Global Heat Stress and Crop Yields

Add one more point to the argument for cutting carbon emissions to near-zero by 2050 ... Scientists are now worried that high global temperatures will cause a "perpetual food crisis" through crop yield decreases of popular and important grains by the middle of this century. Because the high temperatures will be spread throughout the world, there won't necessarily be regions where there are good crop yields to cover for areas where crops are under too much heat stress to grow well, or are perhaps wilted outright.

How will that work, exactly? Well, it starts with this:

6CO₂ + 6H₂O + Sunlight --> C₆H₁₂O₆ + 6O₂

The equation above is the chemical formula for what happens during photosynthesis. It's the most important process in the world for life as we know it, producing both breatheable air and food for us. Carbon is taken from the air in the form of carbon dioxide, reacts with chemical pathways in chloroplasts (specialized cell compartments in plants' leaves*) and the water pulled up from plant roots, and it turns into sugar. That sugar, or its now solid-form carbon backbone, gets turned into other kinds of molecules the plant needs.

This is known as "fixing" carbon, taking it from a gaseous form to solid form. It allows plants to produce their own food, as well as food for other creatures. Whether an animal eats plants directly, or eats animals who eat plants, nearly all the food available in the world, and so, nearly all the living creatures in the world**, come from this reaction with the sun.

Water is brought up to plants' photosynthetic tissues from their roots through a series of vein-like structures known as xylem.

Xylem transports water only one direction, however, and that is out from the roots towards the surface of the leaves. The water either gets incorporated into new tissue growth or it evaporates from the plant's photosynthetic surfaces through its stomata, lip-shaped pores that the plant can open and close as needed.

The stomata serve another function, however, that of letting the plant breathe. They're where the plant takes in carbon dioxide and exhales oxygen or other waste gases, just as we do. When the plant is breathing, it can photosynthesize#, but it's also in a state of permanently giving off moisture, or sweating. This is why the Amazon rainforest makes its own clouds.

If a plant isn't getting enough water and has to conserve what it does have, it must close its stoma, which also means holding its breath. If it holds its breath, no photosynthesis. If these conditions persist, the plant can begin to wilt and will eventually die.

As the article linked at the top describes, crops like wheat, which are very nutritious and feed much of the world, are highly susceptible to this type of heat stress. Not even providing extra water is always enough to bring a plant back if its temperature threshold is exceeded, because some of these processes are set in motion by genetically programmed chemical triggers, and these high temperatures will mean more droughts, anyway.

Another side effect will be the more rapid decomposition of organic matter in soil, a particular bane in the temperate, northern hemisphere zones. Plant communities in the tropics, where high temperatures are routine, have evolved together to keep most of the nutrients and organic matter held in the plant canopy and operate on a waste-not-want-not basis. Plant communities in temperate regions, and crop plants, expect organic matter and the nutrients it helps hold in the soil to be just laying around, as it were.

To sum up: global warming is a threat to everyone's dinner. All the non-masochists of the world need to unite to make it stop.

* Chloroplasts also exist in types of photosynthetic bacteria and algae.

** There are bacteria that fix carbon via chemosynthesis, and whole ecosystems of bacteria that flourished before photosynthetic bacteria started releasing pure oxygen gas into the atmosphere; they're still around today, but are poisoned by oxygen, and so for obvious reasons they thrive in places we find inhospitable. There are also food chains, from bacteria to stationary tubeworms, etc., that get their primary food source from deep oceanic vents. Is it just me, or are those things not just objectively, unspeakably cool?

# Okay, except for plants that breathe at night and finish photosynthesis during the day. There aren't many, and they're mostly desert-dwellers, but they exist.

(Photo credits in order of appearance: Tambako the Jaguar on Flickr, Photohound on Wikimedia Commons.)