The Intergovernmental Panel on Climate Change (IPCC) has just released an important report outlining the evidence for past and future climate change. Unfortunately it confirms our fears; climate change is occurring at an unprecedented rate and humans have been the dominant cause since the 1950s. Atmospheric carbon dioxide (CO₂) has reached the highest level for the past 800,000 years, which has contributed to the increased temperatures and extreme weather we have already started to see.
As a plant scientist, I’m interested in the complicated effects that increased temperatures, carbon dioxide and changes in rainfall will have on global food security. Professor David Lobell and Dr Sharon Gourdji wrote about some of the possible effects of climate change on crop yield last year, summarised below alongside IPCC data.
|Image credit: David Monniaux|
Plants produce their food in a process called photosynthesis, which uses the energy of the sun to combine CO₂ and water into sugars (food) and oxygen (a rather useful waste product). The IPCC reports that we have already increased atmospheric CO₂ levels by 40% since pre-industrial times, which means it is at the highest concentration for almost a million years. Much of this has accumulated in the atmosphere (terrible for global warming) or been absorbed into the ocean (causing ocean acidification) however it may be good news for plants.
Lobell and Gourdji wrote that higher rates of photosynthesis are likely to increase growth rates and yields of many crop plants. Unfortunately, rapid growth can actually reduce the yields of grain crops like wheat, rice and maize. The plants mature too quickly and do not have enough time to move the carbohydrates that we eat into their grains.
The IPCC predicts that by the end of the 21st century, temperatures will be 1.5C to 4.5C higher than they were at the start of it. There will be longer and more frequent heat waves and cold weather will become less common.
Extremely high temperatures can directly damage plants, however even a small increase in temperature can impact yields. High temperatures means plants can photosynthesise and grow more quickly, which can either improve or shrink yields depending on the crop species (see above). Lobell and Gourdji noted that milder spring and autumn seasons would extend the growing period for plants into previously frosty times of year allowing new growth periods to be exploited, although heat waves in the summer may be problematic.
|Image credit: IPCC AR5 executive summary|
Flooding and droughts
In the future, dry regions will become drier whilst rainy places will get wetter. The IPCC predicts that monsoon areas will expand and increase flooding, but droughts will become longer and more intense in other regions.
In flooded areas, waterlogged soils could prevent planting and damage those crops already established. Drought conditions mean that plants close the pores on the leaves (stomata) to prevent water loss, however this means that carbon dioxide cannot enter the leaves for photosynthesis and growth will stop. This may be partly counteracted by the increased carbon dioxide in the air, allowing plants to take in more CO₂ without fully opening their stomata, reducing further water loss and maintaining growth.
Image credit: IPCC AR5 executive summary
These factors (temperature, CO₂ levels and water availability) interact to complicate matters further. High carbon dioxide levels may mean plants need fewer stomata, which would reduce the amount of water they lose to the air. On the other hand, higher temperatures and/or increased rainfall may mean that crop diseases spread more quickly and reduce yields.
Overall Lobell and Gourdji state that climate change is unlikely to result in a net decline in global crop yields, although there will likely be regional losses that devastate local communities. They argue that climate change may prevent the increases in crop yields required to support the growing global population however.
The effect of climate change on global crop yields is extremely complex and difficult to predict, however floods, drought and extreme temperatures will mean that its impact on global food security (“when all people at all times have access to sufficient, safe, nutritious food to maintain a healthy and active life”) will almost certainly be devastating.
On the basis of the IPCC report and the predicted impact of climate change on all aspects of our planet, not just food security, it is critical that we act quickly to prevent temperature and CO₂ levels rising any further.
This blog is written by Sarah Jose, Biological Sciences, University of Bristol
You can follow Sarah on Twitter @JoseSci