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.