Fahenheit 104 (40 degrees C). This is a number everyone should know.
3 July, 2012
According to the UK's Financial Times, Heatwave threatens US grain harvest. Since the US is the world’s top exporter of corn (about half the world's export), soya beans (about one third of the world's export) and wheat, damaging the harvest will have a global impact. This follows mere months since similar problems hit crops in Argentina, Paraguay, Uraguay and Brazil. As a result, the price of corn has risen 30% since mid-June and soy prices are the highest they have been in years.
A paper published in Geophysical Research Letters a few years ago, titled, When can we expect extremely high surface temperatures? (warning PDF) offered some sobering projections. Here is one of their graphs. Given the projections shown above (pay attention to the US Midwest) this recent news should come as no surprise. Yet, the current turn of events apparently caught traders and the USDA off guard. The warmer than usual spring meant more planting was done. However, what was a benefit has now become a liability as the heat, coupled with drought, threatens these crops before harvesting.
“The combination of low subsoil moisture, which is a reflection of the lack of precipitation that we had during the winter, together with the very hot weather that we’re seeing right now could spell a pretty disastrous scenario for corn and soyabeans,” said Hussein Allidina, head of commodities research at Morgan Stanley.
The seriousness of the problem can be demonstrated by the fact the USDA recently declared less than half of US corn was in good or excellent condition while 22 per cent was in poor condition. Even more concerning is the speed with which this problem developed. Only a few months ago, the USDA was projecting US farmers would produce a record corn crop this year. The bottom line: The current heatwave threatens to undermine forecasts of record output after the most widespread US corn plantings in 75 years. This is only two years after Russia suspended grain exports because of droughts that were worse than any they had experienced in half a century. I suspect these kind of historic comparisons will become a thing of the past. Not because we will solve the problem, but because this sort of failure is likely to become increasingly commonplace. Some people may misread this and think I am suggesting that global warming will bring about the end of all photosynthesis on Earth. That is not what I am saying. For starters, the CAM plants will survive quite well at temperatures that will destroy our food crops. Moreover, a lot of photosynthesis occurs in algae, which are located in the water. While global warming will have an effect on the oceans, a lot of that will occur at temperatures well below the ones I am focusing on here. -----------------
: Here's why this magic temperature is a problem for plants. In simple terms, at 40 C you mess up the proteins (in the oxygen-evolving complex of PSII) responsible for splitting water into oxygen, hydrogen and a couple of free electrons. Those free electrons are critical for all the other steps of photosynthesis. Knocking out the machinery that makes this reaction possible is like removing the spark plugs from a car's engine. Different types of plants handle this problem differently. Plants like cacti (CAM plants) have adaptations for bright light, high heat, and low moisture. The plants we are talking about are not CAM plants.Plants like corn (C4 plants) are sensitive to temperature just like soy beans (C3 plants), but corn can deal with low moisture better. Here is an animation that shows the relevant portions of photosynthesis. The temperature-dependent piece of machinery we are focusing on here (responsible for the oxygen-evolving complex) is mentioned at several points in the animation; 1:54, 3:14 and in more detail at 3:54. Unlike the other components discussed, this animation doesn't show the proteins that make up the oxygen-evolving complex. They are stripped away so you can see how the water, oxygen, hydrogen and electrons move around.
Drought conditions just make matters worse because one way the plant protects itself from excessive water loss is by closing up pores in the leaves called stomata. This keeps precious water in, but also prevents damaging heat from getting out.
As noted in the comments (h/t FishOutofWater), this is not limited to crops we use for food. For those interested in primary reference, consider this one (warning PDF). It shows that cotton, which is grown in very hot climates, also has problems with heat stress at this temperature range. This paper by Wise, et al. in "Plant, Cell and Environment" (2004),points out, correctly,
High temperature reduces plant growth and can limit crop yields. Estimates range up to a 17% decrease in yield for each degree Centigrade increase in average growing season temperature. [emphasis added]
It is worth noting that although I am focusing on temperatures that lead to the extreme disruption of photosynthesis, the paper by Wise et al., is a good example of work showing there are other - equally significant -- disruptions that occur at much lower temperatures. Keeping this issue in public awareness will require constant work and frequent reminders. An article (coincidentally published in the Washington Post after this one hit the rec list) underscores the difficulty of maintaining public awareness. The article I am referring to ran under the headline: Global warming no longer Americans’ top environmental concern, poll finds. Nature is not impressed by polling data. The laws of physics don't bend to majority rule. Ignoring biology flies in the face and teeth of reality. I chose photosynthesis as a point of entry to highlight the threat of global warming because the consequences are easy to measure and the impact is easy to understand, even without a technical background. Hopefully, this will help raise a general awareness of just how serious and real the global warming problem can become -- not in a hundred years, not in fifty years, but right now.