It was not by chance that the first two Common Science columns I published when I started this blog in 2011 were about photosynthesis.  The capture of solar energy by plants is the foundation of the food chain and the ultimate source of nearly all of the earth’s energy.  Photosynthesis has also provided the oxygen that we breathe.  However, despite its pivotal role in supporting all life on earth, photosynthesis is subject to the same limits and constraints of any other physical or chemical process.  Global warming about to provide us with a demonstration of those limits, a demonstration are not going to enjoy.

As shown in the graph at the top of the page the rate of photosynthesis is dependent on temperature.  As the temperature increases from 40 to 80 oF the rate of photosynthesis also increases.  The tendency for rate to increase at higher temperatures is observed in most chemical reactions.  However, when the temperature exceeds  80  oF the rate of photosynthesis falls of and then drops to zero at 105 oF. (1)  The reason for this drop in rate stems from the fact that the chemical reactions involved in photosynthesis rely on the action of enzymes.  Enzymes are short-chain proteins which exist in all living things, plant or animal.  Enzymes function as catalysts in biochemical reactions.  
The functionality of proteins in living systems, be they enzymes or other proteins like DNA or hormones, is dependent on them retaining their proper physical structure.  Although typically not as elegant as the famous double helix of DNA, all proteins tend to exist in somewhat coil-like confirmations which can only be maintained a limited temperature range.  If the temperature is too hot the proteins will uncoil, a process known as denaturization.  If you put cream in your coffee (an abomination in my view) you have probably experienced this for yourself.    If add cream to coffee that is very hot the proteins in the cream denature, rendering them to be insoluble in the coffee,  such that they now float to the top of the cup and form rather unattractive globules.
As you have likely already surmised, the reason that the rate of photosynthesis slows at temperatures above 80 oF is due to the denaturization of the enzymes which carry out photosynthesis.  When temperatures exceed 90 oF, the rate of photosynthesis drops of dramatically as nearly all of the enzymes denature.  This is quite a difficult circumstance for a plant.  Without photosynthesis the plant can no longer either grow or make new enzymes to replace the ones which uncoiled. To make matters even worse, at least here in North Carolina, it tends not to rain on days when temperature exceeds 90 oF so the plants, already coping with photosynthesis issues, need to deal with the stress of dehydration as well.
So if you are an Orange County farmer, you would generally want the afternoon high temperature to be about  80 oF every day.   On days cooler than that you lose a little productivity from the field due to slower plant growth stemming from slower photosynthesis, but your crops are not harmed  In contrast, on days were the temperature reaches 90 or even 100 oF, not only do you lose productivity  but your crops are being at least stressed if not damaged.  Therefore, to the extent that a summer season has more cumulative hours with temperatures above 90 oF, the average farmer would expect to come out of the growing season with fewer crops.
Consider now that the world is on pace for a global average temperature increase likely to be in the range of 3-6 oF during this century.  Current average high temperatures for Orange County, NC in June, July, and August are 86, 89, and 88 oF, respectively.  Since these temperatures are near to the 90 oF danger zone for plants, one could make a strong argument that Orange County’s current weather patterns put us right on the cusp of where a 3-6 temperature rise will result in a significant reduction in agricultural productivity. 
Now let’s lift our gaze beyond Chapelboro to look at the potential impact of this temperature rise for the rest of the world.  Orange County is situated a latitude of 360 degrees north of the equator.   As a rough approximation, it would be reasonable to assume that any location as close or closer to the equator as Orange County is likely to experience a reduction in plant productivity, both agricultural and forest/grassland, over the next century as temperatures rise.  This zone of the earth is shown on the map below between the two dashed red lines with a northern limit of Chapel Hill and a southern limit of Buenos Aries, Argentina.

It’s not uncommon to hear someone voice an opinion that global warming will simply shift the zone of agricultural productivity to locations farther from the equator.   I would suggest that you are much more likely to hear this comment from a politician than from a scientist.   Let’s look at the map to examine this proposal more closely.
The first flaw likely to catch your attention relates to the Southern Hemisphere where there is almost no land farther than 36o from the equator for a farmer to go.  The second flaw relates to the influence of ocean currents.  You can see from the map that much of Europe is as the same general latitude as Canada suggesting that they should have similar weather.  We know this not to be the case since the warm waters of the Gulf Stream result in warmer temperatures in Paris compared to Edmonton.  Therefore, farmers in Europe are more threatened by temperature increases that the dashed red lines on my map would suggest.
The most significant short coming of the suggestion that farming operations simply migrate away from the equator as the planet warms stems from the fact that temperature is not the only parameter which effects plant growth.  Plants are highly adapted to the characteristics of their natural ranges including yearly daylight patterns, wet and rainy seasons, soil type, proximity to other plants, bird migrations, and pollinator activity to name only a few.  Therefore, even if Alaska were as warm as Brazil it would not sprout a rain forest.  We will never grow coffee in Kamchatka or bananas in Barrow.
To conclude let’s look at the map one more time and consider the implications that the same zone in which we can anticipate less food production in coming decades overlays almost exactly with the countries in the world already experiencing the most poverty and which are driving the bulk of our global population increase.  It will be the people living here that will have a front row seat to our impending lesson on the limits of photosynthesis.
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  1.  I previously published a column called “Your Mother the Plant” covering the striking similarity of the chemical structure of hemoglobin and chlorophyll and how this similarity stems from humans and rose bushes having a common ancestor. I note for interest hear that 105 oF represents another intersection of the physiology of both plants and humans.   At this temperature, photosynthesis shuts down in plants and the human body shuts down via heatstroke.  For me this is another clear echo of our common evolution with all life on earth.