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.
Usually I read only one book at a time. Lately, I am reading several, one of which is Apocalyptic Planet: Field Guide to the Future of the Earth by Craig Childs. Childs is a sort of combined explorer/adventurer/scientist. He goes to places few go to see what occurs there, the wind, the flora and fauna, the weather, the climate. The next couple of days I intend to share some of his most pithy observations and ruminations. We will start with the desert. He and a friend literally wandered around the most arid and hostile portion of the Sonoran Desert in northwestern Mexico. This desert has enlarged and become more arid due to an extended drought.
Deserts come and go. If you live in a lovely lush green landscape, wait long enough and it, too, may become a desert. Six thousand years ago lakes, marshes, and grassland lay where the Sahara is today. A slight orbital change in earth’s relation to the sun caused nearby oceans to heat up, changing atmospheric conditions. Humans living there had no choice but to move. Forty per cent of the earth’s population lives in semiarid regions. Even a small drought changes survival chances for the people who live there. The Sahara, the Gobi, and the Taklimakan are growing, arable farmland decreasing. Vulnerable areas include southern Spain, Greece, Bolivia, Australia, central Asia, and our own West. The entire American High Plains (I live in the southern part) sits on top a giant desert. Without pivot irrigation, only grass grows here. In the last decade many irrigation wells have dried up or gone too saline. The giant bulges you see in places like the sand hills of Nebraska are really a sand dune sea covered with grass. Take away a little rain and here comes the desert.
Childs and his friend carried water with them and buried them with markers in the sand so they could find them later. As the desert grows in parts of India, women carry water farther and farther, an average of six miles a day, four gallons at a time. In the Sahel just south of the Sahara a difference in rainfall of just an inch or two can mean the difference between survival and starvation. Without water, there is no civilization.
What causes these changes? Human behavior and the increase in greenhouse gases are part of the reason. Humans are creating enough changes that we are moving toward more deserts, not fewer. One climate expert, Jonathan Overpeck, thinks we are seriously underestimating the severity of drought we will face in the not so distant future. Forget five and seven year droughts and think fifty years. Hadley cells also affect climate change. Tomorrow I will explain Hadley cells and how they affect our weather.
Eastern New Mexico