To understand what’s at stake regarding the Mediterranean, here’s a graph I made from the numbers in World Population Prospects: The 2012 Revision: Volume II: Demographic Profiles, which was published in 2013 by the Population Division of the Department of Economic and Social Affairs of the United Nations Secretariat.
The United Nations’ population projections for the continent of Africa are on p. 10 of the paper document (p. 36 of the PDF); the data for the continent of Europe are on p. 23 (p. 49 of the PDF).
(Now, of course, these UN projections are based on the highly arguable premise that the emigration rate out of Africa will decline steadily. The million or so Africans currently massed in Libya waiting to set sail for the EU, where they will invite their relatives back home to join them, would probably not agree with that heroic assumption.)
P.S., I’ve created an updated graph showing the numbers from the U.N.’s new 2015 Revision. In it, I break out population forecasts for Europe, Middle East (North Africa & West Asia), and Sub-Saharan Africa.