Climate change impacts and vulnerability in Europe 2016

Climate change impacts on environmental systems
4.1.2 Ocean acidification
Key messages
• Ocean surface pH has declined from 8.2 to below 8.1 over the industrial era as a result of the increase in atmospheric
CO 2 concentrations. This decline corresponds to an increase in oceanic acidity of about 30 %.
• Ocean acidification in recent decades has been occurring 100 times faster than during past natural events over the last
55 million years.
• Observed reductions in surface water pH are nearly identical across the global ocean and throughout continental
European seas, except for variations near the coast. The pH reduction in the northernmost European seas, i.e. the
Norwegian Sea and the Greenland Sea, is larger than the global average.
• Ocean acidification already reaches into the deep ocean, particularly at the high latitudes.
• Models consistently project further ocean acidification worldwide. Ocean surface pH is projected to decrease to values
between 8.05 and 7.75 by the end of 21st century, depending on future CO 2 emissions levels. The largest projected
decline represents more than a doubling in acidity.
• Ocean acidification is affecting marine organisms and this could alter marine ecosystems.
Relevance
Across the ocean, the pH of surface waters has been
relatively stable for millions of years. Over the last
million years, average surface water pH oscillated
between 8.3 during cold periods (e.g. during the
last glacial maximum, 20 000 years ago) and 8.2
during warm periods (e.g. just prior to the industrial
revolution). Rapid increases in atmospheric CO 2
concentration due to emissions from human
activities are now threatening this stability, as the
CO 2 is subsequently partially absorbed in the ocean.
Currently, the ocean takes up about one-quarter
of the global CO 2 emissions coming from human
activities, e.g. combustion of fossil fuels. The uptake of
CO 2 in the sea causes ocean acidification, as the pH of
sea water declines, even though ocean surface waters
will remain alkaline.
When CO 2 is absorbed by the ocean, it reacts with
water, producing carbonic acid. Carbonic acid
dissociates to form bicarbonate ions and protons,
which further react with carbonate ions. The
carbonate ions act as a buffer, helping to limit the
decline in ocean pH; however, they are being used
up as more and more anthropogenic CO 2 is added
to the ocean. As carbonate ion concentrations
decline, so does the ocean's capacity to take up
more anthropogenic CO 2 . Hence, the ocean's ability
to moderate atmospheric CO 2 and thus climate
change comes at the cost of substantial changes in its
fundamental chemistry.
Ocean acidification can have wide-ranging impacts
on biological systems by reducing the availability
of carbonate (Secretariat of the Convention on
Biological Diversity, 2014). Decreasing carbonate
ion concentrations reduce the rate of calcification of
marine calcifying organisms, such as reef-building
corals, mussels and plankton. pH also affects biological
molecules and processes, e.g. enzyme activities and
photosynthesis. Thus, anthropogenic acidification could
affect entire marine ecosystems. Organisms appear to
be increasingly sensitive to acidification when they are
concurrently exposed to elevated seawater temperature
(Kroeker et al., 2013). Of equal importance is the effect
of acidification on primary producers, as it changes the
bioavailability of essential nutrients, such as iron and
zinc. Primary producers are responsible for a significant
part of global carbon fixation, thereby forming the basis
of marine food webs (Reid et al., 2009).
Past trends
The annual mean atmospheric CO 2 concentration
reached 397 ppm in 2014, which is 40 % above the
pre-industrial level (280 ppm); half of that increase
has occurred since the 1980s. Over the same time
period, ocean pH has been reduced from 8.2 to below
8.1, which corresponds to an increase of about 30 %
in ocean acidity (defined here as the hydrogen ion
concentration). This change has occurred at rates
ranging between – 0.0014 and – 0.0024 per year, which
is about a hundred times faster than any change in
acidity experienced during the last 55 million years
108 Climate change, impacts and vulnerability in Europe 2016 | An indicator-based report