In 1995 "Urban Agriculture" was not listed at the United States Library of Congress, the world's largest repository of information.  On October 21, 2008 I checked Urban Agriculture on Google, there were four million entries.  Next I clicked the organization I work with [The Urban Agriculture Network, TUAN] and found over 420,000 entries. Last I checked my own name [Jac Smit] and found 6,000 entries. 
 
In 1995 if a graduate student wished to research a paper on urban agriculture he or she could not get permission from their professor, as it was not an acceptable topic. 
 
Here is one explanation of what happened in a dozen years.  The beginning may well be a global survey done at the United Nations University [UNU], in the late 1980s titled "The Food Energy Nexus" (FEN).  Somewhat to their surprise, fourteen of the thirty seven papers included in the survey were about urban agriculture, around the globe.  The summary report was not released until the mid 1990s. 
 
The next most recognized publication, Urban Agriculture, Food, Jobs and Sustainable Cities, was launched at the World Urban Forum in Istanbul in 1996 by the United Nations Development Program [UNDP]. This book has been accepted as the "Seminal Document" by many academic and professional organizations.  It is based on visits to twenty countries between 1992 and 1994, visits to another ten countries prior to that date, and research at a dozen institutions in countries on all continents.  I am pleased to have been the lead author of this important work. 
 
Urban Agriculture can be traced back to the "Hanging Gardens of Babylon".  Columbus found urban agriculture practiced in the Caribbean in 1492 and brought advanced methods back to Europe, which are now practiced worldwide.  There were many advances in urban agriculture during the industrial revolution in the 18th Century.  In particular, the "Marais" district of Paris has received a great deal of recognition for applications of urban agriculture. 
 
What might explain the surge of interest and application in the 21st Century? 

A first underpinning may be rapid urbanization.  The world became over 50% urban in 2005 and the most rapid rate of urbanization was in Africa.

The second driver may be the Internet.  The global access to the Internet, beyond universities and government, can be pegged at 1996, the year Agriculture, Food, Jobs and Sustainable Cities was published.  
 
A third element is certainly the advance of relevant technology from the 1970s and on to today. Much of the technology has spread to every corner of the globe, in part due to the Internet and in part due to the acceptance of the distinction by organizations and governments.   
 
Local government endorsement of urban agriculture was strongly expressed at a UNDP global meeting of mayors in New York in 2000.  All but one participant spoke in favor of urban agriculture, from Sweden to Zimbabwe. 
 
A few of the urban agriculture methods that have been recently adopted include: bio-intensive production, plastic greenhouses and plastic ground cover, drip irrigation, hydroponic and aeroponic production, 'eco-sanitation' which safely reuses waste water and solid organic waste, and agriculture production on roofs, walls and fences. 
 
In the majority of countries and urban areas worldwide, agriculture is being re-localized.  This was first recognized by the FEN research during the energy crises of 1970s and 1980s. In 2009, with a much greater food-energy-climate crisis, urban agriculture is spreading at a much faster rate and on a larger scale than ever before. 
 
The energy and climate impact of urban agriculture require considerable explanation.  Briefly, the UN Food and Agriculture Organization [UNFAO] finds that urban agriculture produces seven times as much per acre as rural agriculture world-wide. Rural agriculture today also has a major negative impact on climate.  NASA has found that the urban area in the United States has ten times the potential productivity per acre as the space that is currently being farmed.  
 
Urban agriculture has far less energy demand per calorie of food produced than rural agriculture. Due to urban agriculture's greater productivity, proximity to market, and use of urban waste as a major production input, urban agriculture requires far less fossil fuel calories to produce food calories than does rural agriculture. 
 
Give the expanding world-wide interest in urban agriculture and the need for alternative approaches for feeding a hungry world, it is no surprise that a Google search generated well over four million entries in late 2008.  As the 21st Century progresses, there is no doubt in my mind that interest in and application of urban agriculture policies and methods will accelerate as well.