Environmental Engineering Reference
In-Depth Information
carbon fluxes may reflect as much aggregate events in the past as current
conditions, complicating the task of deconvolving underlying mechanisms.
Experimentally, fertilization of photosynthesis by higher atmospheric carbon
dioxide levels increases plant growth, in many cases substantially, and con-
tributes to carbon uptake. When included in global models, CO
2
fertilization
leads to a modest current sink that may continue for many decades into the
future. Some studies suggest, however, that this effect may be smaller than
earlier thought perhaps due to other limitations such as nitrogen (Long et al.,
2006; Thornton et al., 2009a). Many Northern Hemisphere ecosystems are
also nitrogen limited, and deposition of reactive nitrogen mobilized by fossil
fuel combustion may also cause increased carbon uptake (Lamarque et al.,
2005; Thornton et al., 2009a). Land carbon storage varies on interannual
time scales due to climate variability and in particular variations in tempera-
ture, precipitation, and water availability (Sitch et al., 2008). Wildfire also
plays a major role in the global carbon cycle (Randerson et al., 2006), and
increased tropical fire associated with El Niño droughts may contribute to
increases in the growth rate of atmospheric carbon dioxide concentrations
during recent El Niño years (Van der Werf et al., 2006).
Climate warming may cause land ecosystems to lose carbon because
respiration is more temperature sensitive than photosynthesis, but there is
a wide range of estimates for the climate sensitivity of land carbon stocks.
Larger effects could come if future climates lead to additional disturbances,
especially fire, pests, or widespread replacement of forests to grasslands,
which could rapidly release large amounts of carbon. Current attention
is focusing on both the role of human and climate-caused disturbance in
controlling future ecosystem carbon storage and on physiological processes,
such as carbon dioxide fertilization. Water availability to support photo-
synthesis and primary productivity is thought to be significant, with models
(Fung et al., 2005) and observations (Angert et al., 2005) indicating that
decreasing water balance (drier soil conditions) decreases carbon uptake.
Global effects are a balance between warmer conditions favoring a longer
growing season in the Northern Hemisphere temperate zone, increasing
carbon uptake, and drier soils in the tropics, decreasing carbon uptake (Fung
et al., 2005; Friedlingstein et al., 2006). Past and future land-use change will
also affect land carbon storage and needs to be considered when projecting
future atmospheric CO
2
levels.
The combined effects of ocean and land feedbacks have been explored
in a series of coupled climate-carbon models reported by the international
C
4
MIP (Coupled Climate-Carbon Cycle Model Intercomparison Project;
Friedlingstein et al., 2006; Figure 2.10). Simulations are conducted for the
