Climate Change: Green House Gases and the Green House Effect

Greenhouse Gases

Greenhouse gases let shortwave solar radiation through the atmosphere, but impede the escape of long-wave radiation from the Earth's surface. This process is fundamental to the greenhouse effect — a process by which thermal radiation from a planet's surface is absorbed by greenhouse gases and re-radiated in all directions. Since part of this re-radiation is sent back towards the lower atmosphere, the result is an elevation of the average surface temperature. This process keeps the planet at a livable temperature. Without a suitably balanced mixture of greenhouse gases in the atmosphere, the planet's average surface temperature would be somewhere between -9 and -34 degrees Fahrenheit.

Carbon Cycle

The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Levels of several of these important greenhouse gases have increased by about 25 percent since large-scale industrialization began around 150 years ago. During the past 20 years, about three-quarters of human-made carbon dioxide emissions were from burning fossil fuels.

While natural processes, such as plant photosynthesis, can absorb some of the net 6.1 billion metric tons of human produced (anthropogenic) carbon dioxide emissions each year, an estimated 3.2 billion additional metric tons is added to the atmosphere annually. The Earth's positive imbalance between emissions and absorption results in the continuing growth in greenhouse gases in the atmosphere.

The U.S. produces about 25 percent of global carbon dioxide emissions from burning fossil fuels; primarily because our economy is the largest in the world and we meet 85 percent of our energy needs through burning fossil fuels. The U.S. is projected to lower its carbon intensity by 25 percent through 2025, and remain below the world average.

What About Water Vapor?

By mass and volume, water vapor is the most prevalent greenhouse gas in the atmosphere. According to both the International Panel on Climate Change and many global climate models, water vapor accounts for somewhere between 60 percent and 70 percent of the greenhouse effect. Climate change skeptics have used the following argument to try and punch holes in the notion that human activity is playing any significant role in global warming: Since water vapor is the most potent greenhouse gas, and since this vapor is created through natural evaporation rather than human activity, the current warming trend is nothing to worry about—just the Earth going through a normal climatic cycle.

But this viewpoint ignores the reactive nature of water vapor—in other words, the gas doesn't cause warming all by itself. The amount of water vapor the atmosphere can hold is almost purely a function of temperature—the warmer the air gets, the more vapor it's able to glean from the planet.

The cause of the temperature rise that's resulted in greater evaporation is that global emissions of carbon dioxide have soared. And unlike water vapor, which returns to Earth as precipitation within a week of entering the atmosphere, CO_₂ sticks around for between 50 and 200 years. Carbon dioxide accounts for approximately 25 percent of the greenhouse effect, so it's pretty clear that the dramatic increase in atmospheric CO_₂ is playing a significant role in recent warming.