particular, the amount of carbon dioxide in the atmosphere is now at an all-time high according to the National Oceanic and Atmospheric Administration (NOAA) and so are global temperatures.

Types of greenhouse gases

Water vapor is actually the most common greenhouse gas, but scientists don't fully understand if higher amounts of it in the atmosphere will ultimately have a negative impact on the future climate.

In contrast, carbon dioxide, methane and man-made ozone are considered very threatening to Earth's climate.

Until the Industrial Revolution in the late 18th century, carbon dioxide was produced by natural processes such as animals exhaling and eliminated through natural processes for example, trees take in carbon dioxide to use during photosynthesis.

But mining, fossil fuel consumption and an increasing population have broken that balanced cycle, causing a larger amount of carbon dioxide to remain in the atmosphere.

Although there is much less methane than carbon dioxide in the atmosphere, methane is a more efficient heat absorber. In fact, methane absorbs 23 times the amount of heat as a similar amount of carbon dioxide, according to the EPA.

Methane can be produced naturally, by processes such as decomposition in swamps, but its levels have risen above normal due to human activities, including raising cattle, which belch methane.

Man-made ozone is not the same type of ozone found in the ozone layer , which occurs higher up in the atmosphere in what is called the stratosphere. Man-made ozone is located in the tropospherecloser to Earth's surfaceand is the result of increased carbon and nitrogen from pollution. These elements react with sunlight to produce ozone. It is a particular concern over large metropolitan areas.

Nitrous oxide produced from the agricultural use of nitrogen-based fertilizers, chlorofluorocarbons and volatile organic compounds are dangerous greenhouse gases as well. Chlorofluorocarbons were created in the early twentieth century and used in products like spray cans. According to NOAA, they last for decades and affect the stratosphere. Volatile organic compounds, such as those that exist in oil, can affect ozone production.

Greenhouse gases and climate change

Greenhouse gases trap heat in the troposphere, the part of the atmosphere where weather occurs, and the global warming they cause affects the Earth's climate systems.

This is especially true of carbon dioxide. In the latter part of the twentieth century, scientists recognized that carbon dioxide levels were increasing. The use of fossil fuels such as coal and oil has pushed the levels of carbon dioxide beyond 350 parts per million (ppm), what is considered the upper safety limit.

In 2008, NASA scientist James Hansen and others from research institutions around the world urged action in a paper in the journal Open Atmospheric Science that the world was now in the danger zone.

If the present overshoot of [350 ppm] is not brief, there is a possibility of seeding irreversible catastrophic effects, wrote Hansen and his co-authors.

At the time of the paper, the carbon dioxide level was at 385 ppm. As of May 2010, carbon dioxide levels were at 392.94 ppm, according to NOAA.

Hansen and his team wrote that reducing coal consumption a major contributor to carbon dioxide in the atmosphere by 2030 could halt the rise of carbon dioxide at 400 ppm. But even with decreases in greenhouse gases, they warn that atmospheric carbon dioxide will continue to affect the climate for centuries.

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