Clouds, Volcanoes, Water Vapor and Climate Change

Reader Contribution by Richard Hilderman and Ph.D.

My previous posting discussed how arctic feedback alters the climate. That posting also defined positive feedback as factors that amplify climate change while negative feedback are factors that diminish climate change.  This posting will discuss how clouds, volcanic eruptions and water vapor affect the climate.

Cloudy days are relatively cool but cloudy nights are relatively warm.  Low, thick clouds cool the planet during the day by reflecting incoming solar radiation back into outer space.  High, thin clouds tend to reflect low energy radiation back to the surface of the planet and thus contribute to the greenhouse effect.  This effect occurs mainly at night and helps keep cloudy nights warm.  The role of clouds in climate change is difficult to determine because they exert two competing influences:  they act as both positive and negative feedback factors.  Another reason it is difficult to determine the overall effect of clouds on the climate is that clouds are not constant because cloud covering varies daily over the planet.  They form in some locations and not others depending on how the winds blow.

Volcanic eruptions disperse sulfur dioxide into the atmosphere which reacts with water vapor to form aerosol particles of sulfuric acid.  These particles reflect incoming solar radiation back into outer space and cool the planet for only short periods of time because the particles are eventually removed from the atmosphere.  The length of time the particles remain in the atmosphere depends on how high in the atmosphere the volcanic eruption disperses the sulfur dioxide.  The higher the dispersal the longer the particles remain in the atmosphere and cool the planet.  Volcanic eruptions also release carbon dioxide which is a greenhouse gas.  Volcanic eruptions act as a negative feedback factor for short periods of time but also enhance global warming through the release of carbon dioxide.

The first climate-influencing eruption to be monitored with modern instruments occurred in 1991 when Mount Pinatubo in the Philippines erupted and triggered a mean global surface cooling of 0.5o F.  This cooling effect lasted for only two or three years.  Pinatubo did influence the climate but other eruptions have occurred with more significant consequences.  Several large volcanic eruptions occurred between 1811 and 1814 but the 1815 eruption of Mount Tambora in Indonesia lead to the following year, 1816, being referred to as the “year without summer” in Europe and northeast North America.  The summer of 1816 in northeastern North America was very cold and dry.  The Hudson Bay remained ice covered.  In New England hard frost occurred in June, July and August.  The area from the Gulf Coast to Illinois was unusually warm while in western North America the weather was rather typical.  The summer of 1816 was the coldest in Great Britain since 1750.  However, in Scandinavia the conditions were near normal. These eruptions demonstrate that volcanic eruptions can trigger extremely variable and unpredictable climate responses.  

Water vapor is a greenhouse gas because it reflects low energy radiation back to the surface of the planet and warms the planet.  Increasing the surface temperature increases the rate of water evaporation from the ocean.  This increases the concentration of water vapor in the atmosphere thereby increasing the greenhouse effect and triggering further warming.  Conversely, if Earth’s temperature decreases, water vapor would condense out in the form of rain or snow reducing the concentration of water vapor in the atmosphere.  This decrease in atmospheric water vapor would lower the surface temperature even further.

Clearly, the release of carbon dioxide into the atmosphere from the burning of fossil fuel increases global temperature triggering an increase in the atmospheric concentration of water vapor which in turn induces more warming and the potential for severe weather.  In my next posting we will discuss the role of water on the climate.