Geoscience Reference
In-Depth Information
include the potential impacts of tipping points, or major changes in
global weather patterns. Even given these uncertainties, the pictures of
the grain belt turning into the Sahara Desert are posters for persuasion
rather than the results of careful scholarly studies.
Adverse assessments of the impacts of global warming on farming
rely on two major factors. First, climate change is likely to lead to
warmer climates with declining soil moisture in many regions of the
world where climates are already close to the margin. Work of my Yale
colleague Robert Mendelsohn suggests that current climates in many
parts of Latin America, Africa, and Asia are already warmer than is
optimal for food production, and further warming would reduce
yields in those regions. 8
A second factor is that climate change may lead to adverse impacts
on the “hydrological cycle,” that is, systems that provide water for agri-
culture. Examples of adverse impacts include declines in mountain
snowpack and major changes in seasonal river runoff. These trends
would reduce the availability of water for irrigation, again harming
agricultural productivity. These two elements have been extensively
investigated with climate projections incorporating water and crop
models.
ADAPTATION AND MITIGATING FACTORS
The crystal ball is just as cloudy for agriculture as for other areas,
but there are several factors that could reduce the harmful impacts of
climate change, including carbon fertilization, adaptation, trade, and the
declining share of agriculture in our economies. One important miti-
gating factor for agriculture is carbon fertilization. Carbon dioxide is a
fertilizer for many plants. In the presence of increased CO 2 , yields for
wheat, cotton, and clover—particularly when other inputs were ad-
justed appropriately—have increased sharply in fi eld experiments. One
review of multiple fi eld studies found that doubling atmospheric con-
centrations of CO 2 would increase yields of rice, wheat, and soybeans
10 -15 percent. Certain plants such as corn, which fi x atmospheric car-
bon via what is known as the C4 pathway, are expected to show smaller
increases in CO 2 -induced yields. There are many questions about how
 
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