The US is a major supplier of food and fiber for the world,
for more than 25% of the total global trade in wheat, corn,
cotton. Cropland currently occupies about 400 million acres, or
17% of the
total US land area. In addition, grasslands, and permanent grazing
pasturelands, occupy almost 600 million acres, another 26% of US
area. The value of agricultural commodities (food and fiber)
billion at the farm level and over $500 billion, 10% of GDP, after
processing and marketing.
Economic viability and competitiveness are major concerns for
trying to maintain profitability as real commodity prices have
about two-thirds over the last 50 years. Agricultural productivity
improved at over 1% per year since 1950, resulting in a decline in
production costs and prices. This trend maintains intense pressure
individual producers to continue to increase the productivity of
farms and to reduce costs of production. In this competitive
environment, producers see anything that might increase costs or
their markets as a threat to their viability. Issues of concern
regulatory actions that might increase costs, such as efforts to
the off-site consequences of soil erosion, agricultural chemicals,
livestock wastes; growing resistance to and restrictions on the
genetically modified crops; extreme weather or climate events such
droughts and floods; new pests; and the development of pest
existing pest control strategies. Future changes in climate will
with all of these factors.
The agriculture sector Assessment considered crop agriculture,
livestock, and environmental effects of agriculture. The focus in
document is primarily on crop agriculture which was studied most
intensively in this Assessment. Although extensive, the analysis
yields did not fully consider all of the consequences of possible
in pests, diseases, insects, and extreme events resulting from
change. This analysis assumes continued technological advances and
changes in federal policies or international trade.
Crop Yield Changes and Associated Economic Consequences
It is likely that climate change, as defined by the scenarios
in this Assessment, will not imperil the ability of the US to feed
population and to export foodstuffs. Results of this Assessment
that, at the national level, productivity of many major crops will
increase under the climate scenarios used in these crop models.
showing generally positive results include cotton, corn for grain
silage, soybeans, sorghum, barley, sugar beets, and citrus fruits.
Pastures also show positive results.
For other crops, including wheat, rice, oats, hay, sugar cane,
potatoes, and tomatoes, yields are projected to increase under
conditions and decrease under others. The crop models assume that
fertilization effect will be considerable (see box).
Effects on Crops|
The actual response to increased CO2
differs among crops. Most commercial crops in the US,
wheat, rice, barley, oats, potatoes, and most vegetable
to respond favorably to increased CO2, with a doubling of
atmospheric CO2 concentration leading to yield increases in
range of 15-20%. The crop models used in this Assessment
CO2 fertilization effect in this range, and also assume that
sufficient nutrients and water will be available to support
increases. Other crops including corn, sorghum, sugar cane,
tropical grasses, are less responsive to increases in CO2,
doubling of its concentration leading to yield increases of
In situations where crop yields are
severely limited by factors such as nutrient availability,
enduring CO2 fertilization effect is very likely to be of
Greater concentrations of CO2
generally result in higher photosynthesis rates and may also
water losses from plants. Photosynthesis is enhanced when
carbon is available for assimilation and so crop yields
In the crop yield models, a limited set of on-farm adaptation options
are considered, including changes in planting dates and changes in
varieties. These contribute small additional gains in yields of
crops and greater gains in yields of irrigated crops. The economic
consider a far wider range of adaptations in response to changing
productivity, prices, and resource use, including changes in crops
location of cropping, irrigation, use of fertilizer and
pesticides, and a
variety of other farm management options.
Model simulations of average
changes in crop yields for 16 crops. The yield changes are
as percentages and represent the differences between
yields and those projected for two time periods, 2030 and
Two scenarios of future climate, the Canadian and Hadley,
used. The results consider physiological responses of the
climate under either dryland or irrigated cultivation.
consider either "no adaptation" or
"adaptation" responses by producers to climate change.
Adaptations included changes in planting dates and crop
Only 11 of the 16 crops were actually modeled: cotton,
(winter and summer), corn, hay, potato, orange, soybean,
rice, pasture grass. Results for the other crops are based
extrapolations from the modeled crops.
All agricultural regions of the US are not affected to the same
by the climate scenarios studied in this Assessment. In general,
study finds that climate change favors northern areas. The
and Pacific Northwest exhibit large gains in yields with both
scenarios in the 2030 and 2090 time frames. Crop yield changes in
regions vary more widely depending on the climate scenario and
period. For example, projected wheat yields in western Kansas
under the Canadian scenario.
Model simulations suggest that the net effects of the climate
studied on the agricultural segment of the US economy over the
century are generally positive. The exceptions are simulations
Canadian scenario in the 2030 time period, particularly in the
Economically, consumers benefit from lower prices while
profits decline. Under the Canadian scenario, these opposing
effects are nearly balanced, resulting in a small net effect on
national economy. The estimated $4-5 billion reduction in
profits represents a 13-17% loss of income, while the savings of
billion to consumers represent less than a 1% reduction in the
food and fiber expenditures. This large difference exists because
the final cost of agricultural goods to consumers reflects
transportation, and retailing costs that the models used here
not affected by climate. Under the Hadley scenario, producers'
decline by up to $3 billion (10%), while consumers save $9-12
the range of 1%).
Economic Impacts of climate change under the Canadian and Hadley
climates. The economic index is change in welfare expressed
sum of producer and consumer surplus in billions of dollars.
(light blue bar above) includes sales and purchases in the
Total Surplus (dark blue bar) also includes overseas sales
The major difference between the model outputs is that under the
scenario, productivity increases are substantially greater than
Canadian, resulting in lower food prices, to the consumers'
benefit. The smaller producer losses in the Hadley scenario,
greater productivity gains and price changes, reflect the fact
that the US
farmers' advantage over foreign competitors grows and they are
to significantly increase export volume. Analyses show that
versus consumer effects depend on how climate change affects
elsewhere in the world. The sector Assessment was not able to
estimates on crop and livestock production to other regions of the
but used worldwide shifts in crop and livestock production
Regional production change, the total value of crop and
production, is positive for all regions in both the 2030 and 2090
frames under the Hadley scenario. Adaptation measures have a small
additional positive effect. In contrast, this economic index
regions under the Canadian scenario in both the 2030s and 2090s.
positive for most northern regions, mixed for the northern Plains,
negative for Appalachia, the Southeast, the Delta states, and the
Plains. Adaptation measures help somewhat for the southern
the value of production is lower in these regions under both the
2090 climates considered.
Regional production change (crop
and livestock production weighted by prices) from a year
baseline was positive for all regions in both the 2030 and
timeframes under the Hadley scenario. In contrast this
differed among regions under the Canadian scenario in both
2030s and 2090s.
It was positive for most northern
for the northern Plains, and negative for Appalachia, the
Southeast, the Delta states and the southern Plains.
Changing Water Demands for Irrigation
At the national level, the models used in this Assessment find
irrigated agriculture's need for water declines approximately
2030, and 30-40% for 2090 in the context of the two primary
scenarios. At least two factors are responsible for this possible
reduction. One is increased precipitation in some agricultural
other is that faster development of crops due to higher
results in a reduced growing period and thereby reduced water
the crop modeling analyses done for this Assessment, shortening of
growing period reduces plant water-use enough to more than
the increased water losses from plants and soils due to higher
The picture for future agricultural water demands at the
is less clear and it is possible that it will differ substantially
the national picture. At the regional level, there is the
overall water use will increase in response to climate change.
Without adaptation, predicted irrigated yields from 2030 to 2090 offer a less unified prediction for some crops, notably barely and corn.
With adaptation, predicted irrigated yields from 2030 to 2090 offer a more unified prediction for virtually all crops.
Surface Water Quality
A case study of agriculture in the drainage basin of the
was undertaken to analyze the effects of climate change on
quality. The Bay is a highly valuable natural resource that has
severely degraded in recent decades. Soil erosion and excess
runoff from crop and livestock production have played a major role
decline of the Bay's health.
In simulations for this Assessment, under the two climate
2030, loading of excess nitrogen into the Chesapeake Bay due to
production increases by 17-31% compared with the current
projected effects may not fully represent the effects of extreme
events such as floods or heavy downpours that wash large amounts
fertilizers and animal manure into surface waters. Changes in
practices, such as better matching of the timing of plant need for
fertilizer with the timing of application, could possibly help to
the projected impacts. Because efforts are already underway to
Bay, many of these practices may be required and in use before
The Assessment investigates the relationship between pesticide
climate for crops that require relatively large amounts of
Pesticide use is projected to increase for most crops studied and
states, under the climate scenarios considered. Increased need for
pesticide application on corn is generally in the range of 10-20%,
potatoes, 5-15%, and on soybeans and cotton, 2-5%. The results for
vary widely by state and climate scenario showing changes in
application ranging from approximately -- 15 to +15%.
The increase in pesticide use results in slightly poorer
economic performance, but this effect is quite small because
expenditures are a relatively small share of production costs.
Assessment approach does not consider increased crop losses due to
implicitly assuming that all additional losses are eliminated
increased pest control measures. This may underestimate losses due
pests associated with climate change.
In addition, this Assessment does not consider the
consequences of increased pesticide use and it is possible that
would be substantial. In a complete economic analysis, the costs
negative impacts of pesticides on the environment would be
The consequences of climate change for US agriculture are very
to be affected by changes in climate variability and extreme
Agricultural systems are vulnerable to climate extremes, with
varying from place to place because of differences in soils,
systems, and other factors. Changes in precipitation type (rain,
hail), timing, frequency, and intensity, along with changes in
(windstorms, hurricanes, and tornadoes), are likely to have
consequences. Heavy precipitation events cause erosion,
leaching of animal wastes, pesticides, fertilizers, and other
into surface and groundwater.
A major source of weather variability is the El NiÃ±o Southern
Oscillation (ENSO). ENSO effects vary widely across the country.
prediction of these events would likely allow farmers to plan
altering their choices of which crops to plant and when to plant
value of improved forecasts of ENSO events under their current
and frequency has been estimated at approximately $500 million per
As climate warms, ENSO is likely to be affected. Some models
more frequent El NiÃ±os and stronger La NiÃ±as will have increasing
impacts on US weather. The potential impacts of changes in
strength of ENSO conditions on agriculture were modeled in this
Assessment. An increase in these conditions is found to cost the
million per year if accurate forecasts of these events are
farmers use them as they plan for the growing season. The increase
is projected to be greater if accurate forecasts are not available
Reductions in corn yields often correspond to extreme climate
events including droughts and floods. The record Midwest
1993 resulted from this being the wettest year on record,
out and flooding many corn fields and resulting in late
In 1995, declines in yields resulted from a
of unusual climate events; a cool wet spring delayed
a hot, dry summer affected pollination, and ultimately,
Adaptations such as changing planting dates and choosing longer
varieties are likely to offset losses or further increase yields.
measures are likely to be particularly critical for the Southeast
of the large reductions in yields projected for some crops under
severe climate scenarios examined. Breeding for response to CO2
likely be necessary to achieve the strong fertilization effect
the crop studies. This is an unexploited opportunity and the
selecting for CO2 response are good. However, attempts to breed
single characteristic are often not successful, unless other
interactions are considered. Breeding for tolerance to climatic
already been heavily exploited and varieties that do best under
conditions usually also outperform other varieties under stress
conditions. Breeding specific varieties for specific conditions of
stress is therefore less likely to encounter success.
Some adaptations to climate change and its impacts can have
secondary effects. For example, an examination of use of water
Edward's aquifer region around San Antonio, Texas found increased
pressure on groundwater resources that would threaten endangered
dependent on spring flows supported by the aquifer. Another
relates to agricultural chemical use. An increase in the use of
and herbicides is one adaptation to increased insects, weeds, and
associated with warming. Runoff of these chemicals into prairie
groundwater, and rivers and lakes could threaten drinking water
coastal waters, recreation areas, and waterfowl habitat.
The wide uncertainties in climate scenarios, regional variation
climate effects, and interactions of environment, economics, and
policy suggest that there are no simple and widely applicable
prescriptions. Farmers will need to adapt broadly to changing
in agriculture, of which changing climate is only one factor. Some
possible adaptations more directly related to climate include:
Sowing dates and other seasonal changes: Plant two
instead of one or a spring and fall crop with a short fallow
avoid excessive heat and drought in mid-summer. For already
growing areas, winter cropping could possibly become more
than summer cropping.
New crop varieties: The genetic base is very broad
crops, and biotechnology offers new potential for introducing
tolerance, pest resistance, and general improvements in crop
Water supply, irrigation, and drainage systems: Technologies
and management methods exist to increase irrigation efficiency
reduce problems of soil degradation, but in many areas, the
incentives to reduce wasteful practices do not exist.
precipitation and more intense precipitation will likely mean
some areas will need to increase their use of drainage systems
avoid flooding and water-logging of soils.
Tillage practices: A warmer climate will speed the
soil organic matter by bacteria and fungi. Loss of organic
reduces the capacity of soils to store water and nutrients
for plant growth. Tillage practices that incorporate crop
the soils would likely combat this loss and improve soil
Use near-term climate predictions: Accurate six-month
one-year forecasts could possibly reduce losses due to weather
variability. For example, predictions of El NiÃ±o events have
useful in regions where El NiÃ±o strongly affects weather.
Other management adjustments: Virtually all
components of the
farming system from planting to harvesting to selling might be
modified to adjust to climate change.