Alkalinity From Space

A new
is helping scientists monitor how the
world’s oceans are changing in response to human pressures. As the
level of carbon dioxide in the atmosphere rises, images beamed back
from space show where it’s being absorbed into the oceans, making
them more acidic.

Ocean acidification is a serious but often
overlooked concern facing the world’s oceans and the shellfish,
corals and other creatures that reside in them, say the

The international team of scientists published
some of their early findings and images in the journal
Environmental Science and Technology today.

Acidifying oceans

of the carbon dioxide released into
the atmosphere dissolves into our seas. This changes the seawater
chemistry, making it more acidic. This is known as ocean

Since the start of the industrial era, the pH of
ocean surface water has dropped by 0.1, equivalent to a
26 per cent
increase in acidity.

But acidification isn’t happening at the same pace everywhere,
some places are acidifying faster than others. Observing the earth
from space using satellites can help identify which regions on
Earth are most at risk from ocean acidification.

A global map of total ocean alkalinity, an
indication of the sea surface’s ability to buffer itself against
ocean acidification. Credit: Ifremer/ESA/CNES

Microwave sensors on the satellite measure the
salt content – or salinity – of the water. The scientists can use
salinity, alongside other sea surface measurements, as a proxy for
something known as the total alkalinity. This is a measure of the
ocean’s ability to neutralise acid.

Changes in total alkalinity can’t directly be
used to monitor ocean acidification but they can help identify
areas that will acidify most for a given increase in
carbon dioxide. All other things being equal,
the lower the alkalinity, the more the water will acidify for a
given amount of carbon dioxide.

The team is made up of scientists
from the University
of Exeter
, Plymouth Marine
, Ifremer
, the European Space Agency (ESA) and
international collaborators.

Acidifying hotspots

Coral skeletons are made from a form of calcium
carbonate, which can’t grow when water acidity exceeds a certain
level. At least two thirds of reefs in the Greater Caribbean Region
are threatened by human impacts, including ocean acidification, the
paper notes.

Dr Peter
, an expert in satellite earth observation at the Plymouth
Marine Laboratory and lead author on the new paper, tells Carbon

“Ocean acidification is
expected to have a direct effect on calcifying organisms such as
corals, making it harder for them to grow, with knock-on effects on
organisms that depend on them … As with temperature, each
organism has its own preferred range of pH, just like plants in a
garden. When the whole ocean becomes more acidic, this can leave
some organisms with nowhere to hide.”

The polar
in the Arctic and Antarctic are particularly sensitive
to ocean acidification. The Bay of Bengal is another major focus of
research, partly because of unique sea water water characteristics
and partly because of poor data coverage using traditional

A global approach

Traditionally, scientists monitor ocean
acidification using instruments deployed from research ships at
sea. But information gathered this way is limited by where ships
travel to.

Dr Jamie Shutler
from the University of
Exeter, who heads the research team,

“It can be both
difficult and expensive to take year-round direct measurements in
such inaccessible locations. We are pioneering this data
fusion approach so that we can observe large areas of Earth’s
oceans, allowing us to quickly and easily identify
those areas most at risk from the increasing

Scientists have also put sensors on a network of
free-floating buoys, known as ARGO
. But these don’t yet have global

The benefit of the satellite circling Earth 700
kilometres above our heads is that it gathers information from
remote regions that are hard to reach on the ground, such as the

Satellites are also a very efficient way to
monitor Earth’s surface, as they take a snapshot of the whole
planet every few days.

Satellite futures

Satellites have been making reliable
measurements of sea surface
and chlorophyll, the green
pigment in plants, for a couple of decades now. But they have only
been capable of measuring sea surface salinity since

The ESA Soil Moisture and Ocean Salinity (SMOS)
and Nasa’s Aquarius satellites measure salinity, but these are
nearing the end of their lifetimes, the paper
notes. The new research is the first to build a global map
representative of ocean acidification from a complete set of
satellite sea surface salinity data.

At the moment, Nasa’s Ocean Carbon Observatory
), launched in December 2014, is
designed to monitor atmospheric carbon dioxide. But ocean
acidification is likely to be a focus of future work, say the


Updated 18th Feb 09:15 to clarify the relationship
between total alkalinity and a region’s vulnerability to ocean
acidifcation. Many thanks for the comments leading to this

Updated 17th Feb 2015 15:00 to explain how carbon
dioxide dissolving in the oceans relates to total alkalinity.
Originally, the piece implied carbon dioxide has a direct effect on
alkalinity. In fact, it has an indirect effect through changing the
carbonate chemistry of seawater. This has been clarified with a
couple of extra sentences.