Recent work by Sreeush et al. (2019a) has found that the
Indian Ocean is acidifying due to the accumulation of
anthropogenic CO 2 from the atmosphere, and this storage of
anthropogenic carbon in the Indian Ocean is comparable
(after normalizing for the size of the basins) with the other
major oceans (Sabine et al. 2004). Surface ocean pH over the
Indian Ocean has declined by about 0.1 unit (current mean is
8.1) relative to pre-industrial levels and is larger over the
western Indian Ocean (e.g. Sreeush et al. 2019a). This
increase in ocean acidity may be responsible for the functional collapse of reef building corals. The western Arabian
Sea has undergone more rapid acidification than the rest of
the TIO basin due to strong upwelling in this region drawing
up anthropogenic CO 2 embedded in the deeper ocean.
Moreover, SST warming also accelerates acidification due to
the endothermic nature of CO 2 dissolution in water. An
ocean biogeochemical model-based simulations from 1960
to 2009 show that western Arabian Sea has acidified by
108% due to dissolved inorganic carbon, −36% due to
buffering due to alkalinity, 16% due to SST warming, 6%
due to salinity changes and remaining due to changes in
other minor ions (Sreeush et al. 2019a). Considering that the
western Arabian Sea is a highly productive zone of the
Indian Ocean (Roxy et al. 2016), the role of SST warming in
exacerbating acidification needs to be monitored carefully.
Dissolved O 2 is a major determinant of the abundance
and distribution of the marine habitat. The Indian Ocean
contains one of the oceans’ most pronounced oxygen minimum zones (OMZs) encompassing more than 50% of the
area containing OMZs (e.g. Helly and Levin 2004). The
tropical open ocean O 2 concentration has decreased at a rate
of 0.1–0.3 µ mol kg
−1 year
−1 during the past five decades
(Stramma et al. 2008). Long-term measurements over the
TIO also show a pronounced decrease (at a rate of 20–
30 mol m
−2 per decade) in O 2 concentration (Koslow et al.
Fig. 10.4 Projected changes in
multiple stressor intensity in
2090–2099 relative to 1990–1999
under the RCP8.5 scenario. Red
indicates where sea surface
warming exceeds +3.5 °C,
hatched yellow indicates where
subsurface (200–600 m) oxygen
concentrations decrease by more
than 20 lmol/m
3 and hatched
blue indicates where vertically
integrated annual NPP decreases
by more than 100 gC/m
2
. In
addition, hatched orange indicates
present-day simulated
low-oxygen (<50 mmol/m
3
) in
the subsurface waters
Figure adapted from Bopp et al.
(2013)
Fig. 10.5 Status of the Indian Ocean Observing System (IndOOS) in
2018. The sustained observing system in the Indian Ocean comprises of
the Argo, RAMA, XBT/XCTD, surface drifting buoy and tide gauge
networks. It is supported by satellite observations and the GO-SHIP
program. The empty symbols indicate RAMA sites that were not
implemented due to logistical constraints Source: Beal et al. 2019
200
M. K. Roxy et al.
Indian Ocean is acidifying due to the accumulation of
anthropogenic CO 2 from the atmosphere, and this storage of
anthropogenic carbon in the Indian Ocean is comparable
(after normalizing for the size of the basins) with the other
major oceans (Sabine et al. 2004). Surface ocean pH over the
Indian Ocean has declined by about 0.1 unit (current mean is
8.1) relative to pre-industrial levels and is larger over the
western Indian Ocean (e.g. Sreeush et al. 2019a). This
increase in ocean acidity may be responsible for the functional collapse of reef building corals. The western Arabian
Sea has undergone more rapid acidification than the rest of
the TIO basin due to strong upwelling in this region drawing
up anthropogenic CO 2 embedded in the deeper ocean.
Moreover, SST warming also accelerates acidification due to
the endothermic nature of CO 2 dissolution in water. An
ocean biogeochemical model-based simulations from 1960
to 2009 show that western Arabian Sea has acidified by
108% due to dissolved inorganic carbon, −36% due to
buffering due to alkalinity, 16% due to SST warming, 6%
due to salinity changes and remaining due to changes in
other minor ions (Sreeush et al. 2019a). Considering that the
western Arabian Sea is a highly productive zone of the
Indian Ocean (Roxy et al. 2016), the role of SST warming in
exacerbating acidification needs to be monitored carefully.
Dissolved O 2 is a major determinant of the abundance
and distribution of the marine habitat. The Indian Ocean
contains one of the oceans’ most pronounced oxygen minimum zones (OMZs) encompassing more than 50% of the
area containing OMZs (e.g. Helly and Levin 2004). The
tropical open ocean O 2 concentration has decreased at a rate
of 0.1–0.3 µ mol kg
−1 year
−1 during the past five decades
(Stramma et al. 2008). Long-term measurements over the
TIO also show a pronounced decrease (at a rate of 20–
30 mol m
−2 per decade) in O 2 concentration (Koslow et al.
Fig. 10.4 Projected changes in
multiple stressor intensity in
2090–2099 relative to 1990–1999
under the RCP8.5 scenario. Red
indicates where sea surface
warming exceeds +3.5 °C,
hatched yellow indicates where
subsurface (200–600 m) oxygen
concentrations decrease by more
than 20 lmol/m
3 and hatched
blue indicates where vertically
integrated annual NPP decreases
by more than 100 gC/m
2
. In
addition, hatched orange indicates
present-day simulated
low-oxygen (<50 mmol/m
3
) in
the subsurface waters
Figure adapted from Bopp et al.
(2013)
Fig. 10.5 Status of the Indian Ocean Observing System (IndOOS) in
2018. The sustained observing system in the Indian Ocean comprises of
the Argo, RAMA, XBT/XCTD, surface drifting buoy and tide gauge
networks. It is supported by satellite observations and the GO-SHIP
program. The empty symbols indicate RAMA sites that were not
implemented due to logistical constraints Source: Beal et al. 2019
200
M. K. Roxy et al.