change in total annual precipitation having
the largest impact and leading to a robust increase on its own for most regions (fig. S4). The
larger contribution of change in annual precipitation to the change in mean annual nitrogen flux is attributable to the robustness of
the projected changes in annual precipitation
(fig. S5B) and the larger sensitivity of nitrogen flux to total annual precipitation relative
to extreme precipitation (see supplementary
Overall, we find that regions with high historical loading (which correspond to regions
with high nitrogen inputs and high precipitation) and a robust projected increase in precipitation are most likely to experience a large
and robust future increase in nitrogen loading, at both the watershed and regional scales.
The empirical model used here to relate nitrogen inputs, land use, and precipitation statistics
to nitrogen flux is specific to the continental
United States, precluding its direct application
to other regions of the globe. We may, however,
seek analogs in other regions that meet certain criteria and use those as heuristics to identify other regions where similar conditions exist
and similar outcomes may be expected. If large
increases in nitrogen load are expected for regions with (i) high nitrogen inputs, (ii) high precipitation, and (iii) a robust projected increase
in precipitation within the continental United
States, the same is likely to be true in other
parts of the world. We therefore reexamined
the business-as-usual, far-future precipitation
projections across the 21 available CMIP5 models globally (bias-corrected and spatially downscaled to 1/4°) to identify regions that exhibit
all three risk factors (see supplementary materials). We find that identifying regions with
robust projected precipitation increases (fig.
S6A) and high historical total annual precipitation (>75th percentile globally; 656 mm year−1;
fig. S6B), combined with data on historical fertilizer application rates (as a proxy for nitrogen
inputs) (fig. S6C), provides a good approximation of the regions within the continental United
States that are likely to experience a large and
robust increase in nitrogen flux (stippled region in Fig. 1C versus continental U.S. area in
Applying this heuristic approach globally makes
it possible to identify other regions where changes
in precipitation are likely to engender substantial increases in nitrogen load (Fig. 4). We find
that large portions of East, South, and Southeast Asia, including India and eastern China,
exhibit conditions that are directly analogous to
those in the upper Mississippi Atchafalaya River
Basin, Northeast, and Great Lakes regions of
the continental United States, and these regions are therefore likely to undergo large increases in nitrogen load as a result of projected
changes in precipitation. These regions are also
home to more than half of the world’s population (33) and are heavily dependent on surface water supplies (34). As a result, increased
eutrophication would have widespread impacts.
Among countries in this region, India is especially noteworthy because it exhibits all three
risk factors across more than two-thirds of its
area, is one of the fastest-developing countries in the world, and has one of the fastest-growing populations (33). The precipitation
projections in this region are also highly sensitive to aerosol emission trajectories (35), which
are themselves uncertain (36). Portions of Europe (e.g., Italy, southern France, Denmark, northern Germany) also display all three risk factors.
Other highly agricultural regions (e.g., central
Europe, eastern South America, southern Australia) have comparable fertilizer application
rates (fig. S6C) but have either lower historical
precipitation or less robust projected precipitation changes. In general, this heuristic approach
identifies global agricultural regions that are
particularly susceptible to the impacts of precipitation changes.
We conclude that changes in precipitation patterns will have substantial impacts on nitrogen
loading within the continental United States.
These trends will compound changes due to anticipated intensification of land use (9, 10) or they
may negate the benefits of strategies aimed at
load reductions (9, 10), thereby exacerbating water
quality impairments (37). The same scenario is
likely to play out in East, South, and Southeast
Asia—in particular, in India and eastern China,
which have high precipitation and fertilizer application rates and are projected to experience future
precipitation increases. Our findings imply that
strategies aimed at managing eutrophication and
associated water quality problems must account
for the impact of changing precipitation patterns
on nutrient loading.
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Supported by NSF grant 1313897 (E.S. and A.M.M.) and by the
Cooperative Institute for Climate Science, Princeton University,
under NOAA grant NA08OAR4320752 (V.B.). We thank K. Findell,
J. Ho, M. Lee, Y. Shiga, and three anonymous reviewers for incisive
comments on the manuscript and analysis. We acknowledge the
World Climate Research Programme’s Working Group on Coupled
Modelling, which is responsible for CMIP, and we thank the climate
modeling groups (listed in table S2) for producing and making
available their model output. For CMIP, the U.S. Department of
Energy’s Program for Climate Model Diagnosis and Intercomparison
provides coordinating support and led development of software
infrastructure in partnership with the Global Organization for Earth
System Science Portals. For the global analysis, global climate
scenarios used were from the NEX-GDDP data set, prepared by the
Climate Analytics Group and NASA Ames Research Center using
the NASA Earth Exchange, and distributed by the NASA Center for
Climate Simulation. Data used in this study are freely available
online, as listed in table S3.
Materials and Methods
Figs. S1 to S6
Tables S1 to S3
17 March 2017; accepted 22 June 2017