Higher growing season temperatures
can have dramatic impacts on agricultural productivity, farm incomes, and food
security. We used observational data and output from 23 global climate models
to show a high probability (>90%) that growing season temperatures in the
tropics and subtropics by the end of the 21st century will exceed the most
extreme seasonal temperatures recorded from 1900 to 2006. In temperate regions,
the hottest seasons on record will represent the future norm in many locations.
We used historical examples to illustrate the magnitude of damage to food
systems caused by extreme seasonal heat and show that these short-run events
could become long-term trends without sufficient investments in adaptation.
The food crisis of 2006–2008
demonstrates the fragile nature of feeding the world's human population. Rapid
growth in demand for food, animal feed, and biofuels, coupled with disruptions
in agricultural supplies caused by poor weather, crop disease, and export
restrictions in key countries like India and Argentina, have created chaos in
international markets .Coping
with the short-run challenge of food price volatility is daunting. But the
longer-term challenge of avoiding a perpetual food crisis under conditions of
global warming is far more serious. History shows that extreme seasonal heat
can be detrimental to regional agricultural productivity and human welfare and
to international agricultural markets when policy-makers intervene to secure
domestic food needs.
We calculated the difference between
projected and historical seasonally averaged temperatures throughout the world
by using output from the 23 global climate models contributing to the
Intergovernmental Panel on Climate Change's (IPCC) 2007 scientific synthesis .
Our results show that it is highly likely (greater than 90% chance) that
growing season temperatures by the end of the 21st century will exceed even the
most extreme seasonal temperatures recorded from 1900 to 2006 for most of the
tropics and subtropics. Presently there are more than 3 billion people living
in the tropics and subtropics, many of whom live on under $2 per day and depend
primarily on agriculture for their livelihoods. With growing season
temperatures rising beyond historical bounds, the inevitable question arises:
Will people in these regions have sufficient access to food to meet population-
and income-driven growth in demand in the future, and thus to achieve food
security?
The IPCC concluded that elevated
greenhouse gas concentrations are likely to lead to a general drying of the
subtropics by the end of this century, creating widespread stress on
agriculture. Although much attention is focused on threats of increased
droughts in subtropical agriculture, the potential impacts of seasonal average
temperature changes in both the tropics and subtropics, which are expected to
be large relative to the historical range of variation, are often overlooked.
Experimental and crop-based models for major grains in these regions show
direct yield losses in the range of 2.5 to 16% for every 1°C increase in
seasonal temperature supporting online
material (SOM)]. Large additional losses are expected from sea level rise and
decreased soil moisture caused by higher average temperatures. Despite the
general perception that agriculture in temperate latitudes will benefit from
increased seasonal heat and supply food to deficit areas, even mid-latitude
crops will likely suffer at very high temperatures in the absence of adaptation.
Global climate change thus presents widespread risks of food insecurity.
It is conceivable that the warmest
summers during the past century will represent the norm by the end of this
century. But what if the average future seasonal temperature were to exceed the
hottest seasons on record? Entering a whole new realm of high seasonally
averaged temperatures, not just multiday heat waves, will surely challenge the
global population's ability to produce adequate food in the future or even to
cope physically with chronic heat stress, unless major adaptations are made.
Hypothetical distributions of summer
season temperatures from 1900–2000 and 2080–2100. x axis indicates
seasonal temperature; y axis, probability of occurrence (number of years
in the century). (A) The highest growing-season temperature of the 20th
century represents the median seasonal temperature by the end of the 21st
century. (B) Future temperatures are out-of-bounds hot: that is, it is
certain that the growing season temperature at the end of the 21st century will
exceed the hottest growing season ever observed.
To put in perspective, recall the
record hot summer in Western Europe in 2003 when an estimated 52,000 people
died between June and August from heat stress, making it one of the deadliest
climate-related disasters in Western history. The most intense seasonal
temperature and the majority of fatalities were centered in France and northern
Italy, where over 30,000 people perished from heat-related causes. In France,
the mean summer temperature (June to August) was 3.6°C (3.5 standard
deviations) above the long-term mean. Unfortunately, by the end of the century,
summer heat like that of 2003 is likely to be the norm for the country.
Histogram of summer (June, July, and
August) averaged temperatures (blue) observed from 1900 to 2006 and (red)
projected for 2090 for (A) France, (B) Ukraine, and (C)
the Sahel. Temperature is plotted as the departure from the long-term
(1900–2006) climatological mean. The data are normalized to represent 100
seasons in each histogram. In (A), for example, the hottest summer on record in
France (2003) is 3.6°C above the long-term climatology. The average summer
temperature in 2090 is projected to be 3.7°C greater than the long-term
climatological average, and there is a small chance it could be 9.8°C higher.
Severe heat in the summer of 2003
affected food production as well as human lives in Europe. Record high daytime
and nighttime temperatures over most of the summer growing season reduced leaf
and grain-filling development of key crops such as maize, fruit trees, and
vineyards; accelerated crop ripening and maturity by 10 to 20 days; caused
livestock to be stressed; and resulted in reduced soil moisture and increased
water consumption in agriculture (SOM).
Italy experienced a record drop in maize yields of 36% from a year earlier,
whereas in France maize and fodder production fell by 30%, fruit harvests
declined by 25%, and wheat harvests (which had nearly reached maturity by the
time the heat set in) declined by 21%. These production shortfalls hurt the
region's farmers economically, although global food trade, subsidies, and insurance
compensation helped to avert serious price hikes or reductions in regional or
global food security.
By comparison, extremely high
summer-averaged temperature in the former Soviet Union (USSR) in 1972
contributed to disruptions in world cereal markets and food security that
remain a legacy in the minds of food policy analysts to this day. What sticks
in most people's minds is the towering price spike between 1972 and 1974 that
occurred within a long-term trend decline in grain prices during a 50-year
period after World War II. Nominal prices for wheat—the crop most affected by
the USSR weather shock—rose from $60 to $208 per metric ton in international
markets between the first quarters of 1972 and 1974, and real prices more than
tripled. Although extreme summer averaged temperature in the USSR was among
several factors contributing to the international price spike, this climate
event was largely responsible for setting the dynamics in motion.
The prolonged hot period in the
summer of 1972 in southeast Ukraine and southwest Russia—major breadbaskets in
the former USSR—ranks in the top 10% of temperature anomalies over the
observational period 1900–2006. Summer temperatures in this region ranged from
2° to 4°C above the long-term mean. The vast majority of news reports at the
time focused on drought as opposed to extreme heat, although fully one-third of
summers in this area over the past 100 years were drier than in 1972 (only 0.5
standard deviations below the long-term mean). A peak of high temperatures
exceeding 30°C set in during July and August during key crop development stages
for wheat and coarse grains, causing a 13% decline in grain production from a
year earlier for the USSR as a whole (SOM). Such high summer temperatures in the
region will likely be the norm in 2050 and well below the median of projected
summer temperature by the end of the century.
The USSR had long been known for its
variable climate and crop yields. What changed in 1972 was the Soviets'
unexpected intervention in international markets to compensate for anticipated
crop shortfalls—a marked shift from their earlier policy of internal
adjustments through the culling of livestock herds. The USSR entered the world
grain market at a time when import demand was rising rapidly in Asia because of
expanding populations, low-yield growth, and (in the same year) weak monsoon
rains. Governments in several developing countries, particularly in Asia,
feared political instability with rising grain prices and implemented food
self-sufficiency (minimum trade) policies that remained in effect for decades.
A major lesson from this case and
the recent food crisis is that regional disruptions can easily become global in
character. Countries often respond to production and price volatility by
restricting trade or pursuing large grain purchases in international
markets—both of which can have destabilizing effects on world prices and global
food security. In the future, heat stress on crops and livestock will occur in
an environment of steadily rising demand for food and animal feed worldwide,
making markets more vulnerable to sharp price swings. High and variable prices
are most damaging to poor households that spend the majority of their incomes
on staple foods.
Another region at risk from higher
temperatures is the Sahel, where crop and livestock production play an
essential role in the region's economy, employing roughly 60% of the active
population and contributing 40% to gross national product. The Sahel suffered a
prolonged drought from the late 1960s to the early 1990s that caused crop and
livestock productivity to plummet, and which contributed to countless
hunger-related deaths and unprecedented rates of migration from north to south,
from rural to urban areas, and from landlocked to coastal countries. Although
the Sahel's climate disaster was largely one of extended drought, the specter
of high and rising temperature lurks in the background. Year-to-year
temperature variability in the Sahel has been low during the past century
(particularly in comparison with temperate countries like France and Ukraine),
but the growing season temperature has been very high, with long-term daily
averaged summer temperature ranging from 25°C in the south to 35°C in the
north. Moreover, temperatures have trended upward since 1980. Despite rains
returning to some locations of the Sahel during the past 15 years, the growing
season for staple crops has been reduced, maize yields have remained far below
varietal potential, and millet and sorghum yields continue to stagnate.
Hundreds of thousands of children and infants in the region still die each year
from hunger-related causes, and malnutrition contributes to long-term mental
and physical disabilities. Over recent decades most of the region's poorest
households have lost their livestock or other assets; they remain net consumers
of food and struggle to purchase staples even when they are available in the
market. These households farm at an extreme disadvantage irrespective of
climate change, with limited access to improved crop varieties, seed supplies,
fertilizers, credit, and irrigation and transportation infrastructure.
Most worrisome for the Sahel is that
average growing season temperatures by the end of this century, and even
earlier for some parts of the region, are expected to exceed the hottest
seasons recorded during the past century. Such heat will compound food
insecurity caused by variable rainfall in the region, and it will increase the
incidence of agricultural droughts (as opposed to meteorological droughts)
defined by elevated evapotranspiration, low soil moisture, and high rates of
water runoff from hard pan soils when it rains. Even today, temperatures in the
Sahel can be so high that the rain evaporates before it hits the ground. New
bounds of heat stress will make the region's population far more vulnerable to
poverty and hunger-related deaths and will likely drive many people out of
agriculture altogether, thus expanding migrant and refugee populations.
These historical examples illustrate
the profound damage that can be caused (or in the near future may be caused) by
high seasonal heat, but they also represent short-run impacts. We chose France
(2003) and Ukraine (1972) as examples specifically because temperature
deviations were large in comparison to precipitation deviations relative to the
observational record. In the Sahel case, drought and heat stress are tightly
coupled, with heat stress becoming increasingly important during the past decade.
The threats to food security and human lives caused by unusually high seasonal
temperature in France, Ukraine, and the Sahel in the 20th century were
ameliorated when the extreme temperatures subsided, when markets balanced acute
regional food deficits with food surpluses from other locations, and when
farmers autonomously adapted their practices or migrated. The future, however,
could be entirely different. If growing season temperatures by the end of the
21st century remain chronically high and greatly exceed the hottest temperature
on record throughout the much of the world, not just for these three examples,
then global food security will be severely jeopardized unless large adaptation
investments are made.
Climate model projections from the
IPCC 2007 assessment suggest that this outcome is indeed very likely. Shows
that, as early as 2050, the median projected summer temperature is expected to
be higher than any year on record in most tropical areas. By the end of the
century, it is very likely (greater than 90% chance) that a large proportion of
tropical and subtropical Asia and Africa will experience unprecedented seasonal
average temperature, as will parts of South, Central, and North America and the
Middle East. High seasonal temperatures beyond what has been experienced during
the past century will thus become widespread.
Likelihood (in percent) that future
summer average temperatures will exceed the highest summer temperature observed
on record (A) for 2050 and (B) for 2090. For example, for places
shown in red there is greater than a 90% chance that the summer-averaged
temperature will exceed the highest temperature on record (1900–2006)
Three important conclusions can be
drawn from these projections. First, tropical countries experience less
year-to-year temperature extremes than do temperate countries and therefore
will be the first to experience unprecedented heat stress because of global
climate change. By the end of the century, however, the seasonal growing
temperature is likely to exceed the hottest season on record in temperate
countries (e.g., equivalent to what France experienced in 2003), and the future
for agriculture in these regions will become equally daunting.
Second, the projected seasonal
average temperature represents the median, not the tail, of the climate
distribution and should therefore be considered the norm for the future.
Indeed, the probability exceeds 90% that by the end of the century, the summer
average temperature will exceed the hottest summer on record throughout the
tropics and subtropics. Because these regions are home to about half the
world's population, the human consequences of global climate change could be
enormous.
Lastly, with growing season
temperatures in excess of the hottest years on record for many countries, the
stress on crops and livestock will become global in character. It will be
extremely difficult to balance food deficits in one part of the world with food
surpluses in another, unless major adaptation investments are made soon to
develop crop varieties that are tolerant to heat and heat-induced water stress
and irrigation systems suitable for diverse agroecosystems. The genetics,
genomics, breeding, management, and engineering capacity for such adaptation
can be developed globally but will be costly and will require political
prioritization. National and international agricultural investments have been
waning in recent decades and remain insufficient to meet near-term food needs
in the world's poorest countries, to say nothing of longer-term needs in the
face of climate change. History provides some guide to the magnitude and
effects of high seasonal averaged temperature projected for the future.
Ignoring climate projections at this stage will only result in the worst form
of triage.
References
and Notes
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To
calculate the projected climate for 2090, we first added the observed
temperature departures (in blue) to the change in the summer temperature, taken
to be the mean summer temperature for 2080–2100 minus that for 1980–2000,
simulated by each of the 23 climate models from the IPCC AR4 forced by the
“middle of the road” emission scenario, A1B. We then combined the 23 × 107
projections to create the probability distribution function for summer
temperature in 2090 (see SOM).
The
probability distribution of future summer temperature is calculated as
described in (21). Here, summer is defined north of the equator
as the average temperature from June through August and south of the equator as
December through February. In the immediate vicinity of the equator, values in Fig. 3 are qualitatively insensitive to the
choice of months that define the season.
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