Few people are unaware of the concept of the Greenhouse Effect. Although the idea that human activities contribute towards global warming is not a universally accepted concept (especially if you're a particular petrochemical company) the UN's Intergovernmental Panel on Climate Change (IPCC) itself has studied recent evidence and concluded that climate change is happening, that it is caused by the burning of fossil fuels, and that the world is looking to be getting a lot warmer. If you're currently experiencing an internal dialogue in which you're unsure whether to trust the word of 2500 independent scientists or, say, the largest oil company in the world don't worry, all this particular noder asks of you is to accept that we're pumping A LOT of carbon dioxide into the atmosphere and have been ever since the Industrial Revolution. One would be hard pushed to argue against that.

Now here comes some biology; plants like carbon dioxide. Plants capture carbon dioxide from the air and convert it to carbohydrate by photosynthesis, a process which effectively synthesises the building blocks of just about every biomolecule on Earth. Increasing the levels of atmospheric carbon dioxide has been shown to thus increase the rate of plant growth with a doubled carbon dioxide concentration resulting in an approximate 40% increase in agricultural yield. At this point people reading this may be looking at the title of the node and scratching their heads but bear with me here, it's not a mistake, honestly...

Uptake of carbon dioxide by plants occurs through pores, called stomata, perforrating the epidermis of the leaves and stem. Stomata are also responsile for allowing water vapour to escape from the leaf by evaporation which in turn drives water uptake by the roots via the transpiration stream. You can invisage this as water being sucked up through the plant like drinking through a straw; as water leaves via stomata in the leaf it pulls water up through the roots from the soil. High levels of carbon dioxode promote the narrowing of the stomata, presumably because the plant is able to acheive sufficient carbon dioxide uptake while minimising water loss by evaporation. This in turn leads to a reduction in water uptake through the roots which means that the amount of trace elements taken up from the soil is reduced. Not only are these trace elements not sucked up with the water as normal but they are effectively diluted in the soil due to lack of water uptake from the earth and so ingestion by diffusion down their concentration gradient is also reduced. Furthermore, an increase in the rate of photosynthesis by elevated atmospheric carbon dioxide results in plants making more carbohydrate then they need to fuel their metabolism. This excess is stored in a subcellular compartment called a vacuole which also stores trace elements and so increase in carbohydrate levels leads to a relative decrease in stored trace element levels. So, by increasing atmospheric carbon dioxide we appear to be developing nutritionally dilute plants.

Trace elements that plants take from the soil include iron, zinc, iodine, copper, cobalt and manganese which are vital componants of oxygen carriers such as haemoglobin, elements of the electron transport chain in oxidative phosphorylation, componants of hormones, chemically active componants of many enzymes and constituents of vitamins. Plants deficient in many of these micronutrients appear to grow healthily, presumably due to their ability to store surpluses in vacuoles, however due to the relatively inefficient process of digestion implications further down the food chain are worrying. 84% of our calorie intake comes directly from plants and the majority of the other 16% comes indirectly through animal fodder. Nutritional dilution in plants could be potentially disasterous both biologically and economically. One can imaginge us in the future either having to either eat more calories to compensate for the lack of nutrients in the food or living off of dietary supplements. Neither is a healthy or ideal option.

Much research still needs to be carried out to confirm the extent of the problem of carbon dioxide induced nutrient deficiency in plants but current studies on plants such as rice and potato have begun to confirm the worrying trend. Currently micronutrient malnutrition effects over half of the world's population with iron deficiency alone affecting over 3.5 billion people, mostly in the developing world, impairing the mental and physical development of children while increasing morbidity and maternal mortality. As carbon dioxide levels are set to double by the year 2100 reserach into the agricultural problems related to elevated carbon dioxide levels are set to increase and strategies such as selective breeding and genetic engineering devised to tackle them. As things stand there are numerous potential ways to tackle the problem but it may require the threat of micronutrient malnutrition spreading to Europe and the USA in order for action to be taken. Unfortunately as the levels of carbon dioxide in the atmosphere are set to increase at an ever greater rate combined with an increase in levels of ozone, ultraviolet light and temperature the exact ramifications are unpredictable and much work still needs to be done. Whether the greenhouse effect is real or not, fossil fuels are likely to have much to answer for.


Roberts M B V, Biology, A Functional Approach, Thomas Nelson & Sons Ltd., UK, 1986

New Scientist Vol 176 No 2371, November 2002.

Fangmeier A et al., Effects of elevated CO2 and/or ozone on nutrient concentrations and nutrient uptake of potatoes European Journal of Agronomy 17: pp353-368 (2002)

Loladze I, Rising atmospheric CO2 and human nutrition: toward globally imbalanced plant stoichiometry? Trends in Ecology & Evolution Vol.17 No.10: pp457-461, 2002

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