3. Drought reduces terrestrial ecos

3. Drought reduces terrestrial ecosystem productivity
Research suggests that frequent drought and heat have great
impacts on terrestrial carbon cycling by reducing ecosystem productivity,
although there remain some uncertainties regarding
their effects [17]. Several studies have proposed that dry conditions
may actually boost tropical productivity through increases
in available solar radiation [45,9]. The terrestrial biosphere could
contribute significant amount of CO2 emission during major
drought events, such as those evoked by the 1997–1998 El Niño
event [43]. These impacts occurred on both the global and regional
scales. Table 1 displays a number of major drought events and their
impacts on the carbon cycles of their respective terrestrial ecosystems.
Among those areas presently studied, the Amazon rainforest
is particularly important due to its special status in the global carbon
cycle and climate change. This area can process 18 Pg C annually,
more than twice the amount of annual anthropogenic fossil
fuel emissions [33]. During the 2005 Amazon drought, the total
loss of carbon biomass carbon was 1.2–1.6 Pg C [42]. In 2010, the
Amazon experienced a second 100 year drought, and the carbon
impact of the 2010 drought may eventually exceed 5 billion tons
of CO2 released [31]; this figure accounts for 16.6% of global industrial
emissions (30.1 Pg C) in 2010. The research warns that, if
extreme droughts like these events become more frequent, the
Amazon rainforest may lose its ability to act as a natural buffer
for anthropogenic carbon emissions [31]. Two other critical ecosystems
to Earth’s climate, the boreal forests and the temperate forests,
are facing the same fate, their carbon sinks weakened by
increasingly frequent drought events [32,57].
At the global scale, a reduction in terrestrial NPP of 0.55 petagrams
over the past decade (2000–2009) has been estimated using
the global MODIS NPP algorithm. Furthermore, the drying trend of
the Southern Hemisphere has decreased the NPP in that area,
counteracting the increase in NPP observed over the Northern
Hemisphere [67].
Although a number of studies have examined the influences of
drought on plant productivity and the terrestrial carbon cycle, the
physiological mechanism targeted by drought remain unclear.
Drought has generally been considered to reduce photosynthesis