How To Use This Data Unit
Students will use three data sets in the form of Excel spreadsheets that will allow them to piece together information that will help them understand how the global climate system can affect a specific region of the world due to anthropogenic pollution.
The editable powerpoint can be downloaded here.
A pdf of the powerpoint is also available here.
Introduction
The African Sahel Region has suffered a prolonged drought that lasted 3 decades spanning from the late 1960’s, 1970’s to the 1980’s. During this time period the Sahel had a 30% decline in rainfall, only recently has it experienced a partial recovery. The Sahelian drought is the most severe drought of the 20th century. This severe drought adversely impacted 100 million people that depend of this precious resource for their subsistence.
The changes in climate that are afflicting the Sahel is of global concern. Scientists from across the world began to study this phenomenon and the broader implications with respect to the health of our global climate. According to the latest climate models supported by recorded data the cause of this prolonged drought is the decrease in surface temperature of the North Atlantic Ocean due to an atmospheric phenomena called global dimming. This occurs when highly reflective air pollution particles suspended in the troposphere and in clouds impede the shortwave radiation from the sun to reach the surface of the ocean. The pollutant that has directly been tied to this effect is Sulfur Dioxide (SO2).
The cooling effect of SO2 was observed and recorded when Mt. Pinatubo and Mt. Hudson (1991) ejected 23 million tons of SO2 into the atmosphere circling the globe. The global average temperature decreased by 0.6 degrees C for two years with a gradual recovery. This data confirmed previous theories that SO2 aerosol pollution affects climate. Other recorded observations have confirmed this effect as well. The cooling effect of SO2 particle emissions from ship exhaust produce clouds that have a high density of polluted smaller than average water droplets that increase the clouds reflectivity contributing to the dimming effect. This evidence help narrow the causes of the cooling of certain regional surfaces when the global trend was of warming temperatures.
During the 60’s, 70’s and 80’s the U.S. and Europe power plants and industry injected the atmosphere with thousands of megatons of SO2 annually, making them the largest polluters. Data revealed that during that time surface temperatures of the North Atlantic Ocean decreased its’ average temperature by three tenth’s of a degree Celsius (0.3°C) compared to its’ historical average. Although this may seem insignificant these subtle changes can drastically impact regional climates. The laws of thermodynamics predicate that there is a correlation between available thermal energy and precipitation. Lower sea surface temperatures mean a reduction on evaporative rates reducing the amount of moisture (water vapor) available in the atmosphere to precipitate over land. Conversely, the opposite is true – higher sea surface temperatures cause higher evaporation rates, which produce more moisture hence higher precipitation.
The cooling and warming of the ocean surface can impact how much rain falls on a geographical region. A few tenths of a degree temperature change up or down over large geographical areas causes fluctuations in precipitation. The sea-surface temperature (SST) has an effect on the fluctuations of heat and moisture into the atmosphere. The cooling of the North Atlantic Ocean reduced the moisture available to produce the copious amounts of precipitation the Sahel was accustomed to during the Monsoon rain season during the drought. The Sahel is located north of the tropical rain belt known as the Inter Tropical Convergence Zone that lies 6° north of the equator. The ITCZ shifts naturally north during the summer season and south during the winter season. This system is responsible for bringing rain to the Sahel. The cooling of the North Atlantic Ocean affects the up and down motion of the ITCZ. During the drought the ITCZ shifted further south of what it’s normal latitude location is during the summer months and deprived the Sahel of the much-needed rain.
The U.S. and the European Union have since significantly reduced SO2 emissions in an effort to clean the air due to health hazards and acid rain. This has had a positive effect. As the SO2 levels where dramatically reduced through use of engineered technologies there have been signs of improvement of increased precipitation in the Sahel.
