Geoscience Reference
In-Depth Information
Arctic Ocean with respect to aragonite is likely to
become reality in only a few years (Steinacher
et al.
2009 ) and ocean acidii cation and Arctic undersatu-
ration from baseline 21st century carbon emissions
is irreversible on human timescales (Frölicher and
Joos 2010). Globally, the volume of supersaturated
water decreases for another two centuries after car-
bon emissions stop; the fraction of the ocean vol-
ume occupied by supersaturated water is as low as
8% in 2300 with the 'A2_c' case compared with 42%
for pre-industrial conditions.
The focus of the analysis above is mainly on the
magnitude of change. However, it should be
stressed that rates of change are important. The
rates of change of climate and ocean acidii cation
co-determine the impacts on natural and socio-eco-
nomic systems and their capabilities to adapt.
Earlier analyses of the ice core and atmospheric
records show that the 20th-century increase in CO
2
and its radiative forcing occurred more than an
order of magnitude faster than any sustained
change during at least the past 22 000 years (Joos
and Spahni 2008). This implies that global climate
change and ocean acidii cation, which are anthro-
pogenic in origin, are progressing at high speed. It
is evident from Fig. 14.2 that rates of change in sur-
face-ocean pH
T
and in Ω
a
are much lower for the
range of mitigation scenarios than for the range of
baseline emissions scenarios.
A range of geoengineering options have been dis-
cussed to limit potential impacts of anthropogenic
carbon emissions and climate change. Here, we
summarize a few conclusions from a recent report
( The Royal Society 2009 ). CO
2
removal techniques
address the root cause of climate change by remov-
ing CO
2
from the atmosphere. Solar radiation man-
agement techniques attempt to offset the effects of
increased greenhouse gas concentrations by causing
the earth to absorb less solar radiation. Obviously,
solar radiation management techniques do not con-
tribute in a relevant way to mitigation of ocean acid-
ii cation. Of the CO
2
removal methods assessed,
none has yet been demonstrated to be effective at an
affordable cost and with acceptable side-effects (The
Royal Society 2009). If safe and low-cost methods
can be deployed at an appropriate scale they could
make an important contribution to reducing CO
2
concentrations and could provide a useful comple-
ment to conventional emissions reductions. Methods
that remove CO
2
from the atmosphere without per-
turbing natural systems, and without requirements
for large-scale land-use changes, such as CO
2
cap-
ture from air (IPCC 2005) and possibly also enhanced
weathering, are likely to have fewer side-effects.
Geoengineering techniques are currently not
ready for application, in contrast to low-carbon
technologies.
Experimental evidence has emerged in the past
years that ocean acidii cation has negative impacts
on many organisms and may severely affect cold-
and warm-water corals or high-latitude species
such as aragonite-producing pteropods. Considering
the precautionary principle mentioned in the
UNFCCC, our results may imply that atmospheric
CO
2
should be stabilized somewhere around 450
ppmv or below in order to avoid the risk of large-
scale disruptions in marine ecosystems. A stabiliza-
tion of atmospheric CO
2
at or below 450 ppmv
requires a stringent reduction in carbon emissions
over the coming decades. The results from the IAMs
suggest that such a low stabilization target is eco-
nomically feasible.
14.9 Acknowledgements
This chapter is a contribution to the European Project
on Ocean Acidii cation, EPOCA (FP7/2007-2013; no.
211384). We acknowledge support by the Swiss
National Science Foundation and by the EU pro-
jects CARBOOCEAN (511176) and EUR-OCEANS
(511106-2). Simulations with the NCAR CSM1.4-carbon
model were carried out at the Swiss National Com-
puting Center in Manno, Switzerland. We thank
S. C. Doney, I. Fung, K. Lindsay, J. John, and colleagues
for providing the CSM1.4-carbon code and J.-P. Gattuso,
L. Hansson, and A. Oschlies for helpful comments.
References
Archer, D., Kheshgi, H., and Maier-Reimer, E. (1999).
Dynamics of fossil fuel CO
2
neutralization by marine
CaCO
3
.
Global Biogeochemical Cycles
,
12
, 259-76.
Brewer, P.G. and Peltzer, E.T. (2009). Limits to marine life.
Science
,
324
, 347-8.
Caldeira, K. and Wickett, M.E. (2003). Anthropogenic car-
bon and ocean pH.
Nature
,
425
, 365.
