Greenhouse gases significantly absorb and emit radiation in the thermal infrared band of the electromagnetic spectrum. The most important greenhouse gas, water vapor, drives the water cycle, which moves about 23 percent of incoming solar energy around the atmosphere as water is changed between solid, liquid, and gaseous states. Although water vapor is expected to be the largest net contributor to potential man-made global warming (twice the effect of carbon dioxide), the recent behavior of water vapor suggests that it may represent a less positive feedback on the climate system than generally assumed. Understanding water vapor’s behavior in the atmosphere is especially important because humans have limited ability to control its feedback.
The direct radiative impact of a doubling of carbon dioxide in the atmosphere from pre-industrial levels (280 to 560 parts per million) is generally believed to represent about 1°C of potential warming. As air warms, more water vapor can evaporate into the atmosphere. The potential warming from additional water vapor is anticipated to be about 2°C per 1°C of carbon-dioxide-induced warming. However, recent evidence implies that water-vapor-driven warming may not be occurring at full potential intensity, reducing water vapor’s potential warming effect.
In a paper published in the Journal of Climate (2009),1 researchers examined various models used by the Intergovernmental Panel on Climate Change (IPCC) to simulate the conditions of the Pacific El Niño climate oscillation. These researchers noted that, when compared to observations, the IPCC models tended to overestimate atmospheric water vapor and underestimate cloudiness in the tropical Pacific. Both of these factors would result in a warm temperature bias when compared to real observations. The researchers were unable to determine conclusively whether these results were applicable to larger scale climate change modeling. However, they did indicate that the results were of concern and may require investigation with regard to impacts on global warming scenarios.
Another article published in the Journal of Climate (2012)2, indicates that average relative humidity decreased slightly, almost 0.5 percent per decade, as surface temperatures in North America warmed by about 0.2°C per decade (based on measurements from 1955–2010). Despite an overall increase in water vapor (0.04 g/kg/m3 per decade), this increase was not as large as the potential increases based on the warming temperature rate. In order for the water vapor effect to reach full potential (two times that of carbon dioxide), overall relative humidity values would need to remain constant, which means that the observed water vapor effect was somewhat muted compared to expectations. Much of the reduction in relative humidity has occurred in association with milder winter and spring temperatures, suggesting that the moisture content of Arctic air masses has not significantly increased.
Finally, there is the issue of water vapor in the stratosphere. Compared to the troposphere (lowest 10 km of the atmosphere), the stratosphere (10 to 50 km altitude) is quite dry. Water vapor that reaches the stratosphere can be effective at trapping heat in the atmosphere. In an article published by Science (2010), 3 researchers suggest extra water vapor that reached the stratosphere in the 1980s and 1990s enhanced climate warming in the 90s up to 30 percent. Conversely, a 10 percent reduction in stratospheric water vapor observed during the 2000s may have slowed warming by 25 percent.4
Although the effects of water vapor in future global warming is potentially significant, much more research is needed to properly understand the role of this greenhouse gas on the Earth’s greenhouse effect. Understanding this problem is made more difficult by the characteristically non-homogenous distribution of water vapor throughout the atmosphere and by water vapor’s ability to transport heat through changes in state (latent energy). Continued research should help establish whether water vapor (in its various forms) will exacerbate or reduce anticipated effects of climate change. Nevertheless, the research discussed here suggests that water vapor may be another fine-tuned tool designed to moderate the Earth’s climate.
Kevin Birdwell, PhD
Kevin R. Birdwell received his PhD from the University of Tennessee in 2011 and currently serves as a meteorologist and atmospheric researcher near Knoxville, Tennessee.