Hadley cells, the wind systems in each hemisphere , form patterns of atmospheric circulation in which warm air rises near the equator, cools as it travels poleward at high altitudes, sinks as cold air, and then warms as it travels back to the equator.  They are named after George Hadley, an English scientific writer.  Tropical regions receive more heat from solar radiation than they radiate back to space and such areas have constant temperatures.  More simply, warm air rises (heat rises) and then flows poleward at high altitudes, cools, drops, and flows back toward the equator at lower altitudes.   Then the process repeats itself.  When the air rises and leaves these tropical areas, it loses moisture as it heads to subtropical areas.  The majority of the worlds large deserts lay in these subtropical areas.

Hadley cells are expanding.  Precipitation has declined in tropical areas since 1970.  As I mentioned in yesterday’s post, Southern Asia, the Sahel, Southern Africa, the Mediterranean, and the US Southwest are getting drier and drier.  Even wetter areas now experience long dry spells between extreme events of rain and snow.  Examples in the US include the cold and snowfalls in the Midwest and Eastern Seaboard this past winter.  Texas and New Mexico continue to experience a prolonged drought.  In the next thirty years scientists predict a 30 per cent decline in water resources.

In some places both governments and individuals create innovative and sometimes simple measures to counteract desertification.  In India near the Thar Desert, the government mandated the dispersal of grass seed to hold the ground.  Studies indicate that the grass seed grew better when planted by hand than when dispersed from airplanes.  Also in India orans, small sacred groves, have helped preserve shrubs and grasses and even trees, a genetic bank that would otherwise have been lost.  Childs describes how his friends who live on the outskirts of Tuscon have coaxed their water table fifteen feet higher, using ordinary shovels and hard work.  They built contour traps and “massaged” the ground.  Hardly worth noticing except when it rained, the rain sank into underground catches.  Their properties now look like small areas of refuge in the vast desert.  In some areas of the Sahel people have been able to plant and nurture trees in such a way that areas of green exist where they had disappeared.

Archeologists and geologists know that periods of drought occurred repeatedly for millions of years.  For humans and the animals we know, drought has never been easy.  Large areas of civilization cannot exist without water.  We can affect our future in positive ways and prepare ourselves if we choose.