Will plants actually flourish with more carbon dioxide?

During the photosynthesis GLOSSARY photosynthesisThe biochemical process in which green plants (and some microorganisms such as algae) use energy from light to synthesise carbohydrates from carbon dioxide and water. Photosynthesis can be shown as:
CO₂ + H₂O + energy → [CH₂O] + O₂ process, plants make their energy to grow by converting CO₂ and water to energy, with the help of sunlight. Ignoring the other things (like temperature and water) that limit plant growth, many crops would actually grow better with more CO₂ in the atmosphere. However, the ways in which different plant species convert CO₂ to energy are different, meaning that while some plants benefit from increased CO₂, others do not. Furthermore, ‘real world’ experiments have shown that increased crop yields were not as great as expected from smaller scale greenhouse trials. Additionally, increased CO₂ can increase the vegetative matter of a plant, and not necessarily the number or size of the fruits, seeds or roots, which are the parts that people generally rely on for food.

Faster plant growth can also have side effects. Some types of pests and diseases will flourish in the denser, leafier plants. Faster-growing plants often absorb fewer nutrients, such as nitrogen, so the end food product has a lower nutritional value. Additionally, some plants such as cassava, almonds and sorghum (a common cattle feed) produce toxins in their leaves, fruits or roots as a defence against insects and animals. When these plants grow faster, they produce more of these toxins, and less protein. So, not only do the people who rely on these plants end up eating plants with higher levels of toxins, they also have to eat more of them, to get the same amount of protein.

• Toxins in food crops

  Would you like cyanide with that?

  Many plants produce unpleasant chemicals in their leaves to discourage nibbling insects. The cassava plant, which is a staple food for many people in arid regions of developing countries, contains cyanogenic glycosides in its leaves and root tubers. These can turn into cyanide if the tubers are not processed correctly, with serious consequences for the health of those who depend on cassava for their daily food. Australian researchers, led by Professor Ros Gleadow from Monash University, have found that the cyanide levels of cassava leaves increase dramatically when grown under high-CO₂ conditions. Happily, the cyanide concentration within the root tubers remains comparatively low, but overall yield of the cassava was dramatically reduced under the elevated CO₂ conditions, which is actually a more serious concern than the cyanide.

  Professor Gleadow’s team also looked at the effects of high CO₂ upon sorghum, a common cattle feed that also contains cyanide toxins. In the case of sorghum, it is drought rather than elevated CO₂ that leads to an increase in the leaf toxicity.

  Koalas, too, are threatened with a potentially toxic diet. The eucalypts they feed upon contain not only cyanogenic glucosides, but also other chemicals called phenols. Under elevated CO₂, the protein content of the eucalyptus leaves will drop, while the toxic phenolic and other non-nutrient content increases. Emeritus Professor Ian Hume and other researchers from the University of Sydney have predicted that it will become more and more difficult for koalas to meet their nutritional needs as CO₂ levels increase. The impact of the decreased nutritional content of the leaves will be compounded by other potential changes to their habitats such as declines in their preferred eucalypt species.

  

Warmer... warmer... too hot!

A small amount of warming is expected to improve crop yields in the cooler regions of the world. For example, warmer winters will probably result in areas of northern China being able to grow more wheat. However, warming of greater than 3°C will cause a decrease in crop yields as the benefits of slightly increased temperatures turn into heat stress. After the optimum temperature for growth is exceeded in any one place, the decline in crop yields is very steep. Parts of the tropics are expected to suffer from decreased yields even with warming of 1–2°C.

An increase in night time temperatures, where most of the temperature increase caused by climate change is expected to occur, of just 1°C has seen a 10 per cent loss in rice yields.

Furthermore, higher temperatures during particular stages of plant growth, such as the flowering stage, can result in reduced and poor quality yields. Early flowering, triggered by warmer temperatures, can also present a problem. As the fruiting process of many plants relies on bees to pollinate flowers, reductions in yields can occur if bees are still hibernating when the plants flower.

While warmer temperatures can increase the length of the growing season, they will also result in thirstier plants. About 80 per cent of all agricultural land and virtually all pastures rely on rain, and changes to rainfall patterns may mean this rain does not fall as often or as regularly. Too much rain, and excessive soil moisture, can be as detrimental as not enough, so it’s clear that altered rainfall patterns could have serious consequences.

Hot and bothered animals

About one third of the world’s food supply comes from livestock and fisheries. As average temperatures around the world increase, heat stress may affect animals in a number of ways. Dairy cows in particular don’t like to be too hot, and produce significantly less milk when stressed. Cattle may spend more time looking for shade, and so graze less and also have a decreased feed intake. This means they won’t grow as quickly, or may even lose weight, resulting in less meat. Sheep and pigs will suffer similarly, and hens will produce fewer eggs or even stop laying. Extreme heat stress results in increased mortality of the animals.

A decrease in grain production caused by changing climate conditions may lead to feed shortages for livestock. Animals raised in pastures and rangelands GLOSSARY rangelandsVarious landscapes including woodlands, grasslands, wetlands and deserts, and areas of native vegetation used for livestock. may also suffer as increased temperatures or droughts affect the growth of the grasses and plants they eat.

Fishery stocks will be affected by changes in ocean temperatures, circulation patterns and chemistry. As traditionally colder ocean areas become warmer, some fish may enjoy a broader range of habitable waters, while others’ habitats will shrink. Invasive species can spread to new regions, significantly disrupting ecosystems. Changes in ocean circulation and stratification (the way layers of warmer and cooler water form in the ocean) may affect the availability of food. Altered rainfall patterns will change estuary dynamics, which can affect the food availability and spawning habits of estuarine species.

In Australian waters, we have already seen the East Australian Current reach about 350 km further south than 70 years ago. Many Tasmanian species have already declined in numbers, as warmer species have moved into Tasmanian waters. So, warm temperate water species may benefit from a broadened habitat, but cooler species don’t have as many options for relocating to more hospitable waters.Increased temperatures may also impact the Tasmanian Atlantic salmon industry, as the fish are farmed in waters that are already close to the upper limit of their preferred temperature range.

Oceans also absorb a lot of CO₂ from the atmosphere, which alters their chemical balance. This is known as ocean acidification, and it makes it harder for shellfish to grow their shells and for corals to build their skeletons.

Changing conditions

Double (triple... quadruple?) whammy

Not only will the world’s agricultural systems have to cope with the impacts of higher average global temperatures and altered rainfall patterns, but climate models suggest extreme events, such as heat waves and cold snaps, droughts, floods and bushfires are likely to also increase as the Earth undergoes relatively rapid climate change. The potential impact of these events on agricultural production may be more serious than the effects of changing average temperatures and rainfall patterns.

In Australia, we have lived with droughts for a long time and each summer we prepare for bushfire season. Altered rainfall patterns and generally higher temperatures may make many areas already prone to devastating events more susceptible. Floods can also wipe out huge areas of crops and soil, though the water they provide can be beneficial too.

Warmer temperatures are also tipped to increase the prevalence of diseases and pests, as greater areas of the world become habitable for them. It is likely that wheat will suffer from increased occurrence of diseases such as leaf, stripe and stem rust GLOSSARY stem rustA fungal disease that mainly affects wheat, but can also occur on barley, triticale and some grasses. Stem rust is rare, but can be devastating to crops. Reddish-brown oblong pustules form on the plant stems and leaves. Rusts can be transmitted from one season to the next via living plants, but cannot survive on crop stubble, soil or seed. , powdery mildew GLOSSARY powdery mildewA fungal disease that can affect a wide range of plants. Powdery white spots appear on the stems and leaves of plants. This fungus requires a living host to survive the winter, and be transmitted from one crop to the next. and fusarium head blight GLOSSARY fusarium head blight A fungal disease affecting grasses. Crops that can be affected include wheat, durum and barley. The effects of the disease are floret sterility, which results in major yield losses, production of small shrivelled grains and seed discolouration. Fungal toxins may also be produced, making the grain unsuitable for some end uses. . Coffee crops are predicted to suffer from increased predation from the coffee berry borer. Corn may become more susceptible to the highly toxic and carcinogenic mould Aspergillus flavus, which thrives in warm and dry conditions.

Diseases such as bluetongue GLOSSARY bluetongueA virus spread by Culicoides midges, which causes fever, haemorrhaging in the mouth and nose and excessive saliva production. The animal’s lips, tongue and lower jaw may become swollen. Despite the disease’s name, the blue tongue that can occur is relatively rare. The animal can also become lame. Death occurs about six days after the appearance of symptoms. , which affects sheep and goats and is currently found in the tropics, may soon be found in the mid-latitudes. It’s also thought that Australian cows will face more problems from the cattle tick Rhipicephalus (Boophilus) microplus.

Coping with the changing conditions 

A simple way to adapt to the changing temperature regime will be to alter the timing of crop planting. If other variables are also favourable, warmer winters may mean an increased growing season, which could have positive effects. Similarly, altering irrigation and fertilisation practices may also help farmers to cope with the changing climatic conditions.

Another option is the expansion or relocation of cropping or pasture GLOSSARY pastureAreas of land vegetated with grasses or other plants suitable for grazing by livestock. areas to locations that have become more suitable under the altered climatic regime. This will be an option only for people who have the means to move easily and so will probably not be a solution for many of the world’s poorest people.

• Changing agriculture regions

  It's agriculture, but not as (where) we know it...

  Changing climatic conditions will most likely change the face of agriculture, shifting the regions of the world where certain crops are able to grow. In some extreme cases, we may see the entire globe become inhospitable to certain species.

  Farmers in the USA corn belt are already struggling under prolonged drier conditions, with some Kansas farmers shifting from the traditional crop of corn to sorghum, a less water demanding crop. Meanwhile, around 1100 km north, in Manitoba, Canada, the cropping area devoted to corn production has nearly doubled over the past ten years. Greenland can now produce vegetables that really don’t belong in an Arctic region.

  Climate change is also posing a threat to our coffee supply. We consume 1.6 billion cups of coffee a day, and the species that gives us most of our coffee, Coffea arabica, is a fussy species native to the highlands of Ethiopia. It prefers temperatures between 18 and 21°C, and the quality of the beans deteriorates above 23°C. If subjected to long periods of more than 30°C the plant loses its leaves and tumours grow on its stems. The water requirements for coffee are also very specific – it needs to be dry to build up buds, and then wet to bring on the flowers, but if it’s too wet, the fruit doesn’t set. The berries then require water to grow and mature.

  Increased temperatures and changing rainfall patterns present a serious problem to coffee production. Indeed, some areas of East Africa and Central America are already struggling to maintain their coffee output. Researchers are working on breeding more resilient varieties of coffee, perhaps augmented with genes from the tougher (but also more bitter tasting) Robusta species (Coffea canephora).

  Another favourite beverage, wine, will also encounter challenges in the face of climate change. Increasing temperatures will see current growing areas become unsuitable for many varieties, and we will most likely see a shift in the types of grapes grown in particular regions. Cooler climate varieties, such as pinot noir or sauvignon blanc, may no longer be produced on mainland Australia, but find a home in Tasmanian vineyards. French wine producers, as well, have seen warmer temperatures cause earlier flowering, budburst, and harvests, with the end product of less acidic, more sugary and more alcoholic (and often less palatable) wines. The European wine industry, which revolves around very specific regional variations, is likely to see some huge and unwelcome changes in the coming years.

Work is underway worldwide using genetic modification and selective breeding to develop new strains of crops that can cope with water shortages or heat stress. Scientists are working to find the genes that enable some plants tolerate extreme conditions. They will then attempt to introduce these genes into the standard varieties of food crops, to improve their tolerance of flood, or pests. While a lot of work has been done to develop drought resistant crops, these are unlikely to provide a silver bullet solution to the problem of water scarcity.

Australian agriculture over time

Australian farmers are already well experienced in managing drought and water scarcity. Over recent decades, Australian farmers have developed management techniques that have enabled them to significantly increase their productivity. These revolve around a realistic assessment of water availability, and then development of management practices that use this water in the most efficient ways possible. For example, during the grip of the Millennium Drought of 1997–2009, farmers noticed that while the traditional autumn rains had dried up, there was more rain in summer, and worked to conserve as much of this summer rain in the soils as possible, and plant varieties that could be sowed early.

• Australian agriculture since the 1900s

  Since the beginning of the 20^th century, agricultural production in Australia has steadily increased, with distinct jumps in production related to specific advances in the understanding of plant physiology and improved land management developments. Australian farming has seen a shift from the traditional European methods of tillage and ploughing, to the modern methods of no-till GLOSSARY no-till A farming method where soil is not ploughed or cultivated. Crop residues are retained on the soil surface and soil disturbance is minimised to help maintain soil structure. and conservation farming GLOSSARY conservation farmingA multifaceted approach to sustainable farming that revolves around maximising water use, minimising soil disturbance, keeping crop residues to provide mulch, reduce erosion and increase soil organic matter and minimising soil compaction by reducing traffic on paddocks. .

  As early as the 1900s, plant breeders were working on developing new strains of wheat suited to the Australian climate. Improved knowledge of plant phenology GLOSSARY phenologyThe study of life cycle events, and how they relate to seasonal or climatic changes. – the life cycle of a plant – has enabled farmers to determine the most suitable planting and flowering times to optimise their limited water supply.

  In the 1930s, farmers started to use ‘ley’ pastures, where legumes and clover are planted to overcome problems with weeds that would grow in otherwise fallow GLOSSARY fallowDescribes a cultivated field where nothing is grown in order for the soil to recover and replenish nutrients. fields. The legumes also improve the nitrogen content of the soil, providing nutrients for subsequent grain crops. Additionally, these fields provided pasture for sheep, giving rise to a mixed farming method that diversified the farmers’ output, and improved resilience.

  Even with the establishment of the ley pastures, the repeated cycles of crop residue burning, cultivation and ploughing that were required to eradicate weeds and prepare seed beds resulted in the deterioration of soil quality over time. Erosion was another big problem.

  The 1970s saw the development of herbicides, chemical controls which killed the weeds, meaning manual weed management was no longer required. This significantly helped to increase farm yields. Effective weed control during fallow periods also helps to ensure sufficient reserves of water in the soil at planting times. Old crop residues are now left in place, and they form a mulch layer that helps protect the soil from the impact of raindrops and erosion, retain soil moisture and maintains hospitable soil temperatures for new seedlings. Planting ‘break crops’ of legumes, canola or lupins in between successive crops of wheat helps to reduce the transmission of pests and disease from one crop to the next. These crops also help to improve the nitrogen content of the soil, and also typically leave a smaller amount of residue, making the subsequent seed planting easier.

  As part of the continuing shift to conservation farming, new harvesting and seed spreading machinery was also developed that enabled farmers to effectively process the old crop residues and plant new seeds through the crop detritus left on the fields. Other new management techniques include traffic control, where all machinery driving through the fields is kept along the same tracks, which keeps compaction from tractor tyres to an absolute minimum, along with improving fuel efficiency. GPS-guided planting techniques enable farmers to sow seeds in between the rows of previous years’ crops. Farmers are now able to generate digital maps of soil condition and yield and assess which areas of their fields may need preferential application of fertilisers or specific management treatments.

  More recent advances in plant breeding have led to the development of plant varieties with longer coleoptiles, the part of the plant shoot that sprouts up to the soil surface. This means the seeds can be sown deeper and access water deeper in the soil.

  The way that Australian farmers have transitioned from the traditional European farming techniques of tillage and ploughing to the modern methods of conservation farming illustrates the Australian agricultural sector’s capacity for adaptation. Australia is currently a world leader in the new innovative strategies involved in conservation farming, with nearly 70% of all arable land in Australia devoted to conservation farming methods. Climate change will present challenges to Australian agriculture, but Australian farmers have a proven track record in effective adaptation and application of innovative approaches to continually improve agricultural outcomes.

Agriculture's contribution to climate change

Ignoring electricity use and transport, the agricultural emissions of CO₂ are roughly balanced by the amount of CO₂ that crops take up from the atmosphere. Agriculture’s overall contribution of CO₂ to the atmosphere is less than 1 per cent of global CO₂ emissions.

However, CO₂ is not the only greenhouse gas (GHG). Agriculture is responsible for the emission of significant amounts of other important GHGs – methane (CH₄) and nitrous oxide (N₂O). Overall, the agricultural sector contributes around 14 per cent of all global GHG emissions.

Nitrous oxide is produced when microbes break down soils and manures. Methane mainly comes from ruminant GLOSSARY ruminantAn animal which chews regurgitated food (cud) and usually has a stomach divided into four compartments, one of which is the rumen. Partially digested food is returned from the stomach to the mouth and chewed, allowing further digestion. Ruminants include cows, sheep, goats, deer and giraffes. Their digestive system allows them to digest fibrous plant material which would be indigestible to other animals. animals – the digestive system that enables cows and sheep to eat large amounts of grass also produces this gas as a waste product, which the animals then burp out.

Australian scientists are working on ways to mitigate sheep methane emissions. Including an Australian native plant called tarbush (Eremophila glabra) in a sheep’s diet helps reduce the amount of methane its digestive system produces.

Predicting the future

Climate science is a complicated beast, and making solid predictions about the future is never easy. Forecasting exactly how our agricultural systems will cope with changing climate conditions is not straightforward, but it is clear that we can’t take bountiful harvests for granted. We will need innovative adaptive measures to ensure that our agricultural systems cope with feeding the future generations.

See our infographic on climate change and agriculture.