Global warming

[pbrower2a]

[quote (really an edit of material since deleted)']
North America has a weak monsoon effect in contrast to Asia. It could be that North America is large enough to have a slight monsoon effect. Africa is much larger than North America but had enough of a monsoon effect a few millennia ago (it extended into Arabia) that the Sahara was rich in vegetation:



Today the Earth is closest to the Sun on January 3, around which time the Sun seems to race against the Zodiac through Sagittarius. On July 3 the Sun moves slowly by contrast through Gemini, when the Earth is farthest away; this reflects Kepler's laws of planetary motion along an ellipse.  About 9000 years ago the perihelion of the Earth's orbit coincided more closely to the northern summer, and summers in the northern hemisphere were shorter but more intense (the change of position of the Earth's axial tilt is zodiacal precession). The intertropical convergence zone (ITCZ) followed the intense heating of lands under the sun, putting a tropical low-pressure zone in what is now the southern Sahara. 

Today summers are longer but less intense solely due to the axial tilt putting the winter solstice in the northern hemisphere when the Earth is closest to the sun, and the ITCZ travels only a relatively short distance from the Equator in Africa. About 7000 years ago the Sahara was a great grassland, and not a desert. Except for the lands surrounding the Mediterranean Sea which, as today, get practically no summer rain. Animal and human life were both abundant, with tropical species such as hippos wallowing in the many waterways. Such artifacts as corded-wear basketry common in tropical Africa (the culture was sub-Saharan) appear in places once rich in life but now utterly desolate. Lake Chad, today a salty lake on the border of the Sahara, was gigantic.  As time passed, the summer heating weakened, and so did the summer rains from north to south. Grassland became what the map shows as "semi-desert", which was about like the Mojave or the Australian Outback look today. There are scattered shrubs and not an absolute barren waste as the Sahara is today.

Not on these maps is the easternmost extension of the subtropical desert zone in our time, the northwestern area of the Indian subcontinent, now the Thar Desert in lowlands of Pakistan and nearby sections of India. At the same time in which the Sahara was "green" or at least much "greener", the Thar Desert was "green" or at least "greener".



Around 5000 years ago, such people as there were in what had been the "Green Sahara" either died off, went south into those parts of Africa that still got rain and kept vegetation, or found their way into the Nile Valley, the only place that still had a reliable supply of water. That is where Egyptian civilization started. While the Sahara was moist, the Nile was a forbidding place of impenetrable reeds that made exploration of the Upper Nile Valley impossible to European explorers as late as the latter part of the 19th century. If people are desperate enough to survive, then they will chop down the reeds and dig irrigation and drainage ditches so that they can still eat. (Check Toynbee's Study of History for that explanation of the rise of Egyptian civilization, although one must be aware that Toynbee has nothing good to say about African blacks and their descendants. (Maybe if he weren't such a racist he would have recognized the Harlem Renaissance as having the potential to enrich modern Western civilization... and distinguish the USA from Europe).  That is where Egyptian civilization, which was very sophisticated very early, came into existence.

North and West Africa, and the closely-related Arabian Peninsula are huge. Under the right conditions they can have copious rainfall from a monsoon. Southern Africa is much smaller and does not have that effect even today when the summers are more intense.  

Is Africa at all relevant to North America, or at least the desert and semi-desert zones between the Antelope Valley of southern California (Lancaster and, Palmdale) to north-central Texas (roughly Dallas) for climate change? AGW does not quite work as does zodiacal precession, but it might come close. Monsoon conditions in North America have never been as strong as those in Asia or as they were in Africa during the time of the Green Sahara. It is also possible that the Subtropical High would advance north into North America and turn Dixie into a genuine desert.

... AGW is a dangerous gamble with high risks and slight rewards even if Humanity does win.

(my general source for the maps. There is much commentary to read at this site. I encourage you to read it.
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In accordance with the legend, an extreme desert is a place of extreme barrenness such as most of the Sahara or Arabia. Plant life is extremely sparse due to extreme aridity. The Atacama and Namib deserts are examples of areas of moderate temperatures but extreme aridity. The Gobi and Takla Makan deserts in Asia are cold deserts. At the extreme are the Antarctic dry valleys, where rain is impossible due to the cold, and powerful winds evaporate whatever snow there is. Such places even have hyper-saline lakes and ephemeral streams... and they are the most barren locations on earth that are not outright ice caps.

Semi-deserts have considerable plant life. The Mojave Desert has scattered shrubs, and the Sonoran desert has its giant saguaro cactuses. Other places that have the semi-desert label include the Australian Outback and the Kalahari Desert.

Grasslands can be tall-grass prairie as in much of Illinois (which is arguably a mid-latitude savanna, as there are some trees) because the climate is just dry enough that forest fires would destroy any large groves of trees, or short-grass prairie as in eastern Colorado or western Nebraska. A tall-grass temperate savanna, as in Illinois or eastern Nebraska,  can well be used as excellent farmland. A short-grass prairie, in contrast, is too dry for such use (see western Nebraska) without irrigation.