Peter Roach
November 1, 2012
HCOL 185 I
Professor Beckage
Rising anthropogenic emissions of greenhouse gases are changing the
climate, and it is going to have an impact on agricultural systems. Both the
productivity of farms and geographical distribution of crops will be altered by three
factors: rising air temperatures, increasing severity of storms and droughts, and
increasing concentrations of atmospheric carbon dioxide. Since climate is known to
be the primary driver of agricultural productivity, changes in it are raising many
questions about what the future holds for worldwide crop yields.
Rising temperatures play a large role in agricultural efficiency. On the
positive end, these rising temperatures decrease the risk of an extreme frost in the
beginning of the growing season. In the instance of a frost, the primary buds of any
certain crop can be destroyed. The primary buds store the most energy and
therefore produce the healthiest crops. Secondary and tertiary buds are a crop's
back-up mechanism, but have fewer stored carbohydrates and take longer to
develop than the primary buds. If an early season frost occurs, the crop yield is
typically much lesser than it could have been.
In addition to lowering the risk of frost, increasing temperatures also
lengthen the growing season in most agricultural zones. In the northern and
southernmost agricultural regions, temperature increases have shown a trend of
adding about 2 days to the growing season per decade since the 1940s3. This may
not seem like much, but even a week of extra growth can allow plants to get a
greater yield. This is because these crops have one extra week of vegetative growth.
During the vegetative stage of plant development, plants experience vertical growth
as well as the addition of leaves. This leads to more photosynthesis, meaning more
stored sugars that can be used by the plant for fruit and vegetable production. Also,
a larger plant has the capacity to hold a greater yield.
However, this accelerated growth can be bad for crop yields, too. Crops, such
as grains and corn, rely on a slower growth rate in order to allow for their seeds to
fully mature6. If these crops grow too quickly, yield can be severely reduced. In a
study performed by Dr. David Lobell, a researcher at Stanford, worldwide
production of wheat, corn, soybeans, and rice were all observed with respect to
location, temperature, and precipitation. Since 1980, Lobell found that about 65% of
the locations where these crops are grown experienced a positive temperature
change of one standard deviation from the historical record of year-to-year
temperature fluctuations. It was also found that almost 25% of the countries with
these crops experienced an increase in temperature by two standard deviations. The
20 years prior to 1980 returned values of about 0% temperature change in these
areas7.
Acknowledging that there were other factors involved, Lobell observed a
decrease in wheat and corn yield of 5.5 and 3.8%, respectively, between 1980 and
2008. The soybeans and rice did not follow the same trend, as low yields in some
areas balanced out increased yields in others. Using models to project future crop
yields, the study also estimates a 10% yield reduction in these four crops with each
increase of one degree Celsius7.
Increasing global temperatures can even change the geographical
distribution of certain crops. For instance, lower latitudes in the tropics and
subtropics will experience dramatic decreases in agricultural productivity, while the
mid latitudes will see a less dramatic decrease. Higher latitude areas may even see
an increase in productivity and crop variety with the rising temperatures8. This is
because, in high latitudes, the increasing temperature will allow for new crops to be
grown there due to an extended growing season. In the mid to low latitudes, crops
will experience heat and water stress from the speeding up of vital developmental
stages8. This will force these crops to be grown in higher latitudes. Overall, growing
regions will continue to shift more toward the poles as global temperatures keep
rising.
This temperature change will also increase the rate of transpiration in crops.
Transpiration is comparable to sweating in humans. Plants use transpiration as a
cooling mechanism, as well as a driving force behind pulling up water against the
force of gravity. As temperatures increase, plants need to transpire more, meaning
they need more available water. Unfortunately, this excess water is not available in
most places. Instead, plants turn to their own methods of conserving water. They
manage this by closing their stomata, small openings on the undersides of leaves
used for transpiration and gas exchange. However, closing these stomata mean the
plant experiences more heat stress and takes in less carbon dioxide8. Both of these
results are negative. Heat stress causes plants to die by denaturing the pectin middle
lamella, the glue-like substance between plant cell walls that holds the cells in place.
Without the middle lamella, cell slippage occurs, interfering with cell-to-cell
communications13. This can ultimately kill the plant. With regards to the carbon
dioxide, less uptake means less photosynthesis, resulting in lower growth rates and
a lower yield.
Increasing severity of droughts and storms caused by global warming also
has strong impacts on agricultural productivity, and they tend to be negative. As the
air temperature increases, its capacity to hold water vapor also does. This means
that water accumulates in the air for longer before it falls, resulting in extended
periods of no rainfall. Most crops do not do well without a steady water source.
Water serves many purposes to a plant. First, water is needed as a reactant in the
process of photosynthesis. Without water, this process cannot occur, and the plant
will not survive. Second, as mentioned before, water is used a cooling mechanism
for the plant. Last, and most importantly, water is needed to provide the turgor
necessary to enable living cell growth. Turgor is the term used to describe the
pressure within a living plant cell13. In order to give the plant strength and shape,
each cell must be pressurized. Cells become pressurized when their vacuoles fill up
with water and apply outward pressure on their cell walls. Without turgor, cells
shrivel up and cannot function properly, resulting in the appearance of wilting in the
plant13. This can ultimately kill the plant if the problem is not addressed quickly.
Unfortunately, the only way to avoid these issues is through irrigation. This is
not cheap, however. Modern day irrigation is more efficient than old methods
because it requires less digging. In the past, irrigation was achieved through digging
canals in fields, allowing water to flow into the fields and nourish the surrounding
soil. Surface level irrigation, however, is only about 65-80% efficient10. This method
is still used in places like Africa, where water and money are limited. Modern
irrigation methods are reliant on sprinkler systems. This system is more efficient,
with about 80-90% of water sprayed being taken up by plants10. Modern, large-scale
sprinklers, however, are expensive to buy and maintain, and are not a realistic
solution in third-world countries where global warming's effects on agriculture are
seen the most.
Both of these methods also require a large source of fresh water, as plants
cannot thrive off of salt water. This becomes a challenge in poorer countries where
clean water is already limited. This creates a huge dilemma because these countries
have economies that are reliant on agricultural yield. In most cases, they have to
choose between supplying their people or their crops with clean water. Most
countries that can afford more efficient irrigation have yet to run out of clean water,
but it is not unexpected as global temperatures continue to rise.
When the rain finally does fall, however, it comes down harder and longer.
This may seem like it would be good for these dry plants, but that is not the case.
First, this extreme precipitation has the capability to uproot young, underdeveloped
plants in the early season. This is because these plants have very unsubstantial
roots. Young plants focus most of their energy on vertical shoot growth in order to
maximize photosynthesis13. Root growth comes second to this. In addition, since
water supply is limited, water sits lower underground. Plants recognize this, and
send their roots straight down to try to find water. This sacrifices a lot of strength
because the roots tend to be long and spindly, as opposed to thick and well-rooted13.
In addition to the increased risk of uprooting, intense storms and floods can
kill plants by causing their roots to respire anaerobically, without oxygen.
Respiration occurs in both plants and animals, and is used to break down
compounds into usable energy. In plants, glucose, a product of photosynthesis, is
broken down into usable energy through glycolysis. When oxygen is not present,
plant cells undergo fermentation to convert glucose into energy. However, the
energy yield is much less, and the byproducts can prove harmful to the plant. As a
result of fermentation, the plant produces alcohol. If the plant roots are submerged
in this alcohol for too long, they can break down and rot, effectively destroying the
plants water uptake system, killing the plant13. Even if this does not occur, the
plant's inefficiency at converting glucose to usable energy can severely stunt growth
and development. Either way, this reduces crop yield.
Yet another problem that arises from flooding is phosphorus runoff.
Phosphorus is a vital element to plant development. Most of a plant's energy is used
in the form of ATP, of which phosphorus is a main component. In addition to being
vital in energy transfer, phosphorus also contributes to leaf and shoot expansion13.
Without phosphorus, most plants generally do not thrive. Intense storms and floods
typically dissolve a lot of phosphorus and bring it to the surface, where it becomes a
part of the storm runoff. This is bad because this phosphorus is no longer available
to the crops, stunting yield. Not only is this bad for plants, but for aquatic
ecosystems as well. Excess phosphorus in water promotes algae blooms, which in
turn block light and remove oxygen from the water, killing sub-surface plants and
suffocating fish. Phosphorus runoff is bad for pretty much everything, and needs to
be avoided.
Finally, increasing atmospheric carbon dioxide has both positive and
negative effects on crops. Carbon dioxide is a vital compound in the process of
photosynthesis. The addition of carbon dioxide to the atmosphere is often referred
to as "carbon enrichment" for plants. In C3 plants, common to temperate regions,
this extra carbon allows plants to photosynthesize faster, meaning increasing
growth and sugar storage. Ignoring all other factors, an increase from 350-700 ppm
in atmospheric concentration of carbon dioxide would lead to a 66% increase in
photosynthetic activity in C3 plants, which include wheat, grains, cotton, and tree
crops12. This increase in photosynthetic activity would result in larger plants with
more stored energy, leading to greater crop yield.
However, in C4 plants common to tropical regions, this effect is barely seen.
These crops include corn and sugarcane. Only a 4% increase in photosynthetic
activity is projected for the doubling of atmospheric carbon dioxide. Relative to C3
plants, this is a very gradual increase. This will result in greater competition
between crops and weeds. This is because C3 weeds in C4 crop fields will be outcompeting the crops. However, the reverse is true for C3 crops out-competing C4
weeds12.
Agricultural scientist Dr. Bruce Kimball performed a study in a closed
environment to measure the effects of carbon enrichment12. He observed an average
of 33% yield increase among 37 different C3 crops. In C4 crops, he only observed an
average of a 14% increase in yield. However, all other factors remained the same
throughout this experiment, including temperature and water availability. This is
not the case in the environment today, as rising carbon dioxide levels are also
increasing temperature, therefore decreasing water supply. However, this does
provide strong evidence for the validity of carbon enrichment.
In addition, increasing atmospheric carbon dioxide causes the partial closing
of stomata on plants12. This leads to a decreasing transpiration rate. As stated
before, this will cause plants to warm up. This can have both positive and negative
effects, but it all depends on the crop. If the crop is currently being grown at a lowerthan-ideal temperature, this will be a good thing. However, if the crop is already at
optimum temperature or above, this will decrease yield.
Another interesting effect of stomatal closing is a decrease in
evapotranspiration, the rate of water loss per unit of land area. In 1990, climate
researcher Dr. Norman Rosenberg used climate models and other equations to
predict the effect of this decreasing evapotranspiration12. What he found was that all
of his climate models showed anywhere from a 0-2% increase in evapotranspiration
when accounting for changing temperatures, wind speed, vapor pressure, radiation,
and stomatal conductance as a result of climate change. In this case, carbon
enrichment was observed to have almost no effect on evapotranspiration.
Much is still very unclear about the effects of carbon enrichment, as the
climate is a very sensitive system. Carbon enrichment is effective when all other
conditions remain unchanged. But, when temperature and water availability are
accounted for, this enrichment is much less prevalent. Not to mention, all of these
factors are dependent on the others, and their complex relationships are still
unclear. Nobody knows how much temperature and carbon dioxide concentration
will increase in relation to each other, so the only things to rely on are climate
models. Unfortunately, these models only take into account the information that
climate scientists already know and understand, which is not everything.
As far as the social and economical effects of climate change on agriculture
are concerned, much is unclear about what the future holds. Some areas, such as
northern Africa, have already experienced many of the effects of climate change
previously explained. These countries need to turn to alternative practices in order
to survive. Irrigation is becoming more and more important, but it is very expensive
and hard to control. Also, agricultural practices are beginning to shift away from
monoculture, the cultivation of one crop. Crop variety allows for better crop yield in
some instances. This is because with changing conditions, farmers will need to redetermine which crops are best for their area. By growing a lot of different plants,
this increases the odds of finding the optimal crop. However, this can be expensive
because all crops take different care and different equipment.
In places where the effects are starting to show, like the southwestern US, it
is still uncertain as to what steps will be taken to increase productivity. The shift to
urban farming has become very popular in countries that can afford it, as it is easier
to control and manipulate indoor growing environments. Crops can now be grown
in every area of the world in greenhouses. This also allows crops to be grown where
population density is greatest, lowering shipping costs, decreasing fossil fuel
emissions from transportation, and increasing the quality of the produce. It is clear
that much is still unsure about what the future climate holds for our agricultural
system, but it does not look good if changes are not made in our methodology.
References
1. Plant Ecology - Volume 104-105, Number 1 (1993), 65-75
B. A. Kimball, J. R. Mauney, F. S. Nakayama and S. B. Idso
2. Perspectives in World Food and Agriculture - Volume 2, 2005, 185-209
John A. Miranowski, Colin G. Scanes
3. The American Economic Review - Volume 84, Number 4 (1994), 753-771
"The Impact of Global Warming on Agriculture: A Ricardian Analysis"
Robert Mendelsohn, William D. Nordhaus and Daigee Shaw
http://www.jstor.org/stable/10.2307/2118029
5. Climate Change: Picturing the Science - 2009
Gavin Schmidt, Joshua Wolfe
6. http://www.epa.gov/climatechange/impacts-adaptation/agriculture.html
7. "Climate Trends and Global Crop Production Since 1980" - Science, July 29, 2011
David B. Lobell, Wolfram Schlenker, Justin Costa-Roberts
http://www.sciencemag.org/content/333/6042/616.full
8. Climate Research - Volume 11: 19-30, December 17, 1998
"Effects of global climate change on agriculture: an interpretative review"
Richard M. Adams, Brian H. Hurd, Stephanie Lenhart, Neil Leary
http://www.int-res.com/articles/cr/11/c011p019
9. Climate Change 1995-IPCC Second Assessment - 1995
"A Report of the Intergovernmental Panel on Climate Change"
http://www.ipcc.ch/pdf/climate-changes-1995/ipcc-2nd-assessment/2ndassessment-en.pdf
10. "Irrigation Efficiency" - United States Department of Agriculture
Terry A. Howell
http://www.cprl.ars.usda.gov/pdfs/Howell-Irrig%20EfficiencyEncy%20Water%20Sci.pdf
11. "Potential impacts of climate change on world food supply" - Nature, 1994
Cynthia Rosenzweig and Martin L. Parry
http://ecoethics.net/cyprus-institute.us/PDF/Rosensweig-FoodSupply.pdf
12. "Effects of increasing atmospheric CO2 on vegetation" - Vegetatio, 1993
B. A. Kimball, J. R. Mauney, F. S. Nakayama, and S. B. Idso
13. Plant Physiology - Fifth Edition
Lincoln Taiz, Eduardo Zeiger