Agriculture Reference
In-Depth Information
environmental benefits. We close by reviewing the means to encourage greater
environmental stewardship both within current U.S. legal structures and beyond.
Managed Ecosystems and Human Impacts
The impact of human domination of Earth's ecosystems is well documented
(Vitousek et al. 1997). Our large ecological footprint owes much to the effects of
intentional ecosystem management (Farber et al. 2006). Among managed ecosys-
tems, agricultural systems cover the largest area, with estimates ranging from 25
to 38% of Earth's land area (Wood et al. 2000, Millennium Ecosystem Assessment
2005), and arguably have the greatest environmental impact (Robertson and
Swinton 2005). Not only does agriculture compose over half of the land area that
is not either desert or permafrost, but also agricultural systems are increasing in
area. Indeed, the prospect of a growing reliance on biofuels is likely to drive greater
global growth in cultivated land area than even the ~20% growth that Tilman et al.
(2001) predicted by 2050.
Agricultural impacts on global ecosystem services are significant. Smith
et al. (2007) estimate that 10-14% of total global GHG emissions originate
from agriculture, and that does not include land-use change. Land-use change,
mostly associated with deforestation for agriculture, is responsible for another
12-17%. Watershed biogeochemical models supported by empirical evidence
suggest that agriculture is responsible for over 70% of the phosphorus and
nitrogen carried by the Mississippi River to the hypoxic zone of the Gulf of
Mexico (Alexander et al. 2008), and similar dead zones exist in other coastal
regions around the world (Diaz and Rosenberg 2008). Groundwater reserves
that serve drinking water wells and recharge surface streams have been signifi-
cantly contaminated by agriculture during the twentieth century (Böhlke 2002).
On the plus side, carbon reserves in U.S. agricultural soils are estimated to
have risen during 1982-1997 due to the replacement of moldboard plowing by
conservation tillage practices (i.e., reduced or no tillage), land retirement from
agriculture, and reduced use of bare-soil fallow periods (Eve et al. 2002). The
magnitude of changes in ecosystem services across air, water, and land indi-
cates the importance of agricultural management effects on services at global,
regional, and local scales. The recent evolution of U.S. agriculture helps to
explain these environmental impacts.
Trends in United States Agriculture
The past half century of U.S. agriculture has seen rising economic efficiency at
producing marketed products. Particularly striking has been the trend of rising
productivity (Fig. 13.1). During 1948-2001, the 1.9% annual increase in total
factor productivity permitted the real value of agricultural output to rise steadily
without the use of additional inputs (Ball et al. 1997, Dumagan and Ball 2009).
For example, fertilizer and pesticide use have remained essentially constant
