How will climate change affect the extent of the Sahara Desert?

Based on past events supported by archeological finds climate change will affect it. Here is a short quote from an about.com referencing National Geographic

"... recent excavations at the site of Gobero in Niger indicate. Gobero is a cemetery site, including at least 200 human burials located on top of a ridge or set of ridges, sand dunes with a hard calcrete-fringe. These burials occurred in two periods of settlement: 7700-6200 BC (called Kiffian culture) and 5200-2500 BC (called Tenerean culture).

There, explorations by a team led by National Geographic Explorer-in-Residence and University of Chicago paleontologist Paul C. Sereno, have illuminated some small part of the last 10,000 years of the Saharan ecosystem. "

Some references cited include

• Sereno, Paul C., et al. 2008 Lakeside Cemeteries in the Sahara: 5000 Years of Holocene Population and Environmental Change. PLoS ONE 3(8):e2995. Free article download available

  Green Sahara: National Geographic Magazine September 2008.

People lived on the edge of the desert thousands of years ago since the last ice age. The Sahara was then a much wetter place than it is today. Over 30,000 petroglyphs of river animals such as crocodiles[8] survive, with half found in the Tassili n'Ajjer in southeast Algeria.... The modern Sahara, though, is not lush in vegetation, except in the Nile Valley, at a few oases, and in the northern highlands, where Mediterranean plants such as the olive tree are found to grow. The region has been this way since about 1600 BCE (aka as BC or 4600 years ago), after shifts in the Earth's axis increased temperatures and decreased precipitation.

• Citation: American Geophysical Union. "Sahara's Abrupt Desertification Started By Changes In Earth's Orbit, Accelerated By Atmospheric And Vegetation Feedbacks." ScienceDaily. ScienceDaily, 12 July 1999. .

Within the Holocene era, roughly the last 11,000 years, meaning the time frame within which the Western Sahara was sufficiently habitable to support a cemetery, we have experienced a "little ice age". The Holocene is generally described as beginning at the end of the last glacial period.

There has been climate variation during the Holocene such that the Sahara could support human life. It is fair to extrapolate that any future climate change will also affect the Sahara.

If you look at a world map you will see that the great dry deserts pretty much center around the 20 degree latitude. Proximity to a large body of water does not mean that the landmass can support vegetation. The Namib desert is south Africa stretches to the Atlantic coast as the Atacama in Chile has a very long boundary with the Pacific. (The Atacama is also the driest place on earth)

If there is going to be an affect on a desert from climate change , it is possible that the desert could extend to a major coast line. It would be reasonable that the North African coast could undergo desertification. It is already coterminous with the east and west coastal borders of Africa. If it expands , it can go north or south. It already approaches the Mediterranean coast

Presumably the Sahel could also become drier and there is some indication this has been happening. The Sahel is where we find the great savannahs. That region is most at risk for increased temperature or aridity. Movement in eh other direction- milder temperatures and more rain would be beneficial and supportive of animals and human habitation.

The arid region and the mean temperatures in the western Sahara are sufficiently large that it seems unlikely the Hadley cell formation would be affected, but I am not a meteorologist, so I defer to them on that point.

Facts:
The strip along the equator receives more solar radiation and hence there is more moisture in the air. This reduces the difference between maximum and minimum temperatures. As there exist more clouds the maximum temperatures are not as high as in other temperate regions of the Earth.
An If:
If there is global warming then the equatorial region becomes larger and, in the extreme limit all the earth would have equatorial climate.
Then:
With global cooling the process would be the reverse: The equatorial area reduces, the air is dryer, increase of the contrast between maximum and minimum temperatures (diurnal and annual), increase the erosion and greater intensity of local winds. So, in this global cooling scenario deserts get bigger. Because the maximum temperatures are increasing, most people and also many scientists, get the wrong impression that we are living in a global warming cycle.

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About the Sahel:
Most of the hot deserts extend from latitude 15 to 35º degrees because they depend on the trade winds. As explained above the long term evolution of the equatorial zone should be shrinking from a global cooling viewpoint and then the desert land strips will approach the equator invading the Sahel and the south of Angola.

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EDIT ADD:
after 4 downvotes without any explanation on physics I will copy/past from ENCYCLOPEDIA of PALEOCLIMATOLOGY AND ANCIENT ENVIRONMENTS - ARID CLIMATES AND INDICATORS

where you can find what I said above:

Pleistocene accentuation of aridity
Although deserts may have existed before the Pleistocene, there is evidence that aridity was intensified in many regions in the late Pliocene and Pleistocene. It appears to have become a prominent feature of the Sahara in the late Cenozoic, partly because of the cooling of the oceans and partly because the buildup of ice caps created a steeper temperature gradient between the Equator and the Poles. This in turn led to an increase in trade-wind velocities and in their ability to mobilize dust and sand. DeMenocal (1995) recognized a great acceleration in dust loadings in ocean cores off the Sahara and Arabia after 2.8 Ma, and attributed these to a decrease in sea surface temperatures associated with the initia- tion of extensive Northern Hemisphere glaciation. Likewise, loess deposition became more intense in China after around 2.5 Ma ago and eolian activity in the American High Plains dates back beyond 1.4 million y BP . The study of sediments from the central parts of the North Pacific suggest that eolian processes (dust deposition) became more important in the late Tertiary, accelerating greatly between 7 and 3 Ma (Leinen and Heath, 1981). It was around 2.5 Ma ago that the most dramatic increase in eolian sedimentation occurred – an increase that accompanied the onset of Northern Hemisphere glaciation. In Arabia, humid conditions in the late Tertiary caused deep weathering and fluvial incision of lava flows dated to 3.5 Ma. Such signs are absent in early Pleistocene flows (Hötzl et al., 1978).
Environmental fluctuations within the Pleistocene
All deserts show the impact of Pleistocene climatic changes; no deserts were immune. On the one hand, the deserts expanded from time to time, covering areas that are now wooded and forested. As a consequence, stabilized sand seas occur in areas where rainfall levels are currently in excess of 800 mm. A large tract of dead ergs occurs on the south side of the Sahara from Senegal in the west to Sudan in the east, while in southern Africa the Mega-Kalahari extended into Angola, Zambia, Zimbabwe and the Congo Basin.

As seen, the Deserts are a very recent feature on Earth, which means that the temperature in the past was hotter (and less global winds).

I'm consistent with data in the above topic and in the new model here presented (hoho it is vixra, do not read it, it is poison):

A Model of Past Earth’s Climate from Isotopic and Biologic Data and Its Relationship with Orbits’ Expansion

The first model of the past hot Earth’s climate consistently indicated by isotopic and biologic data is here established. This model, here named Evolving Climate Model (ECM), accurately matches a 3 Gy long compilation of isotope $^{18}O$ data. An important consequence of the model is the fast increase of oxygen atmospheric level between 2 and 1 Ga (Gy ago); this is a well-known but until now mysterious occurrence, the Great Oxygenation Event. A solution is presented for the two centuries old “dolomite problem” and new explanations arise for a number of long lasting problems, such as the origin of petroleum or of proto-continents. Differently from the usual climate scenarios, the ECM presents ideal conditions for the massive production of long organic molecules. Critical occurrences of life evolution, such as the Cambrian explosion, are explained and fitted by the ECM, exposing a previously unknown connection between the evolution of life and climate. The most likely cause for this hot past is the expansion of orbits; it is verified that this phenomenon can explain the ECM, the receding of the Moon and the water on early Mars for the same value of H0 = 48 km s-1 Mpc-1, which, if not a coincidence, is a non-negligible result.

Sahara greening is generally driven by the strength of the North African Monsoon, which follows the earth's orbital cycles pretty neatly. (Piquet's answer covers this).

http://en.wikipedia.org/wiki/North_African_climate_cycles

The answer to how much man made climate change will affect the North African Monsoon extent isn't clear. Man made climate change behaves differently than orbital cycles. Orbital cycles change the amount of sunlight different parts of the earth receive while carbon emissions inherently trap heat. The oceans were likely colder 10,000 years ago when the Sahara was green and getting lots of rain. Anthropogenic climate forcers will make the oceans warmer. It's a different scenario and hard to predict with any certainty. Global weather patterns are complicated.

It's worth pointing out that a measurable rise in CO2 helps plants retain water. Plants retain more water during transpiration in high CO2 concentrations because their stomata don't need to open as much to get the desired CO2. Plants still need water, but they hold onto the water they have better in higher CO2 environments. What effect that has on the greening deserts, it's too early to say. We also shouldn't confuse human efforts to withdraw more water from aquifers and make areas more green with natural greening.

http://www.sciencedaily.com/releases/2011/03/110303111624.htm

and you might think "good" plants hold water better, but we actually rely on plant transpiration as part of the water cycle. Warmer air needs more water to get saturated enough for clouds to rain and if plants release less water into the air (and simultaneously there's less snow runoff), less water is returned to the air over land. The result is increased dry areas and growing deserts, if not around the Sahara, then elsewhere. So, even if the Sahara or parts of the Sahara get greener, other areas could see increased deserts and drought. This effect is particularly important in areas that rely on atmospheric water vapor returned from land (not off the ocean) such as the mid-west US, much of china, parts of Australia.

Over time, oceans will also be rising, warming, and acidifying. So, any greening in arid areas will be dwarfed by other problems.

Land use patterns affect desertification in the short run. However increase in temperature along with more Carbon dioxide will Green the desert with more applicable vegetation. Right before the 8.2 kiloyear event the Sahara wasn't a desert but a grassland and tree dominant area, much like the savanna. Some simulations of global warming and increased carbon dioxide concentrations have shown a substantial increase in precipitation in the Sahel/Sahara. Because atmospheric humidity from the Atlantic and Mediterranean Sea and Indian oceans will provide More humidity. Using NOAA AVHRR NDVI-data from 1982 to 1999 the Sahel region below the desert has been greening.