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Global Warming
and Agriculture |
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It has been widely recognized that developing countries in
general stand to lose more from the effects of global warming on
agriculture than do industrial countries. |
John Steinbeck's The Grapes of Wrath
provides a verbal mural depicting America's experience in the Dust Bowl of the
1930s, with its migration of "Okies" from ruined farmlands in Oklahoma and
Texas to a not-so-promised land in California. This historical experience and
perhaps the present-day drought of biblical proportions in Australia should
alert international policymakers to the risks to world agriculture of a hotter
and drier world by late this century as a consequence of unarrested global
warming.
In the long list of potential problems
from global warming, the risks to world agriculture stand out as among the
most important. Yet there has been a tendency in the climate economics
literature in recent years to downplay this risk, and even to argue that a
couple of degrees Celsius warming might benefit world agriculture. But such
studies typically have too short a time horizon (generally out to about 2050).
They also focus on overall temperature change (which includes oceans), rather
than on the changes that will occur over land (which warms more easily and
quickly than water)—and specifically agricultural land.
It has been widely recognized that
developing countries in general stand to lose more from the effects of global
warming on agriculture than do industrial countries. Most developing countries
have less capacity to adapt than do their wealthier neighbors. Most are in
warmer parts of the globe, where temperatures are already close to or beyond
thresholds at which further warming will reduce rather than increase
agricultural output. And agriculture is a larger share of developing economies
than of industrial economies. But it has been difficult to estimate just how
much individual countries are likely to be affected.
For that reason, this study (Cline,
2007) was undertaken both to get a better long-term fix on overall world
effects under current policies (the so-called baseline or business-as-usual
scenario) and to understand the likely impact on individual countries and
regions. The time frame stretched out to the average for 2070–99, what is
called the "2080s." Climate model projections are available on a comparable
basis for this period, which is far enough in the future to allow sizable
warming and potential damage to materialize but close enough to the present to
elicit public concern. The study, which is explored in this article, suggests
that there is good reason not to downplay the risks to agriculture from global
warming.
How Climate
Affects Agriculture:
Climate change can affect agriculture in a variety of ways. Beyond a certain
range of temperatures, warming tends to reduce yields because crops speed
through their development, producing less grain in the process. And higher
temperatures also interfere with the ability of plants to get and use
moisture. Evaporation from the soil accelerates when temperatures rise and
plants increase transpiration—that is, lose more moisture from their leaves.
The combined effect is called "evapotranspiration." Because global warming is
likely to increase rainfall, the net impact of higher temperatures on water
availability is a race between higher evapotranspiration and higher
precipitation. Typically, that race is won by higher evapotranspiration.
But a key
culprit in climate change—carbon emissions—can also help agriculture by
enhancing photosynthesis in many important, so-called C3, crops (such as
wheat, rice, and soybeans). The science, however, is far from certain on the
benefits of carbon fertilization. But we do know that this phenomenon does not
much help C4 crops (such as sugar-cane and maize), which account for about
one-fourth of all crops by value.
Crunching the
Numbers: To
estimate the country-specific impact of global warming on agriculture if
carbon emissions continue to grow unabated, the study combined two sets of
existing models—one from climate science and the other from agronomy and
economics. Six leading climate models provided estimates of future changes in
temperature and precipitation at a typical detail of about 2,000 land-based
areas, or grid cells. These changes were added to information on present
climate (about 22,000 land cells) and then averaged to obtain a consensus
climate projection at a detail of about 4,000 land cells. These estimates were
fed into crop impact models from agronomy and economics to produce the
yield-impact estimates, which were then averaged up to the level of countries
and regions.
The consensus of the six models shows
that a doubling of atmospheric carbon concentration will produce an eventual
overall warming of 3.3°C. This is close to the UN Intergovernmental Panel on
Climate Change (IPCC) estimate of what is called "climate sensitivity," or the
amount of long-term global warming to be expected from a doubling of carbon
dioxide in the atmosphere above preindustrial levels —an indication that the
models are producing mainstream forecasts.
To develop these estimates, the baseline
emissions projections from the most widely used scenario in the IPCC's Third
Assessment Review in 2001 were fed into the climate models. Currently, annual
fossil-fuel emissions amount to about 7 billion tons of carbon. Under the
IPCC's business-as-usual estimate, they would rise to about 16 billion by 2050
and 29 billion by 2100, partly because of a greater use of coal. The
corresponding atmospheric concentrations of carbon dioxide would reach 735
parts per million (ppm) by 2085, in contrast to the preindustrial level of 280
ppm and today's level of 380 ppm.
The study divides the world into 116
countries and regions. By the 2080s, the six climate models predict an average
surface temperature increase of nearly 5°C weighting by land area and about
4.4°C weighting by farm area. This is higher than a global mean warming of
3°C, because land areas warm more than the oceans. Precipitation also rises,
but only by about 3 percent.
The climate change projections are then
applied to the agricultural impact models to develop two sets of assessments
of the effect of climate change on agricultural productivity. One set, the
"crop models," relates farm output to land quality, climate, fertilizer
inputs, and so forth (Rosenzweig and Iglesias, 2006). The other, "Ricardian
models," statistically infers the contribution of temperature and
precipitation to agricultural productivity by examining the relationship of
land price to climate—agricultural productivity improves as temperatures go
from cold to warm, then deteriorates going from warm to hot (Mendelsohn and
Schlesinger, 1999). Models relating county- or farm-level data on land values
or net revenue to such influences as soil quality as well as temperature and
rainfall are now available for Canada, the United States, India, and many
countries in Africa and Latin America. Both sets of models tend to produce
similar results. The study combined them to create a consensus estimate of
crop yields both under conditions in which there is no benefit from increased
carbon dioxide on crop yields and under assumptions that result in positive
effects from carbon fertilization.
The Impact on
Crop Yields:
The results give little support to
the optimists. Globally, the overall impact of baseline global warming by the
2080s is a reduction in agricultural productivity (output per hectare) of 16
percent without carbon fertilization, and a reduction of 3 percent should
carbon fertilization benefits actually materialize—when results are weighted
by output. The losses are greater when weighted by population or country.
The sharp concentration of losses is in
the developing countries. Whereas the industrial countries experience outcomes
ranging from 6 percent losses without carbon fertilization to 8 percent gains
with it, developing country regions suffer losses of about 25 percent without
carbon fertilization and 10–15 percent if carbon fertilization is included.
For developing countries, the median loss would be 15–26 percent, and the
output-weighted average loss, 9–21 percent. Losses could reach devastating
levels in some of the poorest countries (greater than 50 percent in Senegal
and Sudan).
Damage will generally be greater in
countries located closer to the equator, where temperatures already tend to be
close to crop tolerance levels. Country elevation also matters. For example,
because of higher elevation and lower average temperatures, Uganda faces
smaller losses (17 percent without carbon fertilization) than Burkina Faso (24
percent) even though the latter is situated about 10 degrees farther north of
the equator. Whereas the major losses are concentrated in the lower latitudes,
the gains, where they occur, are toward the higher latitudes. In the absence
of any boost from carbon fertilization, the most severely affected countries
are in Africa, Latin America, and south Asia, although most of the world
registers a decline in agricultural productivity. Will carbon fertilization
benefits help much? The answer appears to be yes and no. There are still very
adverse outcomes for countries in Africa, Latin America, and south
Asia—although some individual countries and subregions would fare much better.
On a more detailed country and regional
breakdown, the study shows the following:
In South America, there are
potentially sizable losses in Argentina and Brazil if the carbon fertilization
effect does not materialize, and moderate losses in Brazil even with carbon
fertilization—although Argentina would be better off.
In North America, there is a
tremendous variation from the north to the south. For the United States,
the overall outcome would average from a 6 percent overall decline without
carbon fertilization to an 8 percent increase with it. But this average masks
large potential losses in the southeast and in the southwest plains, where the
weighted average of the Ricardian and crop models shows losses ranging from 25
percent if there is a carbon fertilization benefit to 35 percent without the
benefit. Canada, like the United States, shows small losses without
carbon fertilization and moderate gains with it. And Mexico shows losses of
25–35 percent.
In Africa there are large losses.
Nigeria's losses range from 6 to 19 percent, and South Africa's and
Ethiopia's losses are much larger.
In Europe, Germany has
small losses without carbon fertilization and moderate gains with it. For
Spain, the results are somewhat less favorable, once again reflecting
latitudinal location. For Russia, without carbon fertilization,
productivity declines by 8 percent; with it, Russia shows gains of 6 percent.
In Asia, the contrasting picture
for the two largest and most dynamic developing countries is stark. India
produces perhaps the most disturbing single set of results, with losses
ranging from about 30 to 40 percent. China would have a more moderate
range, from a 7 percent loss to a 7 percent gain. Its results resemble those
of the United States: in both countries, the average effects are relatively
neutral, but there are sizable losses in the southern regions. The contrast
with India is strictly consistent with India's location closer to the equator.
Both China and the United States have a latitude center of about 38° north,
whereas India's latitude center is about 22° north.
A Technological
Rescue?:
There are those who argue that rapid technological change will raise
agricultural yields so much by late this century that any reduction caused by
global warming would easily be more than offset. But technological change is a
false panacea for several reasons.
First, the green revolution has already
slowed. Calculations based on UN Food and Agricultural Organization data show
that grain yields, which rose at an annual rate of 2.7 percent in the 1960s
and 1970s, have risen at only a 1.6 percent annual rate in the past quarter
century. Although rising agricultural prices might provide incentives that
would slow or reverse this decline, such a response is not assured.
Second, even if there is no further
slowdown, there is likely to be a close race between rising food demand and
rising output. Global food demand is expected to approximately triple by the
2080s because of higher world population and higher incomes. It also seems
quite likely that a sizable share of land will be shifted to the production of
biomass for ethanol fuel. As a result, there is a rather precarious balance
between supply and demand, which would be seriously worsened by a major
adverse shock from global warming.
The Stakes Are
Large: This
study's estimates underscore the importance of coordinated international
action to limit carbon dioxide emissions and avert warming and damage that
will likely otherwise occur, not only in agriculture but also from sea level
rise and increased intensity of hurricanes, among other things.
Moreover, it is likely that actual
global losses will be worse than those portrayed here. Neither crop nor
Ricardian models can account for the influence of what are likely to be
increases in extreme weather, such as droughts and floods, and insect pests.
Nor do the estimates take account of agricultural losses associated with
rising sea levels, a major consideration in countries such as Bangladesh and
Egypt. More fundamentally, by taking a snapshot of the 2080s, the estimates do
not capture the much greater damage that could be expected from the still more
severe global warming that would occur by the 22nd century if no steps are
taken to curb carbon emissions.
The developing countries are most at
risk, so it is strongly in their own interest that they participate actively
in international abatement programs. China already produces larger carbon
dioxide emissions than the European Union and will soon surpass those of the
United States. Global emissions from developing countries (including from
deforestation) are already equal to those from industrial countries, and are
growing faster.
It is striking that the two largest
developing countries, India and China, seem to have potentially conflicting
interests in their approach to international abatement efforts. With broadly
neutral or even positive effects on its agriculture, China could be less
interested in international efforts to restrain emissions than India, which
faces major potential losses if there is no change in global emissions
policies. But even in China, some key subregions are at risk.
It is fortunate that at the December
2007 UN Climate Conference in Bali, Indonesia, nations agreed to pursue
negotiations toward a new international agreement to succeed the Kyoto
Protocol by 2009. Of the two leading industrial countries that had refused to
sign the Kyoto accord, Australia has recently changed governments and signed
on, and, in the United States, the leading presidential candidates of both
parties have called for relatively aggressive reductions in U.S. emissions of
carbon dioxide. If leading developing nations, such as Brazil, China, and
India, also become more disposed to take steps to reduce emissions, the new
negotiations could set the stage for meaningful international abatement
measures in the post-Kyoto period. |
