Download Overview of Ocean pH Acidity is one of the properties of matter, and

Overview of Ocean pH
Acidity is one of the properties of matter, and it is usually associated with liquids. We can
easily measure the acid-base properties of a liquid solution, and determine whether it is
acidic, neutral or basic. Pure water is neutral. Pure water has an acidic component, but it is
exactly balanced by pure water’s basic component.1
Vinegar is an acid. Vinegar has about 40,000 times the acidity of pure water. Household
ammonia is basic. It has 10,000 times less of water’s acidic component and 10,000 times
more of water’s basic component. A strong acidic solution, such as battery acid, is
1,000,000,000,000 (one thousand billion) times more acidic than a strong basic solution,
such as household lye or oven cleaner.
Scientists use what is called the pH scale to classify the acid-base properties of a solution.
Absolutely pure neutral water has a value of 7 on the pH scale. An acidic solution has a pH
value of less than 7, and a basic solution has a value greater than 7.
The pH scale is what mathematicians call a logarithmic scale. A change of one unit (say
from pH 7 to pH 8, or from pH 7 to pH 6) represents a change in ten times the amount. A
solution with a pH of 6 has ten times the acidity of neutral water. A solution with a pH of 8
has one tenth the acidity of neutral water. The chart below shows how much the acidity
increases for different decreases in pH. Note that as pH goes down, acidity goes up.
How Much the Acidity Increases for Different Changes in pH
pH Decrease
Amount of Increase in Acidity
0.1 pH decrease
25% more acidity
0.2 pH decrease
60% more acidity
0.3 pH decrease
2 times more acidity
0.7 pH decrease
5 times more acidity
1.0 pH decrease
10 times more acidity
2.0 pH decrease
100 times more acidity
3.0 pH decrease
1,000 times more acidity
With respect to climate change, the main reason we care about acidity is that when carbon
dioxide dissolves in water, it increases the acidity of water. Carbon dioxide (CO2) is the
most important heat-trapping greenhouse gas that humans are adding to the atmosphere.
When we burn oil, coal or gas, we are taking carbon from under the ground, changing it
into CO2, and putting that CO2 into the air. CO2is the main gas in the atmosphere involved in
human-caused climate change, and CO2 is now causing the pH of the oceans to change.
In 1958 we started systematically measuring the amount of carbon dioxide in the
atmosphere. As a result of these measurements, we know that humans are causing the
1
The acidic component of water is H+ and the basic component is OH-.
amount of CO2in the atmosphere to increase. As of 2011, humans have caused about a 40%
increase in the amount of CO2 in the atmosphere.
We can make good estimates of how much CO2we are adding to the atmosphere. We know
that we are adding more CO2 to the atmosphere than is actually staying there. In other
words, based on how much fossil fuel we have been burning and other human activities,
the amount of CO2in the atmosphere should have increased even more than it has. It turns
out that each year oceans absorb about 30% of the extra CO2that we are putting into the
atmosphere.
At first, the scientific community considered this to be good news. The oceans were helping
us by absorbing CO2, and this absorption of the heat-trapping gas was reducing the amount
of climate change. Now we realize that this good news may not be so good. The extra
CO2absorbed by the oceans has caused the ocean pH to decrease by about 0.1 pH units.
While a change of 0.1 pH units does not seem to be a lot, it represents about a 25% increase
in acidity.
Why do we care about this increase in ocean acidity?2 The shells and skeletons of many
ocean plants and animals are built from carbonate, a chemical that naturally occurs in the
ocean. As the acidic component of ocean water increases, the concentration of carbonate
decreases and it becomes more difficult for these organisms to build their shells and
skeletons. As a result, increases in ocean acidity threaten microscopic phytoplankton,
shellfish and huge coral reefs, as well as the food webs that depend on them.
We are currently on a path to double or even triple by the end of this century the preindustrial concentration of carbon dioxide in the atmosphere. In addition to causing large
changes in climate, these high concentrations of CO2could cause the ocean pH to have
decreased by 0.3 to 0.4 pH units. The resulting more than doubling of the ocean’s acidity
could cause very severe problems for life that depends on shell-building organisms.
Because of the increase in the ocean’s acidic component, this issue is generally called
ocean acidification. The natural pH of ocean water is about 8.0. Adding carbon dioxide to
the atmosphere increases the acidic component of oceans, but it will not make the oceans
acidic (pH less than 7). The very negative impacts of ocean acidification will occur long
before the ocean pH could drop that low.
2