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.
Increased CO₂
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| 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.
High temperatures
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.
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| 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.
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Image credit: IPCC AR5 executive summary
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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.
You can follow Sarah on Twitter @JoseSci
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Sarah Jose
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