New Research Opportunities in the Earth Sciences - New Research Opportunities in the Earth Sciences - page 80

water cycle and related biogeochemical cycles caused by climate change as well as by
land use change and other human impacts.
Human Impacts on Water, Carbon, and Nitrogen Cycles
Humans have altered the terrestrial water cycle through activities like
reservoir construction, agriculture, groundwater extraction, and urbanization. Over
half (52 percent) of the world's largest rivers are regulated by dams, including 85
percent of the most biogeographically diverse large river systems (systems that span
five or more biomes; Nilsson et al., 2005). Regulation and fragmentation of rivers by
dams also strongly impact sediment storage and the discharge of terrestrial sediment
to the coastal ocean. While surface freshwater resources exceed global water demand
at present, variations in water availability and demand in time and place result in
regions of high water stress. In these water-stressed regions, groundwater withdrawal
often exceeds recharge, with recent estimates suggesting that groundwater depletion
(withdrawal in excess of recharge) has more than doubled since the 1960s (Wada et
al., 2010). Virtual trade of water used in the production of goods or services is likely
to become increasingly important in supporting human populations in water-stressed
regions, especially during drought, but may also facilitate unsupportable population
growth in regions of water scarcity (D'Odorico et al., 2010a). Accurate assessments
of water availability, water demand, and sustainable water use require more complete
global hydrological data sets, compilations of operational data regarding water use,
and advances in modeling coupled with hydrological and socioeconomic systems.
Because of the centrality of the carbon cycle to climate, it is critical that the
effects of human activities on the carbon cycle be quantified, that the response of the
carbon cycle to disturbance be determined, that potential future impacts on carbon
cycling and carbon pools (e.g., ocean acidification and methane dynamics) be
evaluated, and that possible mitigation strategies be considered (Canadell et al.,
2010). The potential for rising atmospheric carbon dioxide levels to significantly
impact climate, ecosystems, and human populations has given rise to a variety of
ideas for slowing rates of future increases in atmospheric carbon dioxide, ranging
from energy-saving measures and use of renewable energy sources to schemes for
increasing terrestrial and marine carbon storage (Gussow et al., 2010). Proposed
engineered approaches to reducing atmospheric carbon dioxide include ocean iron
fertilization, large-scale forestation using nonnative species and injection of carbon
dioxide in deep-sea sediments and aquifers. Geoengineering proposals for carbon
storage can involve substantial risks, possible unintended consequences, and
potentially limited benefit (Bala, 2009; Finzi, 2011). Both the American
Meteorological Society (AMS) and the American Geophysical Union (AGU) have
adopted position statements on geoengineering that recommend further research on
the intended and unintended Earth system response to geoengineering proposals and
coordinated, interdisciplinary study of the relevant scientific, social, legal, and ethical
issues (AMS, 2009; AGU, 2009).
Humans have also significantly impacted other biogeochemical cycles. As
noted above, industrial production of fertilizer, fossil fuel combustion, and cultivation