From Jason Clay, World Wildlife Fund in a NYT debate on population:
We currently use 33 percent of the Earth's surface for food. As 25 percent isn't usable (deserts, cities, roads) and 12 percent is set aside for national parks and the like, we continue to expand the food production frontier each year. At the current rate of habitat loss, after 40 years, we will have "eaten" nearly all the remaining natural habitat on the planet. Whatever is sustainable with 7 billion people will not be with 10 billion.
So you add up 33 + 12 + 25 and you're talking 30 percent of the surface that theoretically gets exploited. Population growth (we hit 7 billion around Halloween) to come by 2050 (40 years) is approximately 2.5 (not 3 to make 10B), but let's take the three and say we'll have 40% more people.
Honestly, considering how low yields are in most ag environments around the world, the notion that we can't support 40 percent more if we boost current land yields and get access to good land freed up by global warming/climate change (unmentionable to any WWF because of the species loss that will necessarily occur) is a huge supposition, given recent history. For example, America now produces 50% more corn on the same land as it did in 2000. Remember the corn fields you ran through as a kid. Impossible today! Why? Dense rows of plants.
Clay then goes on to sound ominous notes about food production in 2100 due to per capita (he has to switch his argument there because the pop growth will level off and end mid-century) and he comes up with this meaningless stat that we'll "need to produce an amount of food that is 2.5 times the amount that all human societies have produces in the last 8,000 years." That one is a pure scare tactic. Human population was negligible until about 200 years ago and hunter-gathering was the prime route for a major portion of that sub-billion population, so stacking up the previous 7,800 years of ag production is a goofy standard. Almost as unintelligent as saying we've got more humans alive today than have ever lived!
I do like the stats on the land use, so I blog to remember.