The earth's ice-free terrestrial area is 149,000,000 km2. According to the United Nation's Food and Agriculture Organization (FAO), croplands in 2015 cover 15,300,000 km2 (represented by the small square on the map) and pasture for animal grazing covers another 33,800,000 km2, a total of 49,100,000 km2 (38% of the earth's ice–free land). Theoretically, there remains some 26,780,400 km2 of land with potential for crop production worldwide. 1 Assuming all this land (represented by the middle square on the map) is under production by 2100 (as will be necessary to feed a global population of circa 10 billion) it would leave 75.8 million km2 or 50% of the earth's ice free land for other uses — such as sequestration, evapotranspiration and biodiversity. If however grazing rangelands expand into this remaining 50% as would also be likely then its capacity to provide ecosystems services will reduce accordingly.
However, into this calculation needs to be factored that 33% (49 million km2) of the earth's terrestrial surface is desert — by definition land unsuited to forestry and of limited biodiversity value. If we subtract the deserts from the equation, we are left with 40.2 million km2 or 17% of the earth's ice-free terrestrial area precisely that which the Convention on Biological Diversity aims to secure by 2020.
This breakdown of the global foodscape is based on reaching the limit of the world's remaining arable land. The other, more problematic way to approach it is to imagine that our hypothetical 10 billion people would, (if they could) consume what the average American does today.
The average American requires 1.4 hectares of land for their total food requirements. 10 billion such consumers would therefore require 140.4 million km2 of land for food production which is 93% of the earth's ice-free terrestrial surface (including land for crops and grazing according to current yields). In this case not only would all the world's arable land be used for agriculture but an additional 64 million km2 (43%) would need to be converted to arable land, essentially the entirety of the world's deserts, plus some. According to this breakdown a mere 7% of the earth's terrestrial surface would then be left for biodiversity — essentially a big zoo in the midst of a global monoculture of corn and cattle, much of it irrigated by desalination plants.
Put another way, if the world's total arable land (not including expansion into deserts) were distributed evenly, each of 10 billion people would have 0.14 hectare-about four basketball courts-from which to survive. 2 By 2050 people in developed countries are predicted to consume food requiring almost triple this amount. 3
1 Future Directions International, "The Future Prospects for Global Arable Land" (May 19, 2011), http://www.futuredirections.org.au/publication/the-future-prospects-for-global-arable-land/ (accessed June 24, 2016).
2 The World Bank, "Arable land (hectares per person)," http://data.worldbank.org/indicator/AG.LND.ARBL.HA.PC (accessed June 26, 2016).
3 Christine Chemnitz & Jes Weigelt (eds), The Soil Atlas 2015 (Berlin & Potsdam: Heinrich Böil Foundation & Institute for Advanced Sustainability Studies, 2015). Available at http://globalsoilweek.org/wp-content/uploads/2014/12/soilatlas2015_web_141221.pdf.