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Water Vapor, CO2, and Global Warming

By Anita Dotson

H20MYTH: Water vapor is the most important, abundant greenhouse gas. So if we’re going to control a greenhouse gas, why don’t we control it instead of carbon dioxide (CO2)?

This is a common misconception in the debate over greenhouse gases and the causes of global warming. Both water vapor and carbon dioxide are important greenhouse gases that play a crucial role in atmospheric warming. A greenhouse gas is a gas that absorbs and re-emits infrared radiation in Earth’s atmosphere, thereby increasing temperatures. Which gas then is to blame for global warming and should be controlled?

Water vapor accounts for 60-70% of the greenhouse effect while CO2 accounts for 25% —a notable difference when numbers alone are compared. It would seem then that water vapor should be climatologists’ primary focus. However, water vapor cannot be controlled by human intervention; it is simply a product of its environment.

The amount of water vapor the atmosphere can hold is dependent on temperature. Under normal conditions, most of the heat emitted from Earth’s surface in the form of long wave radiation goes into the atmosphere and out into space.  However, the presence of increased greenhouse gases traps more long-wave radiation, which means there is more energy in the atmosphere to warm the Earth’s surface.

As the atmospheric temperature rises, more water is evaporated from ground storage, such as that found in our rivers, oceans, soils, and reservoirs. The released water vapor becomes a greenhouse gas where it then absorbs more energy radiated from the Earth and thus warms the atmosphere. The warmer atmosphere results in further water evaporation and the cycle continues. This mechanism is known as a Positive Feedback Loop.

Scientists then need to focus on what is causing air temperatures to rise in the first place. Heat from other greenhouse gases is causing atmospheric warming, leading to an increase in water evaporation and compounding the greenhouse effect. Anthropogenic, or human-derived, CO2 serves as the primary source of warming with water vapor playing a secondary role. While CO2 occurs naturally in the atmosphere, human interference has interrupted the carbon cycle through activities, such as burning forests, mining, and burning coal. These activities artificially release more carbon from their solid storage to its gaseous state in the lower atmosphere. The rapid increase in CO2 volume has exceeded the amount oceans and vegetation are able to re-absorb. Furthermore, as deforestation continues around the world, there is less vegetation every year available to sequester the carbon. Thus, excess CO2 remains in the atmosphere where it traps heat and stimulates water evaporation.

Since the late 1700s, natural and man-made gases have been rising due to the birth of the industrial revolution. Prior to this time, atmospheric CO2 concentrations were steady at 280ppm (parts per million). Today they have increased by over 30% to 370ppm. Although it was calculated in the late 19th century to take a few millennia before temperature increases would be observed, human activities and CO2 emissions did not hold steady. A surge in atmospheric CO2 concentrations came in the 1960s due to car exhaust, industrial emissions, fuel production, and deforestation. Carbon dioxide was the first of the greenhouse gases to show a rise in atmospheric concentration from these activities. Ocean absorption began to lag behind as CO2 was being produced much too rapidly. Saturation and consequential warming over the oceans has been demonstrated by the fact that the total water vapor content over the oceans has risen 0.41kg/sq meter every 10 years since 1988.

While water vapor may be the most dominant greenhouse gas by mass and volume, it certainly is not the primary culprit responsible for global warming. Rather it is part of an amplifying effect. As other greenhouse gases such as CO2, warm the atmosphere, the air is able to hold more water vapor. The water vapor traps more heat and further warms the atmosphere. Human activities have increased CO2 output, and responsible human activities can reduce its production. Unlike water vapor that returns to Earth as precipitation within one week of entering the atmosphere, CO2 stays in the atmosphere between 50-200 years. Therefore, the best way to control global warming is to reduce CO2 emissions.

Appel, Arianne (2005, November 10). Global Warming Supercharged by Water Vapor? National Geographic News. Retrieved October 26, 2009, from http://news.nationalgeographic.com/news/pf/48158147.html

Koerner, Brendan I. (2008, January 22). Is Global Warming Caused by Water Vapor? Slate. Retrieved October 26, 2009, from http://www.slate.com/id/2182564/

National Oceanic and Atmospheric Administration (2008, August 20). Greenhouse Gases. NOAA Satelliteand Information Service.  Retrieved October 26, 2009, from http://lwf.ncdc.noaa.gov/oa/climate/gases.html

Weart, Spencer (2009, June). The Carbon Dioxide Greenhouse Effect. The Discovery of Global Warming. Retrieved October 26, 2009, from http://www.aip.org/history/climate/co2.htm

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