In research that Wolfram Schlenker and I have conducted using the Hadley III climate model, we project yield declines of about 20% over the next 20 years, holding all else the same.
These findings have now been complemented by Dr Knight's project, which made use of climateprediction.net, a network of personal computers on which processing time is volunteered by members of the public, to compile 57, 000 different runs of a global-climate model developed at the Hadley Centre.