The Earth is heating up. The effects of human-caused global climate change are becoming more and more apparent as we see more record-breaking heat waves, intense droughts, shifts in rainfall patterns and a rise in average temperatures. And these environmental changes touch every part of crop production.
Around the world, agricultural practices have developed as a function of topography, soil type, crop type, annual rainfall, and tradition. This montage of six images from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) sensor on NASA’s Terra satellite shows differences in field geometry and size in different parts of the world. Credits: NASA's Earth Observatory
NASA, along with partner agencies and organizations, monitors all of these
environmental changes happening today. In addition, NASA uses advanced computer
models that pull in satellite data and then simulate how Earth’s climate will
respond to continued greenhouse gas emissions in the future. Researchers do
this for a range of future scenarios – and then they use the resulting climate
projections to see how climate change will affect global agriculture.
“When we look at future climate change, it's not the same as the current
hot years that we experience,” said Alex Ruane, co-Director of the Climate
Impacts Group at NASA’s Goddard Institute for Space Studies (GISS) in New York
City. He coordinates and leads the climate team for the Agricultural Model
Intercomparison and Improvement Project (AgMIP), an international
project connecting climate science, crop modeling and economic modeling to look
at the potential future of crop yields and food security.
“If we were to find a location and look at a hot year that was recently
experienced, it would likely have been a heat wave that would have raised the
overall temperature,” Ruane said. “Climate change is different. Climate change
is every day, a little bit more and more. When those heat waves come [in the
future], they're just a little bit more intense or extreme, and that has a
different physiological impact [on plants].”
Those physiological changes on plants can be complex and are tied to crop
type and the climate effects seen at the regional and local level.
Carbon Dioxide as a
Fertilizer
Carbon dioxide is the primary greenhouse gas responsible for the increase
in Earth’s global temperature. Emitted from the burning of fossil fuels, it can
stay in the atmosphere for hundreds of years, which means that every year we
are adding carbon dioxide to the amount that has accumulated since the start of
the Industrial Revolution over 200 years ago.
The U.S. Department of Agriculture conducts experiments on the rate of crop growth in controlled environment chambers, including with glasshouses and field plots in which they control the temperature, humidity and atmospheric carbon dioxide. Credits: USDA
Carbon dioxide is
removed from the atmosphere by plants during photosynthesis, (though not in
quantities sufficient enough to remove everything humans emit.) In fact, greenhouse and field experiments have
shown that higher levels of carbon dioxide in the atmosphere can act as a
fertilizer and increase plant growth. The amount of benefit a crop receives
depends on its type. Wheat, barley and rice for example benefit more from
higher carbon dioxide concentrations than corn. More carbon dioxide in the air
makes the plant more efficient at absorbing the gas, and consequently it loses
less water during the process, which is better for the plant’s growth. With
sufficient water and other nutrients, crop yields can increase
significantly.
However, those higher
yields often come with drawbacks for nutrition. “Crops grow faster and bigger
under higher CO2,” said Jonas Jäegermeyr, the coordinator for
the Global Gridded Crop Model
Intercomparison project under AgMIP at GISS. “But the
protein and micronutrient content is proportionally lower.”
Quantity versus quality
is one complication when looking at climate effects on crops. Another is that
while higher carbon dioxide levels bring some benefits, they also bring the
heat.
Turning up the Heat
Increases in regional
temperatures due to climate change, especially in the tropics, can lead to heat
stress for all types of crops. Many crops start feeling stressed at
temperatures above about 90 to 95 degrees Fahrenheit (32 to 35 degrees
Celsius), said Jäegermeyr, although this will vary by crop type and depend on
water availability. Heat stress’s most visible sign is wilting from water loss,
and can lead to permanent damage to the plant.
This color-coded map in Robinson projection displays a progression of changing global surface temperature anomalies. Normal temperatures are the average over the 30 year baseline period 1951-1980. Higher than normal temperatures are shown in red and lower than normal temperatures are shown in blue. The final frame represents the 5 year global temperature anomalies from 2016-2020. Scale in degrees Celsius. Credits: NASA’s Scientific Visualization Studio
Different regions will experience different heat intensities in the future
climate, especially during extreme events like heat waves. “The pattern of
where crops are grown decides the pattern of impacts,” Jäegermyer said. “The
more you grow in the tropics, the harder you will be hit. Because it's already
pretty warm, an additional amount of warming will be more severe than at high
latitudes.”
A 2019 model study simulated future global wheat
production with projected global temperatures
1.5 degrees Celsius and 2.0 degrees Celsius above pre-industrial temperatures.
Taking into account carbon dioxide’s fertilization effect, the results showed
that grain yields for winter or spring-planted wheat rose by about 5% in more
temperate regions such as the United States and Europe, and declined by about 2
to 3% in warmer regions such as Central America and parts of Africa.
Additionally, in hot regions including India, which produces 14% of global wheat,
they more frequently saw years with low wheat yields.
Temperature also affects the life cycle of crops. A small increase in
every-day temperatures during the growing season accelerates the plant’s
lifecycle, said Ruane. “So what ends up happening is the plant matures more
rapidly and at the end of the season when it puts the grain down, it just has
not spent as much time building up leaves, collecting sunlight and making that
energy that you need for the grain.” The result is fewer grains and smaller
crop yields.
Show Me the Water
The last major piece of the puzzle is water. Climate change is affecting
rain and snowfall patterns and giving rise to more extremes in droughts and
rainfall.
“Some areas will see additional rainfall and therefore benefits,” said
Jäegermeyr. “Some regions will receive too much additional rainfall and then
see adverse effects from excess rain. And a ton of regions will actually see
drought.” For example, monsoons may bring more rain to Southeast Asia, and
droughts may become more intense in the Western United States, Australia,
Africa and Central America.
The amount of water available for irrigation is already seeing climate
change impacts. Mountain snow packs are shrinking in the Himalayas and California’s Sierra Nevada, which are primary
sources of both drinking and irrigation water.
Groundwater levels are also sensitive to changes in climate like persistent
drought and excessive rain. A 2018 study showed that where groundwater is used for
agriculture, groundwater levels are generally decreasing both from the
water having been extracted and its sensitivity to change. Additionally, plants
access water in the soil, which in hotter regions and a hotter future is more
prone to evaporation, leaving less for plants to use.
Access to water has a direct effect on crop health, and satellite
observations are one of the key inputs to tools that NASA researchers and
partners are building to help manage our warmer future.
Adaptation
“We care about climate change not because of degrees Celsius or parts per
million CO2, but because those in turn affect all sectors and our
lives,” said Ruane, referring to not only the large-scale agricultural sector
and economy, but also the everyday changes that will happen as communities
respond to climate change.
In addition to looking at the direct consequences of environmental factors
of climate change on crops, research teams within AgMIP are also looking at the
potential for adaptations, management practices and economic incentives that
will help mitigate the worst outcomes.
There are three types of adaptation strategies, said Ruane: things decided
upon every year, such as when to plant and a field’s crop rotation; longer term
investments, such as a new tractor, improved irrigation systems or new
irrigation infrastructure in currently rain-fed areas; and transformative
actions, such as breeding new crop varieties or responding to large-scale
shifts in a population’s diet.
“We can test different options in the virtual fields [of the model],” Ruane
said. “We can also ask questions about how do the prices [calculated in] our
economic models shift if people adopt the type of diet that we have here in the
U.S. versus the Mediterranean diet or east Asian diet.” For example, what
happens when a population eats more or less meat, or shifts from eating more
wheat-based foods to eating more rice-based foods, or vice versa? The models
can also explore other secondary effects of these big changes, especially unanticipated
ones.
Ruane adds, “If we really want to know what's going to happen to farmers or
consumers, we have to bring in the economics of the situation.” As climate
change impacts food systems in the future, the effects will ripple out through
the economy and into households, shaped by how people respond.
Banner image: Climate change is impacting
agriculture in a number of ways. Researchers use satellite data and computer
modeling to monitor and mitigate these impacts. Credits: NASA/Earth
Observatory/USDA/Jesse Kirsch
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