For some time now, the general trend has been for a contraction in farmland, at the same time as an increase in farm produce, due to higher yields. There is simply no way that this contraction in farmland could be met by growth in developed land, there just are not enough people, concrete or houses to built them up. The way that governments across the world have dealt with the agricultural surpluses (once they have exhausted all the P.L.480 dumping possibilities) is to manage the retirment of land from farming, and since farming is the pricniple kind of land use, that means a lot of land being retired.
The main way of retiring land from farm use is to turn it into national park, forest, or specifics like the EU 'set aside' scheme. There is in fact a veritable explosion in National Parks underway, and has been for about twenty years. Below are some examples.
Unesco lists 507 biosphere reserves in 102 countries, approved between 1977 and 2007 http://www.unesco.org/mab/BRs/BRlist.shtml
UK
Since 1997 there have been political moves towards designating new national parks in SE England and Scotland. Loch Lomond and the Trossachs National Park was the first of these, and was designated in July 2002. New Forest National Park and Cairngorms National Park followed shortly, and it seems likely that there will fairly soon be a South Downs National Park too. http://www.naturenet.net/status/npark.html
On top of that there are the growing Green Belts and National Trust properties, areas of Scientific Interest etc etc
Europe
Europe has 359 National Parks, newly reorganised under the European National Parks Centre
A total of just over 8% of Sweden's territory is protected in one way or another. http://www.internat.naturvardsverket.se/
"Between 5 and 15% of eligible land is ordered by the EU into set-aside every year, typically 10%."
US
<EVERGLADES NATIONAL PARK - In another milestone for restoration of America's Everglades, Florida today donated the last parcel of state-owned property needed to complete the massive expansion of Everglades National Park launched by President George H. W. Bush in 1989. A total of 42,000 acres of state land was donated to the federal government to accomplish plans to grow the park by 109,000 acres - a move necessary to restore the natural flow, habitat and wildlife of the famed River of Grass. "More than fifty years ago, Florida donated land to the federal government to create the Everglades National Park," said Governor Jeb Bush. "More than 1 million acres later, Florida's commitment to the restoration and protection of this natural treasure remains steadfast."> http://www.floridadep.com/secretary/news/2003/oct/1010.htm
Shenandoah Valley: Cedar Creek and Belle Grove National Historical Park Cedar Creek and Belle Grove National Historical Park encompasses approximately 3,500 acres across 3 counties. Jan. 2002 http://home.nps.gov/applications/release/Detail.cfm?ID=355
Canada is creating new National Parks in the Northwest Boreal Uplands and the Western High Arctic http://www.pc.gc.ca/progs/np-pn/cnpn-cnnp/itm1-/page1_e.asp
Central Africa
The Central African heads of state have agreed to protect at least 10% of their lands as National Parks. This is part of the "Yaounde Process". According to Gabonese law, the surface area of a National Park cannot be less than 1,000 hectares.
President El Hadj Omar Bongo has created a National Park network based on the findings of a two-year biological and socio-economic evaluation by field teams from the DFC, WCS and WWF. http://gabonnationalparks.com/gnp-home/gnp-nationalparks
I could go on, but I think you get the point.