The temperature changes even in the worst case do not seriously effect things for several decades. Here are some things to understand in terms of how agricultural productivity is still increasing and how we could handle increased desertification pressures.
Continue to boost agricultural productivity and use genetically engineered crops that are more drought resistant (can introduce new biological pathways that enable massive crop toughess), reduce irrigation challenge, and domed cities and domed land and weather control.
1. Understand how agriculture has been and is changing
By the year 2020 it is believed the world's rice crop will increase by an additional 60 percent. Current dwarf varieties have 15 productive panicles, or seed clusters per stalk (out 25 or so total stalks) that produce about 100 grains (seeds) each. New strains will have fewer, but stronger and thicker, stalks that will yield 200 or more grains each. These new plants are expected to account for most of the increased productivity. Genetically modiified crops will vastly increase yield and we will have all of the crops fully sequenced for more rapid modification
2. Alter photosynthesis for another boost in productivity
3. Adapting plants to survive on the moon and Mars. Can also be applied to deserts etc...
Gluthathione Reductase (GOR) from Colwellia psychrerythraea was cloned and overexpressed in E coli and is cold active.
[Times were from a late 2006 timeframe, but even if delayed this type of research looks workable in the timeframes]
They are putting GOR into plants and selecting the transformed plants over the next 10 months.
Develop plants to resist ROS stress under extreme environment of low Oxygen and pressure. This is for the next 3 years
Produce transgenic plants with improved growth and productivity at low temperature and water availability. This is targeted for 5 years time.
4. Waterproof sand is being produced in thousands of tons per day now. Put it into plastic sheets and bury it to create artificial water table and reduce irrigation needs by 75% in the deserts. This is already being done.
5. There are ways to pull CO2 out of the air. Also, should go after the low hanging fruit and reducing black carbon (soot). Easier to do.
Also, should put out underground coal fires and stop flaring natural gas in Africa.
We produce too much CO2 as a side effect of our industry. Directly countering this effect is cheaper with good plans.
Both for taking CO2 and soot out and for reducing temperature.
Can also have more local weather control with large domes (air supported structures).
Major CO2 Mitigation methods
CO2 removal from the atmosphere
Whatever the plans are for weather control, agriculture etc... They have to be big enough to address the 31 billion tons of CO2 per year (2010) and still increasing to 41 billion tons or so for 2030. (EIA reference case IEO2009) Just recycling and living leaner is nice and should be done but will not match the scale of what will may need to be done.