Human-caused emissions directly affect plants, as higher CO2 levels generally increase photosynthesis and plant growth. It is almost certainly because of this ‘fertilising’ effect that land ecosystems take up more than a quarter of the CO2 emitted by human activities[1].
Some crops, especially in temperate regions, are expected to grow faster and have higher yields as a result of such increases in CO2. However, because raised CO2 levels are also the cause of climate change, their impacts on plants are not straightforwardly positive. While crops in temperate regions could benefit from warmer weather during their growing season as a result of global warming, the effect of climate change impacts such as droughts and heat-stress are expected to have net negative impacts on crops in many warmer regions of the world[2].
Photosynthesis depends directly on the amount of light absorbed by leaves, and higher CO2 levels help plants use the light they absorb more efficiently to convert CO2 into biomass[3]. In addition, it also makes plants use water more efficiently. This improved efficiency increases vegetation cover – which further increases the amount of light plants absorb. These effects are part of the reason for an increase in green vegetation cover that can be seen from space; although intensive human use of land for growing crops, particularly in China and India, is also contributing to this ‘global greening’, and could account for up to a third or more of observed net increase in global vegetation cover[4].
Scientists have explored the effects of CO2 through experiments on ecosystems, including both forests and food crops. They show that increasing CO2 by a further 150–200 parts per million – up from today’s level of around 410 parts per million – increases the rate of photosynthesis in leaves growing under natural light conditions by around 12% on average[5],[6].
However, whether specific plants are able to grow faster as CO2 levels rise also depend on several other factors. Some plants – so-called C4 plants, including many tropical grasses, maize and sugarcane – have a mechanism that concentrates CO2 inside their leaves. This means higher CO2 concentrations will not increase their rates of photosynthesis and their growth. This is why yields for maize are not generally expected to increase[7] – except in dry areas where crops are not artificially supplied with water, and where the increased efficiency in using water (due to raised CO2) will benefit growth. Plants’ ability to grow faster in response to increased CO2 concentrations also depends on whether they can access the extra nutrients they need to grow more[8].
Non-C4 crops, like wheat, soybean and rice, can grow more in rising CO2 concentrations. In one sense, this phenomenon makes increasing CO2 levels ‘good’ for agriculture. On the other hand, risingCO2 levels are causing climate change, which is likely to have a harmful effect on crop growth, for example through heat-stress, especially in regions that are already warm. Future scenarios for crop yields suggest that we will see a mixed outcome, with higher average agricultural production in some regions, but increased risks of crop damage in other regions, particularly in the developing world[9].
The effects of raised CO2 concentrations on natural ecosystems are also not straightforwardly ‘good’ or ‘bad’. Rising CO2 levels tend to increase tree cover in grasslands, for example, which is ‘good’ for removing carbon from the air but ‘bad’ for grazing animals, as savannas become less grassy. And as raised CO2 levels makes plant water use more efficient, plant coverage can increase so much that it ends up using as much or even more water than before – which can add to the pressures on fresh water supplies in ecosystems with limited water[10].
That a higher level of CO2 has some positive effects on plant life does not change the fact that continued climate change will have increasingly harmful effects on many aspects of human activity across the globe, including crop growth and agriculture in warmer regions. The outcomes of policies to limit these impacts will take time to come into effect, which makes action towards net-zero emissions an urgent priority.
References
[1] Le Quéré, C. et al. (2018) Global carbon budget 2018. Earth System Science Data 10, 2141–2194
[2] Liu, B. et al. (2018) Global wheat production with 1.5 and 2.0˚C above pre-industrial warming. Global Change Biology 25, 1428–1444
[3] Cernusak, LA et al. (2019) Robust response of terrestrial plants to rising CO2. Trends in Plant Science 24, 578–586
[4] Chen, C., Park, T., Wang, X.et al.(2019). China and India lead in greening of the world through land-use management,Nat Sustain2,122–129 doi:10.1038/s41893-019-0220-7
[5]Ainsworth, E. A. and Long, S.P. (2005). What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2. New Phytologist 165, 351-71
[6] Broberg, M.C. et al. (2019) Effects of elevated CO2 on wheat yield: non-linear response and relation to site productivity. Agronomy9, 243
[7] Leakey, A. D. B. (2006) Photosynthesis, productivity, and yield of maize are not affected by open-air elevation of CO2concentration in the absence of drought. Plant Physiology140, 779-790
[8] Terrer, C. et al. (2016) Mycorrhizal association as a primary control of the CO2fertilization effect. Science 353, 72-74
[9] Deryng, D. et al. (2014). Global crop yield response to extreme heat stress under multiple climate change futures. Environmental Research Letters 9, 034011
[10] Ukkola, A. M.et al. (2015)Reduced streamflow in water-stressed climates consistent with CO2effects on vegetation.Nature Climate Change6, 75–78
