Idiot Tracker has a post on food security that uses food security as a means of focusing the reader on the challenges that climate change are likely to present in the near future.  In short, the argument goes that climate change will negatively impact our future agricultural productivity, making it difficult to increase that productivity as our population grows.  If we do hit nine billion people by mid-century (barring cataclysm this seems to be the minimum number we will hit), the author calculates that we will need to come up with 14.5 trillion calories per day, and notes that climate change is likely to present significant barriers to meeting this need.
I agree . . . in a general way.  We are losing huge amounts of arable land each year to soil degradation, and we are running out of productive places in which to extend new farms that do not create really problematic ecological tradeoffs (like massive deforestation that speeds climate change).  Climate change is likely to force the transformation of entire agricultural regimes in otherwise sustainable areas – for example, by changing temperatures and precipitation such that most strains of maize will have difficulty germinating in Southern Africa in a few decades.  This is all a very big deal.  But this post is also very, very thin on support for its argument.
As the post does not present any hard data, including how the 14.5 trillion calorie per day figure was derived, I cannot be sure if the author did any real math on our current production or the likely loss of caloric production that might occur under any number of likely climate scenarios (a problem unto itself, at global circulation models are much better for temperature than they are for rainfall, and there are few regional circulation models that can correct this problem – see the fascinating recent work of FEWS-NET on modeled versus empirically-measured patterns of precipitation in East Africa).  All of these might create significant error bars around likely future caloric production.  Further, I cannot tell if the author has considered whether or not crops will migrate as their ecological zones shift – surely farmers that previously could not raise a certain crop will start to take it up as the local environment allows and as other producing areas fall out of favor.  We know that some ecosystems will at least start to migrate if corridors for such movement are available – and agricultural systems are just another form of (heavily managed) ecosystem.  As cropping areas shift, what will the net caloric impact be?  It is not enough to say that we will lose a lot of calories when maize stops germinating in southern Africa.  We will need to get a net figure by calculating in all of the new areas in which maize will germinate.
Of course, such math only works at the global scale, and issues of hunger have very little to do with global production – hunger is local, shaped more by the intersection of markets, the environment, politics and society.  So noting that maize will germinate in new areas does nothing for the people in southern Africa who will be without maize.  However, we have to obtain another net figure: the lost calories from maize versus the new calories from new crops that people can grow, but chose not to before.  This may still total a net decrease in calories (indeed, it probably will), but this is not the same as simply subtracting maize from the equation.
Finally, what of plants that are edible, but that we currently choose not to eat?  The clearest analogy, to me, is the evolution of seafood here in the US.  I like to explain to my students that these new, exotic fish that are showing up at restaurants are the species that no self-respecting chef would touch two decades ago.  But when you wipe out the cod, you start getting creative.  And don’t get me started about tilapia.  It’s the rat of fish.  Seriously, it likes murky, stagnant water.  It will grow anywhere.  There is nothing I find funnier than hearing a server say “we have a very nice tilapia today.”  Yeah, I’d love to pay $20 for the swimming pigeon, thanks!  That said, people do eat tilapia and all sorts of other hilarious species because they are hungry and willing to pay.  So what new species of plant and animal will we be willing to eat a decade from now?  Three decades from now?  This is hard to predict, but I’ll bet quite a lot that we will find new species to exploit and offset even more of this caloric loss.
Despite all of this, I do think we face significant food challenges in the next three to four decades.  These will be felt very unevenly around the world, but they will be felt in significant ways.  To figure out what these impacts will look like, and who will experience them, requires that we carefully think through not only the exposure of crops to climate change impacts, but also the sensitivity and adaptive capacity of the agroecological system to those impacts.  It is only when we understand how such systems are likely to respond that we can begin to really plan for the challenges ahead.