(41-45)
Since long before the beginning of the Industrial Age,
oceans were effectively absorbing carbon dioxide. Recent
studies show that in addition to rising sea levels and a 0.5
degree Fahrenheit increase in ocean surface temperature,
excess Carbon dioxide in the atmosphere is increasing ocean
pH levels, a process known as ocean acidification. This change
in chemistry appears to be affecting the ability of some
mollusks and other marine organisms to build calciferous
shells, and threatens to shift the ranges of marine organisms
and alter entire food webs in ways not yet fully understood.
Several species of marine birds that depend on California
habitats—including already endangered breeding populations
of ashy storm-petrels, Craveri’s murrelets and black-vented
shearwaters—are expected to experience declines as rising
sea levels inundate the rocky areas where they breed on and
near the coast. In addition, critical foraging for at least 70 bird
species could be disrupted by the effects of rising sea
temperatures on the food chain.
Corals, calcareous phytoplankton, mussels, snails, sea
urchins and other marine organisms use calcium and
carbonate in seawater to construct their calcium carbonate
shells or skeletons. As the pH decreases, carbonate becomes
less available, which makes it more difficult for organisms to
secrete Calcium carbonate to form their skeletal material. For
animals in general, including invertebrates and some fish,
carbon dioxide accumulation and lowered pH may result in
acidosis, or a buildup of carbonic acid in the organism's body
fluids. This can lead to lowered immune response, metabolic
depression, behavioral depression affecting physical activity
and reproduction, and asphyxiation. Since the oceans have
never experienced such a rapid acidification, it is not clear if
ecosystems have the ability to adapt to these changes. Effects
of ocean acidification on organisms and ecosystems are still
poorly understood. Over the last few years, research has
intensified significantly to fill the many knowledge gaps.
Nearly 500 million people depend on healthy coral reefs
for sustenance, coastal protection, renewable resources, and
tourism, with an estimated 30 million of the world's poorest
people depending entirely on the reefs for food. Coral reefs
face two challenges from increasing atmospheric carbon
dioxide. First, higher carbon dioxide concentrations in the
atmosphere are linked to warmer global temperatures, which
in turn lead to warmer water temperatures. Corals are very
sensitive to temperature change: a 1–2º Celsius change in local
temperature above their normal summer maximum can lead
to a phenomenon called ‘bleaching’, whereby the corals expel
their vital algal symbionts (algae which live in the cells of the
coral), leaving the coral tissues translucent.
In 1998, a single bleaching event led to the loss of almost
20% of the world’s living coral. Corals can recover from these
events but repeated episodes are likely to weaken the coral
ecosystem, making them more susceptible to disease and
causing a loss of biodiversity. The second challenge faced by
corals is the increasing acidity of the water caused by higher
carbon dioxide concentrations. Lowered calcification rates
affect the reef’s ability to grow its carbonate skeleton, leading
to slower growth of the reef and a more fragile structural
support, which makes it more vulnerable to erosion.
By the middle of this century, the estimated reduction in
calcification rates may lead to more reef area erosion than can
be rebuilt through new calcification.
【題組】44. Which of the following statements about “bleaching” is
NOT true?
(A)It is a contagious disease.
(B)It is an effect caused by warmer sea temperature.
(C)It is a phenomenal change.
(D)It is a loss of algae symbionts.
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