Acid Rain Destroying Plant, Marine and Human Life

Since the beginning of the industrial revolution the release of acidic carbon dioxide (CO2) from human activities has resulted in atmospheric CO2 concentrations that have increased from approximately 280 to 385 parts per million (ppm). The atmospheric concentration of CO2 is now higher than experienced on Earth for at least the last 800,000 years and probably over 20 million years, and is expected to continue to rise at an increasing rate, leading to significant temperature increases in the atmosphere and oceans in the coming decades. The oceans have adsorbed and absorbed approximately 525 billion tons of carbon dioxide acid from the atmosphere, or about one third of the anthropogenic carbon emissions released. This absorption has benefited humankind by significantly reducing the greenhouse gas levels in the atmosphere and minimizing some of the impacts of global warming. However, the ocean's uptake of carbon dioxide acid is having negative impacts on the chemistry and biolog y of the oceans. Hydrographic surveys and modeling studies have revealed that the chemical changes in seawater resulting from the absorption of carbon dioxide acid are lowering seawater pH. The pH of ocean surface waters has already decreased by about 0.1 units from an average of about 8.21 to 8.10 since the beginning of the industrial revolution. Estimates of future atmospheric and oceanic carbon dioxide acid concentrations, based on the Intergovernmental Panel on Climate Change (IPCC) CO2 emission scenarios and coupled ocean-atmosphere models, suggest that by the middle of this century atmospheric carbon dioxide acid levels could reach more than 500 ppm, and near the end of the century they could be over 800 ppm. This would result in an additional surface water pH decrease of approximately 0.3 pH units by 2100.

The ocean absorbs approximately 1/3rd of the CO2 acid emitted to the atmosphere from the burning of fossil fuels. However, this valuable service comes at a steep ecological cost - the acidification of the ocean. As CO2 acid dissolves in seawater, the pH of the water decreases, which is called "acidification".

While these pH levels are not alarming in themselves, the rate of change is cause for concern. To the best of our knowledge, the ocean has never experienced such a rapid acidification. By the end of this century, if concentrations of CO2 acid continues to rise exponentially, we may expect to see changes in pH that are three times greater and 100 times faster than those experienced during the transitions from glacial to interglacial periods. Such large changes in ocean pH have probably not been experienced on the planet for the past 21 million years.

When the CO2 acid reacts with seawater, the reduction in alkaline seawater pH also reduces the availability of alkalizing carbonate ions, which play an important role in shell formation for a number of marine organisms such as corals, marine plankton, and shellfish. This phenomenon, which is commonly called "ocean acidification," could have profound impacts on some of the most fundamental biological and geochemical processes of the sea in coming decades. Some of the smaller calcifying organisms are important food sources for higher marine organisms. Declining coral reefs due to increases in temperature and decreases in carbonate ion would have negative impacts on tourism and fisheries. Abundance of commercially important shellfish species may also decline and negative impacts on fin fish may occur. This rapidly emerging scientific issue and possible ecological impacts have raised serious concerns across the scientific and fisheries resource management communities.

When CO2 acid dissolves in seawater, it forms carbonic acid, which releases hydrogen ions into solution. Acidity is a measure of the hydrogen ion concentration in the water, where an increase in hydrogen leads to an increase in acidity (and a decrease in the pH scale used to quantify acidity). These hydrogen ions then combine with carbonate ions in the water to form bicarbonate. Carbonate ions are the basic building blocks for the shells of many marine organisms. Thus the formation of bicarbonate through this chemical reaction removes carbonate ions from the water, making them less available for use by organisms. The combination of increased acidity and decreased carbonate concentration has implications for many functions of marine organisms, many of which we do not yet fully understand.

The details of the reactions look like this:

CO2 + H20 <=> H2CO3

When CO2 acid dissolves in seawater, carbonic acid is produced via the reaction:

H2CO3 <=> H + HCO3

This carbonic acid dissociates in the water, releasing hydrogen ions and bicarbonate:

The increase in the hydrogen ion concentration causes an increase in acidity, since acidity is defined by the pH scale, where pH = -log [H+] (so as hydrogen increases, the pH decreases). This log scale means that for every unit decrease on the pH scale, the hydrogen ion concentration has increased 10-fold.

One result of the release of hydrogen ions is that they combine with any carbonate ions in the water to form bicarbonate:

This removes carbonate ions from the water, making it more difficult for organisms to form the CaCO3 they need for their shells.

H + CO3 <=> HCO3

How acidic are the oceans?

The oceans are not, in fact, acidic, but slightly basic.

Acidity is measured using the pH scale, where 7.0 is defined as neutral, with higher levels called "basic" and lower levels called "acidic".

Historical global mean seawater values are approximately 8.16 on this scale, making them slightly basic.

To put this in perspective, pure water has a pH of 7.0 (neutral), whereas household bleach has a pH of 12 (highly basic) and battery acid has a pH of zero (highly acidic).

However, even a small change in pH may lead to large changes in ocean chemistry and ecosystem functioning. Over the past 300 million years, global mean ocean pH values have probably never been more than 0.6 units lower than today. Ocean ecosystems have thus evolved over time in a very stable pH environment, and it is unknown if they can adapt to such large and rapid changes.

What can we expect in the future?

Based on the emissions scenarios of the Intergovernmental Panel on Climate Change and general circulation models, we may expect a drop in ocean pH of about 0.4 pH units by the end of this century, and a 60% decrease in the concentration of calcium carbonate, the basic building block for the shells of many marine organisms.

Changes in atmospheric CO2 acid (acid rain) under the "business as usual" scenario to the year 2100 and associated changes in ocean pH and carbon chemsitry.

Today, the surface ocean is saturated with respect to calcium carbonate (including its several mineral forms, i.e., high-magnesium calcite, aragonite, and calcite), meaning that under present surface conditions these minerals have no tendency to dissolve and that there is still enough calcium and carbonate ions available for marine organisms to build their shells or skeletons. Colder and deeper waters are naturally under-saturated with respect to calcium carbonate, where the water is corrosive enough to dissolve these minerals. The transition between saturated surface waters and under-saturated deep waters is called the saturation horizon. Because of the increase in CO2 acid entering into the ocean from the atmosphere, the saturation horizons for calcium carbonate have shifted towards the surface by 50-200 meters compared with their positions before the industrial revolution. This means that the zone occupied by under-saturated deep waters is growing larger and the zone occup ied by the saturated surface waters is growing smaller.

By 2050, this saturated surface zone will begin to completely disappear in some areas of the ocean. High-latitude surface waters, already naturally low in calcium and carbonate ion concentration, will be the first to have under-saturated surface waters with respect to aragonite, with under-saturations for the calcite phase of calcium carbonate expected to follow 50-100 years later.

Before the industrial revolution, we see large bands of the tropical ocean that are optimal for growth. By 2040, these same bands are only adequate, and by 2100 most areas are only marginal at best.

Many scientists believe that stabilizing atmospheric CO2 acid concentration at 550 parts per million (ppm) may avoid the worst impacts from climate change. Atmospheric concentration of CO2 is currently 380 ppm and, if no precautionary action is taken, is expected to reach 550 ppm by the middle of this century. However, if we consider the impacts of CO2 acid on ocean chemistry and ecosystems rather than on climate considerations alone, there are strong arguments to be made for a lower stabilization target of 450 ppm.

In order to prevent changes that would lead to under-saturation of aragonite and put marine ecosystems at risk, it has been suggested that the average pH of surface waters should be prevented from dropping by more than 0.2 units below the pre-industrial value. Stabilization of atmospheric CO2 acid concentrations at 450 ppm by the year 2100 would lead to a pH decrease of about 0.17; stabilization at 540 ppm by the year 2100 would lead to a decrease of 0.23 pH units. With stabilization at 450 ppm, about 7% of the Southern Ocean will still become under-saturated with respect to aragonite. At 550 ppm, about half of the Southern Ocean will be under-saturated.

According to Dr. Robert O. Young, Director of Research at the pH Miracle Living Center, in San Diego County, California, "acid rain is not only acidifying are oceans and putting marine life in harms way but is also destroying are green forests, such as in Denali, Alaska, placing human life in harms way."

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