“… the Intergovernmental Panel on Climate Change (IPCC)’s Fifth Assessment Report has announced some challenges for global agriculture: Yields are expected to decline by 2 percent per decade due to climate change, while the demand for food is expected to increase by 14 percent per decade.”  Genetically Engineered Crops: Key to Climate Adaptation and Food Security in Africa? Brookings Institute  by Jessica Pugliese, Mwangi S. Kimenyi, and Temesgen Deressa/ Brookings Institution - September 4, 2014

“In a departure from an earlier assessment, the scientists concluded that rising temperatures will have some beneficial effects on crops in some places, but that globally they will make it harder for crops to thrive — perhaps reducing production over all by as much as 2 percent each decade for the rest of this century, compared with what it would be without climate change.” Climate Change Seen Posing Risk to Food Supplies by Justin Gillis/New York Times - November 1, 2013

“Global warming will cut crop harvests by 2% each decade, researchers say” “Global warming will cut crop harvests by 2% each decade, researchers say” by Oliver Milman/The Guardian - March 19, 2014

Now for what the Intergovernmental Panel on Climate Change actually says:

“Without adaptation, local temperature increases in excess of about 1 degree C above pre-industrial is projected to have negative effects on yields for the major crops (wheat, rice and maize) in both tropical and temperate regions, although individual locations may benefit (medium confidence). With or without adaptation, negative impacts on average yields become likely from the 2030s with median yield impacts of 0 to -2% per decade projected for the rest of the century, and after 2050 the risk of more severe impacts increases (medium confidence). These impacts will occur in the context of rising crop demand, which is projected to increase by about 14% per decade until 2050. Regional chapters 22 (Africa), 23 (Europe), 24 (Asia), 27 (Central and South America) and Box 7-1 show crop production to be consistently and negatively affected by climate change in the future in low latitude countries, while climate change may have positive or negative effects in northern latitudes (high confidence).”

So, per the IPCC, without adaptation, there is medium confidence in a 0 to -2% median yield impact per decade for this century for the major crops (wheat, rice, and corn). There is high confidence the effect on crop production will be consistently negative in the low altitudes, while "climate change may have positive or negative effects in northern latitudes".

Question: what does it mean to have "high confidence" that something may be good or bad? I'm guessing that means high confidence there will be change and change is either good or bad, ergo...

Main lesson:  when possible, go to the original source. Then interrogate it.

Next: what adaptations may reduce or eliminate the effect of climate change on crop yields?