Photosynthesis and CO2 Enrichment

Photosynthesis and CO2 Enrichment

Published June 13, 2009 Uncategorized 11 Comments

Tags: carbon capture, carbon dioxide, climate change, CO2, CO2 enrichment, crops, food, global warming, photo-respiration, photosynthesis, transpiration, water use efficiency

The benefits of increased atmospheric CO2 on crops are so extensive that a long article or book needs to be written to do justice to the subject and to the results of thousands of research trials. The improvement in photosynthesis efficiency at higher CO2 levels does not tell the whole story, but it is a good place to start, since all plant growth relies on this process.

Photosynthesis is the process by which plants utilize visible light energy (e.g. sunlight) to convert aerial CO2 and water (from roots) into plant matter. This process also requires phosphorus and nitrogen.

There are three photosynthesis ‘pathways’, known as C3, C4 and CAM. CAM is unimportant for food crops, being the method used by cacti, succulents and agaves. Pineapple is the only food crop of any importance to use CAM, so CAM can be neglected for the present purposes. World food security depends on C3 and C4 photosynthesis.

Less than 1% of all plant species in the world use the C4 photosynthesis pathway. Of the 86 plant species that supply most of the world’s food, only five use the C4 photosynthetic pathway, of which only four are of much importance (corn [=maize], sorghum, millet, and sugarcane) yet these four constitute some 20% of all the food crops grown. Because of their high photosynthetic efficiency, the C4 crops corn and sugarcane are favoured for ethanol production by those who want to produce liquid biofuels rather than food, thus increasing food prices and poverty.

Those crops using the C3 pathway include nearly all cereals (wheat, rice, barley, oats, rye, triticale etc), all legumes (dry bean, soybean, peanut, mung bean, faba bean, cowpea, common pea, chickpea, pigeon pea, lentil etc), nearly all fruits (including banana, coconut etc), roots and tubers (potato, taro, yams, sweet potato, cassava etc). C3 is also the pathway for sugar beet, for fibre crops (cotton, jute, sisal etc) and oil crops (sesame, sunflower, rapeseed, safflower etc), and for trees.

At present atmospheric levels of CO2, C4 plants are more efficient at photosynthesis than C3: in absolute conversion efficiency of light energy to stored chemical energy they are around 7% efficient, compared to 4% for C3. C4 plants typically use less water per weight of biomass produced, and can tolerate greater water and temperature stress than C3 plants. Accordingly, C4 crops are most often grown in tropical and equatorial regions.

The advantage that C4 plants have in terms of photosynthesis does not always translate into higher harvest yields, however, as only parts of the plant are edible. In terms of ground use, C3 crops can produce some of the highest amounts of edible calories and protein per acre: for example, potatoes and soybeans respectively.

C4 plants show a relatively small improvement in photosynthesis rate with increasing atmospheric CO2 above present levels; however, at increased levels of CO2 the leaf pores (stomata) of both C4 and C3 plants increasingly close up, which also reduces the amount of water lost by the plant (transpiration). Thus C3 and C4 plants significantly improve their water efficiency as CO2 levels increase. This is shown below for C4 (corn) and C3 (soybean).

C3 photosynthesis is less efficient than C4 partly because of an effect known as photo-respiration, which results in the loss (to the atmosphere or soil) of a substantial proportion of the carbon that has been extracted from the atmosphere by photosynthesis. C3 photo-respiration increases under heat stress and drought, which is a major factor behind the choice of C4 crops for hot dry climates. However, as CO2 levels increase, photo-respiration is suppressed, such that at double today’s levels of atmospheric CO2 the efficiencies of C3 plants (in photosynthesis rate and water use) are as good as or better than C4 plants. Moreover, at higher levels of CO2, C3 plants can maintain efficient photosynthesis rates at considerably higher temperatures than today’s conditions – their optimal temperatures for photosynthesis increase.

The effect of CO2 concentration on photosynthetic rate at constant temperature is shown below for C3 and C4 crops.

As CO2 concentrations increase, the photosynthetic efficiency gap between C3 and C4 plants rapidly closes, and at double today’s CO2 concentration (i.e. at 780 ppm instead of today’s 390 ppm), the photosynthesis rates are the same. Incidentally, the majority of the world’s most troublesome weeds use the C4 pathway, and so have a competitive advantage over C3 crops at current CO2 concentrations. At higher CO2 concentrations, competing for the same resources on the same patch (light, water, CO2, nutrients etc), C3 crops increasing out-compete the weeds.

The photosynthesis rate with temperature is shown below for C3 plants at today’s CO2 levels (Low CO2), and at double CO2 level (High CO2).

The upper curve is the same for C4. From this it is clear that at double CO2 concentration, not only has the efficiency of C3 crops improved tremendously, but the temperature at which optimal photosynthesis occurs in C3 increases up to that of C4. Thus the vast majority of food crops will benefit hugely by increased CO2, and even more so by increased CO2 coupled with warming.

A dangerous combination would be increased warming without increased CO2. Since there is no proof whatsoever that increasing CO2 is having any significant effect on climate (any climate changes might be taking place by completely natural means over which we have no control), but there is incontrovertible evidence that increasing CO2 is positively beneficial with or without warming, then on the basis of risk mitigation and precaution it is utterly foolish to be reducing carbon emissions. As S.A. Cowling put it in Plants and temperature – CO2 uncoupling (Science, 1999, 285, 1500-1501)

We should be less concerned about rising CO2 and rising temperatures and more worried about the possibility that future atmospheric CO2 will suddenly stop increasing

The article Global Temperature Change and Terrestrial Ecology in the Encyclopedia of Water Science (CRC Press, 2007) has the matter stated correctly:

[It is a] well-established fact that CO2 is a powerful aerial fertilizer, which when added to the air can substantially increase the vegetative productivity of nearly all plants…numerous studies have demonstrated that the percent increase in growth produced by an increase in the air’s CO2 content typically rises with an increase in air temperature. In addition, at the species-specific upper-limiting air temperature at which plants typically die from thermal stress under current atmospheric CO2 concentrations, higher CO2 concentrations have been shown to protect plants and help them stave off thermal death…[and] increase the species-specific temperature at which plants grow best. Indeed it has been experimentally demonstrated that the typical CO2-induced increase in plant optimum temperature is as great as, if not greater than, the CO2-induced global warming typically predicted…Hence, [with] an increase in the air’s CO2 concentration – even if it did have a tendency to warm the earth (which is hotly debated) – …[plants] …would grow equally well, if not better, in a warmer and CO2-enriched environment.

We will set out the full range of benefits of increasing CO2 in future posts, but as an initial summary the following is helpful by Vaclav Smil from China’s environmental crisis: an enquiry into the limits of national development (M.E. Sharpe, 1993)

There could also be important beneficial effects, above all a roughly 30 percent higher crop productivity brought by 600ppm of atmospheric CO2 and higher tropospheric temperatures. And the benefits may not end with higher productivity. As photosynthesis is predicated on a very uneven CO2—H2O exchange, higher atmospheric CO2 levels would significantly boost the water use efficiency of all plants. This reduction [in water use] would also average about 30 percent.

Other notable benefits or a higher CO2 level include lower photorespiration (which would increase both the optimum as well as the upper temperature range for photosynthesis), substantially improved symbiotic fixation of nitrogen in leguminous plants, increased resistance to lower temperatures, and air pollution, and a better tolerance of soil and water salinity. A combination of these responses would mean that all major crops would yield more in their current environments while using less water and, when rotated with leguminous species, less fertilizer—or they could be grown in areas considered today too arid for continuous field farming, or that they may be able to outperform the current yields in those regions where precipitation may decline…

Policies such as reducing carbon dioxide emissions, carbon capture and storage, taking land out of food use for biofuels (or onshore wind farms), increasing energy costs, grossly inefficient and poisonous ‘organic’ farming methods etc all serve to destroy the capability of this planet to support an increasing population. Behind this surely is the stated neo-Malthusian and Green policy to wipe out billions of lives by the sheer force of economics. As hunger increases, it will be blamed on man-made climate change, and the screw will be turned ever more tightly to introduce policies that will accelerate the destruction of mankind.