It’s not just about the weather
Global warming has been a high profile issue in recent months and Al Gore’s recent film has helped raise the profile of this issue. Simply, burning fossil fuels releases carbon dioxide. Human use of fossil fuels has significantly increased the levels of carbon dioxide in the atmosphere over the last 200 years or so. Carbon dioxide acts like a blanket. It slows the loss of heat from the earth, preventing it escaping into space with the result that the atmosphere gets warmer. The impact of this on sea levels, glaciers and polar ice is now well known.
One of the less publicized issues of concern about ‘global warming’ is the impact of higher levels of atmospheric carbon dioxide on the oceans. When it dissolves in water, carbon dioxide forms weak carbonic acid. Some of the extra atmospheric carbon dioxide dissolves in the sea. The scientific community has been modeling and monitoring this possibility for some time and it is now clear, the oceans are becoming more acidic.
This has a direct effect on the marine ecosystems, and is especially significant in the Southern Ocean. The main impact is on the tiny plants and animals at the bottom of the food chain. Disrupting the food chain affects the whole Antarctic ecosystem up to, and including, whales.
Gases in our atmosphere exist in equilibrium with the oceans, following a physical rule known as ‘Henry’s Law’. The amount of carbon dioxide in the atmosphere is in equilibrium with the amount of carbon dioxide that is dissolved in the sea. In pre-industrial times ocean life generated excess carbon dioxide which was then released into the atmosphere to reach a balance.
Today, this is not the case. With the increasing levels of carbon dioxide emitted by our fossil fuel burning society, more carbon dioxide is entering the ocean than is leaving it. The ocean absorbs about one million tons of carbon dioxide every hour, which is about ten times the natural pre-industrial rate. The oceans are a carbon dioxide ‘sink’ and this is believed to have acted as a ‘buffer’ that has slowed the rate of global warming. It is estimated that the oceans have absorbed about 30% of the carbon dioxide released by our industrial society. In the next 50 years or so, the oceans will be absorbing an increasing amount of the carbon generated by human society, especially if no changes are made to reduce that generation.
There is another simple but important piece of chemistry at work though. Acids (including carbonic acid) dissolve carbonates such as calcium carbonate, which is the mineral in bones, teeth, seashells and the skeletons of marine organisms. When acid reacts with carbonate, it releases more carbon dioxide.
The increase in dissolved carbon dioxide thus reduces the concentration of dissolved calcium carbonate that is available to marine life in the ocean. And here is the sinister point. If you are small and float in the ocean, a way to protect yourself is by having a shell. Just like corals. Most marine animals make their shells out of calcium carbonate. These little animals are among the most important parts of the ecosystem. They make up the critical bottom end of the food chain. Other small animals eat them, then other predators eat these small animals, and so on. With increased acidity, the problems are twofold. First, it decreases the amount of calcium carbonate available to these animals to make their shells and secondly, the acidity erodes and weakens the shells once they are made.
In the same way that there is a dynamic balance between the amount of carbon dioxide in the atmosphere and in the sea, there is also a balance related to the amount of calcium carbonate dissolved in the sea. In ages past, the sea was abundant with calcium carbonate. Coral reefs grew, built from the shells of small animals. Nowadays, as the level of carbon dioxide in the seawater changes, the water becomes under-saturated in calcium carbonate. When this happens, animals that use calcium carbonate are less able to make their shell and are in trouble.
The widely held belief has been that a change in calcium carbonate levels would take hundreds of years to occur, but recent work shows that ecologically destructive changes can happen in decades. Modeling suggests this will be critical between 2050 and 2100.
The Southern Ocean will be hit the hardest because there are limited sources of carbonate to replenish the sea. One of the significant sources available is known as ‘aragonite’ and Antarctic animals rely on it. One of the most abundant animals is the pteropod, a snail. It is near the bottom of the food chain and many animals depend on it for food. Current research suggests that by 2100, aragonite will not be available for pteropods to make their shells. They are likely to disappear.
This loss will effect other forms of sealife. The abundance of small fish will change, which will in turn effect animals such as larger fish, penguins, and seals. The abundance of krill (which also have shells composed of calcium carbonate) will change, altering local ecosystems that rely on them for food and affect whales that migrate to feed in Antarctica.
It is difficult to predict what changes might happen when these tiny, but important animals disappear. What is known though, is that the changes will be wide-ranging and profound, and they are likely to happen in my life-time. They will certainly affect my children.
The problem with global warming is that it is a growth game. The earlier we make cuts to greenhouse gases the less reductions we have to make. The less we do now, the more we have to do later. Just a few years ago, scientists were estimating the melting of the Antarctic ice sheets would take thousands of years, now they say it is a few hundred. The spectre of icebergs drifting up our coast is a message it is hard to ignore.
Raelene Jonstone is a BEd(teaching) student at Massey University College of Education.