feeding a hot, hungry world - agriculture in the face of climate change
Key text
increasing population.
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How will we feed the world’s population as the planet gets warmer?
In 2010, global agricultural production amounted to more than eight and a half billion tonnes of grains, vegetables, meat and other bio-products. Slightly more than eight billion tonnes (that’s more than eight trillion kilograms!) of this was food. Despite this staggering number, more than 870 million people in the world are still hungry.
Global food producers need to meet the needs of a continually rising world population. Our agricultural systems must become even more efficient and produce ever increasing amounts. This is a big ask, and the effects of climate change are going to make it tougher still. The increased levels of CO2 in the Earth’s atmosphere, produced by the burning of fossil fuels and changes in land use, are predicted to cause higher temperatures, altered rainfall patterns, and an increase in extreme weather events (see the Nova - Climate change topics, Climate change - questions and answers, and Nova - Feeding the future - sustainable agriculture for more information).
The complex nature of agricultural systems means there are a number of different variables we must consider when we try to assess how these changes in the Earth’s climate system will affect global food production.
Will plants actually flourish with higher levels of atmospheric CO2?
During the photosynthesis 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 often increases only the vegetative matter of a plant, and not 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 (Box 1: Would you like cyanide with that?).
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, 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.
seen a 10% decrease in rice yields.
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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% 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 (see Nova - Getting the buzz on the value of honeybees for more about bees).
While warmer temperatures can increase the length of the growing season, they will also result in thirstier plants. About 80% 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. The general expectation is that wet areas of the world will get wetter and dry areas drier. 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 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 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 the balance of the existing ecosystem. 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 benefit from a broadened habitat, but cooler species don’t have as many options for relocating to more hospitable waters.
increased oceanic uptake of CO2 will make it harder
for shellfish like oysters to grow their shells.
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Increased temperatures will 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 (see Nova - Acid test for the seas for more about ocean acidification).
Double (triple … quadruple?) whammy
Not only will the world’s agricultural systems have to cope with the impacts of higher global temperatures and altered rainfall patterns, but it is also expected that extreme events, such as heat waves and cold snaps, droughts, floods and bushfires will also increase as the Earth feels the effects of climate change. The potential impact of these events is thought to be even more serious than the effects of changing temperatures and rainfall patterns.
In Australia, we have lived with droughts for a long time and each summer we brace ourselves for bushfire season. Altered rainfall patterns and generally higher temperatures will make many areas already prone to these devastating events even more susceptible. Floods can also wipe out huge areas of crops, 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, powdery mildew and fusarium head blight. 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, 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. With warmer winters, this will mean an increased growing season, which will have some 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 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 (Box 2: It’s agriculture, but not as (where) we know it).
A lot of work is underway 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, pests or poor salty soils. 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 (see Nova - More food, cleaner food, and Nova - Integrated pest management for more about gene technology).
As for coping with hotter and drier conditions, 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 (Box 3: Australian agriculture over time).
How does agriculture contribute to global warming?
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% of global emissions.
However, agriculture is also responsible for the emission of significant amounts of other greenhouse gases (GHGs) – methane (CH4) and nitrous oxide (N2O). About half of the global emissions of these GHGs comes from the agricultural sector. In 2010, this was an amount equivalent to around 4.5 million gigatonnes of CO2. Nitrous oxide is produced when microbes break down soils and manures.
reduces the amount of methane they burp into the atmosphere.
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Methane mainly comes from ruminant 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 (see Nova - Excuse me! The problem with methane gas for more about methane).
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.
Boxes
1. Would you like cyanide with that?
2. It's agriculture, but not as (where) we know it
3. Australian agriculture over time
Credits
Writer:
Academy staff
Reviewers:
Dr John Passioura FAA
Plant Industry
CSIRO
Dr T.J. Higgins FAA
Plant Industry
CSIRO
Posted May 2013.